Respiratory System 423
2. During inspiration, contraction of the muscles of inspiration The normal respiration rate for a newborn is around 40 breaths
increases the volume of the thoracic cavity. The increased per minute. Tu’s high respiratory rate is stimulated by high blood CO2
thoracic volume causes the lungs to expand, resulting in an levels and low blood O2 levels (see “Chemical Control of Breathing”
increase in alveolar volume (see “Changing Alveolar later in this chapter). Her blue lips, tongue, and nail beds are signs of
Volume” later in this section). As the alveolar volume cyanosis caused by deoxygenated blood. The nasal flaring occurs to
increases, alveolar pressure becomes less than atmospheric maximize air intake.
pressure, and air flows from outside the body through the If too little surfactant has been produced by the time of birth,
respiratory passages to the alveoli (figure 15.11, step 2). the lungs tend to collapse, and the muscles of respiration must
exert a great deal of energy to keep the lungs inflated; even then,
3. At the end of inspiration, the thorax and alveoli stop ventilation is inadequate. The thoracic cage in newborns is very
expanding. When the alveolar pressure and atmospheric pliable. During labored inspiration, the increased inferior movement
pressure become equal, airflow stops (figure 15.11, step 3). of the diaphragm causes such a decrease in thoracic cavity pressure
that the thoracic cage is pulled inward as the abdomen expands. The
4. During expiration, the thoracic volume decreases, producing more labored the breathing, the more exaggerated the expansion of
a corresponding decrease in alveolar volume. Consequently, the abdomen and the inward movement of the thoracic cage.
alveolar pressure increases above atmospheric pressure, and An expiratory grunt is a gruff, throaty sound made during
air flows from the alveoli through the respiratory passages to expiration. It is caused by partial closure of the vestibular and
the outside (figure 15.11, step 4). vocal folds. Expiratory grunting increases airway pressure and helps
prevent alveolar collapse.
As expiration ends, the decrease in thoracic volume stops, and The shake test determines the presence of surfactant in lung
the process repeats, beginning at step 1. fluid. Fetal lung fluid is either swallowed by the fetus or passed
through the mouth into the amniotic fluid, so 30 minutes after
Lung Recoil delivery the fetus’s gastric fluid still contains swallowed lung fluid
and amniotic fluid. To perform the shake test, a sample of gastric
During quiet expiration, thoracic volume and lung volume decrease fluid is collected and then mixed with saline and alcohol, placed
because of lung recoil, the tendency for an expanded lung to decrease in a tube, and shaken. A positive shake test produces bubbles,
in size. The thoracic wall also recoils due to the elastic properties of but a negative shake test does not, which indicates that very little
its tissues. Lung recoil is able to occur because the connective tissue surfactant is present because the bubbles collapse.
of the lungs contains elastic fibers and because the film of fluid lining Without specialized treatment, most babies with IRDS die
the alveoli has surface tension. Surface tension exists because the soon after birth due to inadequate ventilation of the lungs and
oppositely charged ends of water molecules are attracted to each other respiratory muscle fatigue.
(see chapter 2). As the water molecules pull together, they also pull on Treatment strategies include forcing enough oxygen-rich air
the alveolar walls, causing the alveoli to recoil and become smaller. into the lungs to inflate them and administering purified, natural
Two factors keep the lungs from collapsing: (1) surfactant and surfactant via an intubation tube directly into the lungs.
(2) pressure in the pleural cavity.
Pleural Pressure Respiratory
Surfactant
When pleural pressure, the pressure in the pleural cavity, is less than
Surfactant (ser-fak′ tănt; surface acting agent) is a mixture of alveolar pressure, the alveoli tend to expand. This principle can be
lipoprotein molecules produced by secretory cells of the alveolar understood by considering a balloon. The balloon expands when the
epithelium. The surfactant molecules form a single layer on the pressure outside it is less than the pressure inside. This pressure
surface of the thin fluid layer lining the alveoli, reducing surface ten- difference is normally achieved by increasing the pressure inside
sion. Without surfactant, the surface tension causing the alveoli to the balloon by blowing into it. This pressure difference, however,
recoil can be ten times greater than when surfactant is present. Thus, can also be achieved by decreasing the pressure outside the balloon.
surfactant greatly reduces the tendency of the lungs to c ollapse. For example, if the balloon is placed in a chamber from which air is
removed, the pressure around the balloon becomes lower than atmo-
A Case in Point spheric pressure, and the balloon expands. The lower the pressure
outside the balloon, the greater the tendency for the higher pres-
Infant Respiratory Distress Syndrome sure inside the balloon to cause it to expand. In a similar fashion,
decreasing pleural pressure can result in expansion of the alveoli.
Tu Soun was born 3 months prematurely. She presented with a Normally, the alveoli are in the expanded state because pleural
respiration rate of 68 breaths per minute; blue lips, tongue, and pressure is lower than alveolar pressure. Pleural pressure is lower
nail beds; nasal flaring during inspiration; inward movement of than alveolar pressure because of a suction effect caused by fluid
the thoracic cage and outward movement of the abdomen during removal by the lymphatic system (see chapter 14) and by lung recoil.
inspiration; an expiratory grunt; and a negative shake test. Tu As the lungs recoil, the visceral and parietal pleurae tend to be pulled
Soun has infant respiratory distress syndrome (IRDS), caused by apart. Normally, the lungs do not pull away from the thoracic wall
too little surfactant, a substance that covers the lining of the lung because pleural fluid holds the visceral and parietal pleurae together.
alveoli, where it helps reduce surface tension. IRDS, also called Nonetheless, this pull decreases pressure in the pleural cavity. You
hyaline membrane disease, is common in premature infants because can appreciate this effect by putting water on the palms of your
surfactant is not produced in adequate quantities until about the hands and then placing them together. As you gently pull your hands
seventh month of gestation. Thereafter, the amount produced apart, you will feel a sensation of negative pressure.
increases as the fetus matures. Pregnant women who are likely to
deliver prematurely can be given cortisol, which crosses the placenta
into the fetus and stimulates surfactant synthesis.
424 Chapter 15 Thorax Atmospheric pressure
Atmospheric pressure expands. During inspiration
End of expiration Air moves in.
No air
movement
Alveolar pressure Alveolar pressure
equals atmospheric is less than
pressure. atmospheric pressure.
Diaphragm
Rib 9 Diaphragm
contracts.
Rib 9
1 At the end of expiration, alveolar pressure is 2 During inspiration, increased thoracic
equal to atmospheric pressure, and there is volume results in increased alveolar volume
no air movement. and decreased alveolar pressure.
Atmospheric pressure is greater than
alveolar pressure, and air moves into the
lungs.
Atmospheric pressure Thorax Atmospheric pressure
End of inspiration recoils. During expiration
No air Air moves out.
movement
Respiratory Alveolar pressure Alveolar pressure
equals atmospheric is greater than
pressure. atmospheric pressure.
Rib 9 Diaphragm
relaxes.
Rib 9
3 At the end of inspiration, alveolar pressure is 4 During expiration, decreased thoracic
equal to atmospheric pressure, and there is no volume results in decreased alveolar
volume and increased alveolar pressure.
air movement. Alveolar pressure is greater than
atmospheric pressure, and air moves out of
the lungs.
PROCESS Figure 15.11 Alveolar Pressure Changes During Inspiration and Expiration
the combined space of all the alveoli is represented by a large “bubble” (blue). the alveoli are actually microscopic and cannot be seen in the illustration.
Respiratory System 425
CLINICAL IMPACT Pneumothorax
A pneumothorax (noo-mo¯- the lung surface rupture, as can occur in a outside the body. When pleural pressure
tho¯r′aks) is the introduction of air into patient with emphysema. When the pleu- and alveolar pressure are equal, there is
the pleural cavity, the space between the ral cavity is connected to the outside by no tendency for the alveoli to expand,
parietal and visceral pleurae that normally such openings, the pressure in the pleural lung recoil is unopposed, and the lungs
contains only pleural fluid. Air can enter by cavity increases and becomes equal to the collapse. A pneumothorax can occur in
an external route, as when a sharp object, air pressure outside the body. Thus, pleural one lung while the other remains inflated
such as a bullet or a broken rib, penetrates pressure is also equal to alveolar pressure because the two pleural cavities are sepa-
the thoracic wall, or air can enter the pleu- because pressure in the alveoli at the rated by the mediastinum.
ral cavity by an internal route if alveoli at end of expiration is equal to air pressure
When pleural pressure is lower than alveolar pressure, the Respiratory Volumes and Capacities Respiratory
alveoli tend to expand. This expansion is opposed by the tendency
of the lungs to recoil. Therefore, the alveoli expand when the Spirometry (spı-rom′ -tr ) is the process of measuring volumes
pleural pressure is low enough that lung recoil is overcome. If of air that move into and out of the respiratory system, and the
the pleural pressure is not low enough to overcome lung recoil, spirometer (spı-rom′ -ter) is the device that measures these respi-
the alveoli collapse, as is the case with a pneumothorax (see the ratory volumes. Measurements of the respiratory volumes can
Clinical Impact: “Pneumothorax”). provide information about the health of the lungs. Respiratory
volumes are measures of the amount of air movement during dif-
Predict 6 ferent portions of ventilation, whereas respiratory capacities are
sums of two or more respiratory volumes. The four respiratory
Treatment of a pneumothorax involves closing the opening into volumes and their normal values for a young adult male are shown
the pleural cavity that caused the pneumothorax, then placing in figure 15.12:
a tube into the pleural cavity. In order to inflate the lung,
should this tube pump in air under pressure (as in blowing 1. Tidal volume is the volume of air inspired or expired with
up a balloon), or should the tube apply suction? Explain. each breath. At rest, quiet breathing results in a tidal
volume of about 500 milliliters (mL).
Changing Alveolar Volume
2. Inspiratory reserve volume is the amount of air that can
Changes in alveolar volume cause the changes in alveolar pressure be inspired forcefully beyond the resting tidal volume
that are responsible for moving air into and out of the lungs (see (about 3000 mL).
figure 15.11). Alveolar volume changes result from changes in
pleural pressure. For example, during inspiration, pleural pressure 3. Expiratory reserve volume is the amount of air that can
decreases, and the alveoli expand. The decrease in pleural pressure be expired forcefully beyond the resting tidal volume
occurs for two reasons: (about 1100 mL).
1. Increasing the volume of the thoracic cavity results in a 4. Residual volume is the volume of air still remaining in the
decrease in pleural pressure because a change in volume respiratory passages and lungs after maximum expiration
affects pressure. (about 1200 mL).
2. As the lungs expand, lung recoil increases, increasing the The tidal volume increases when a person is more active.
suction effect and lowering the pleural pressure. The Because the maximum volume of the respiratory system does not
increased lung recoil of the stretched lung is similar to change from moment to moment, an increase in the tidal volume
the increased force generated in a stretched rubber band. causes a decrease in the inspiratory and expiratory reserve volumes.
The events of inspiration and expiration can be summarized Predict 7
as follows:
The minute ventilation is the total amount of air moved into
1. During inspiration, pleural pressure decreases because and out of the respiratory system each minute, and it is equal
of increased thoracic volume and increased lung recoil. to the tidal volume times the respiratory rate. The respiratory
As pleural pressure decreases, alveolar volume increases, rate is the number of breaths taken per minute. Calculate
alveolar pressure decreases, and air flows into the lungs. the minute ventilation of a resting person who has a tidal
volume of 500 mL and a respiratory rate of 12 respirations/min
2. During expiration, pleural pressure increases because of and the minute ventilation of an exercising person who
decreased thoracic volume and decreased lung recoil. has a tidal volume of 4000 mL and a respiratory rate of
As pleural pressure increases, alveolar volume decreases, 24 respirations/min.
alveolar pressure increases, and air flows out of the lungs.
426 Chapter 15
6000
Maximum Volumes Capacities
5000 inspiration
4000 Inspiratory reserve volume Inspiratory capacity Total lung capacity (5800 mL)
(3000 mL) (3500 mL)
Volume (mL) Vital capacity (4600 mL)
3000
Tidal
volume
(500
mL)
2000 Maximum Expiratory Functional residual capacity
1000 expiration reserve (2300 mL)
volume
(1100 mL)
Residual
volume
(1200 mL)
0
Time
Figure 15.12 Respiratory Volumes and Respiratory Capacities
The tidal volume shown here is during resting conditions. Respiratory volumes are measurements of the volume of air moved into and out of the lungs during
breathing. Respiratory capacities are the sum of two or more respiratory volumes.
Respiratory Values of respiratory capacities, the sum of two or more pul- a greater vital capacity than short people, and thin people have a
monary volumes, are shown in figure 15.12: greater vital capacity than obese people. Well-trained athletes can
have a vital capacity 30–40% above that of untrained people. In
1. Functional residual capacity is the expiratory reserve patients whose respiratory muscles are paralyzed by spinal cord
volume plus the residual volume. This is the amount of air injury or diseases such as poliomyelitis or muscular dystrophy, the
remaining in the lungs at the end of a normal expiration vital capacity can be reduced to values not consistent with survival
(about 2300 mL at rest). (less than 500–1000 mL).
The forced expiratory vital capacity is the rate at which
2. Inspiratory capacity is the tidal volume plus the inspiratory lung volume changes during direct measurement of the vital
reserve volume. This is the amount of air a person can capacity. It is a simple and clinically important pulmonary test.
inspire maximally after a normal expiration (about The individual inspires maximally and then exhales maximally
3500 mL at rest). as rapidly as possible into a spirometer. The spirometer records
the volume of air expired per second. This test can help identify
3. Vital capacity is the sum of the inspiratory reserve volume, conditions in which the vital capacity might not be affected but
the tidal volume, and the expiratory reserve volume. It is the expiratory flow rate is reduced. Abnormalities that increase the
the maximum volume of air that a person can expel resistance to airflow slow the rate at which air can be forced out
from the respiratory tract after a maximum inspiration of the lungs. For example, in people who have asthma, contraction
(about 4600 mL). of the smooth muscle in the bronchioles increases the resistance to
airflow. In people who have emphysema, changes in the lung tissue
4. Total lung capacity is the sum of the inspiratory and result in the destruction of the alveolar walls, collapse of the bron-
expiratory reserves and the tidal and residual volumes chioles, and decreased elasticity of the lung tissue. The collapsed
(about 5800 mL). The total lung capacity is also equal bronchioles increase the resistance to airflow. In people who have
to the vital capacity plus the residual volume. chronic bronchitis, the air passages are inflamed. The swelling,
increased mucus secretion, and gradual loss of cilia result in narrowed
Factors such as sex, age, and body size influence the respira- bronchioles and increased resistance to airflow.
tory volumes and capacities. For example, the vital capacity of
adult females is usually 20–25% less than that of adult males.
The vital capacity reaches its maximum amount in young adults
and gradually decreases in the elderly. Tall people usually have
Respiratory System 427
15.4 Gas Exchange Partial Pressure
Learning Outcomes After reading this section, you should be able to Gas molecules move randomly from higher concentration to lower
concentration until an equilibrium is achieved. One measurement
A. Explain the factors that affect gas movement through the of the concentration of gases is partial pressure. The partial pres-
respiratory membrane. sure of a gas is the pressure exerted by a specific gas in a mixture
of gases, such as air. For example, if the total pressure of all the
B. Describe the partial pressure gradients for O2 and CO2. gases in a mixture of gases is 760 millimeters of mercury (mm Hg),
which is the atmospheric pressure at sea level, and 21% of the
Ventilation supplies atmospheric air to the alveoli. The next step mixture is made up of O2, the partial pressure for O2 is 160 mm Hg
in the process of respiration is the diffusion of gases between the (0.21 × 760 mm Hg = 160 mm Hg). If the composition of air is
alveoli and the blood in the pulmonary capillaries. As previously 0.04% CO2 at sea level, the partial pressure for CO2 is 0.3 mm Hg
stated, gas exchange between air and blood occurs in the respira- (0.0004 × 760 = 0.3 mm Hg) (table 15.1). It is traditional to
tory membrane of the lungs (see figure 15.8). The major area of gas designate the partial pressure of individual gases in a mixture with
exchange is in the alveoli, although some takes place in the respira- a capital P followed by the symbol for the gas. Thus, the partial
tory bronchioles and alveolar ducts. Gas exchange between blood pressure of O2 is Po2, and that of CO2 is Pco2.
and air does not occur in other areas of the respiratory passageways, When air is in contact with a liquid, gases in the air dissolve in
such as the bronchioles, bronchi, and trachea. The volume of these the liquid. The gases dissolve until the partial pressure of each gas in
passageways is therefore called anatomical dead space. the liquid is equal to the partial pressure of that gas in the air. Gases
The exchange of gases across the respiratory membrane is in a liquid, like gases in air, diffuse from areas of higher partial pres-
influenced by the thickness of the membrane, the total surface sure toward areas of lower partial pressure, until the partial pressures
area of the respiratory membrane, and the partial pressure of gases of the gases are equal throughout the liquid. In other words, gases
across the membrane. diffuse down their pressure gradient; when atmospheric air comes
into contact with the water-based fluid in the lungs, CO2 and O2 dis-
Respiratory Membrane Thickness solve in the fluid and each diffuses down its pressure gradient.
The thickness of the respiratory membrane increases during cer- Diffusion of Gases in the Lungs Respiratory
tain respiratory diseases. For example, in patients with pulmonary
edema, fluid accumulates in the alveoli, and gases must diffuse The cells of the body use O2 and produce CO2. Thus, blood return-
through a thicker than normal layer of fluid. If the thickness of the ing from tissues and entering the lungs has a decreased Po2 and
respiratory membrane is doubled or tripled, the rate of gas exchange an increased Pco2 compared to alveolar air (figure 15.13). Oxygen
is markedly decreased. Oxygen exchange is affected before CO2 diffuses from the alveoli into the pulmonary capillaries because
exchange because O2 diffuses through the respiratory membrane the Po2 in the alveoli is greater than that in the pulmonary capillar-
about 20 times less easily than does CO2. ies. In contrast, CO2 diffuses from the pulmonary capillaries into the
alveoli because the Pco2 is greater in the pulmonary capillaries than
Surface Area in the alveoli (figure 15.13, step 1) .
When blood enters a pulmonary capillary, the Po2 and Pco2 in
The total surface area of the respiratory membrane is about 70 square the capillary are different from the Po2 and Pco2 in the alveolus.
meters (m2) in the normal adult, which is approximately the floor By the time blood flows through the first third of the pulmonary
area of a 25- × 30-ft room, or roughly the size of a racquetball capillary, an equilibrium is achieved, and the Po2 and Pco2 in the
court (20 × 40 ft). Under resting conditions, a decrease in the sur- capillary are the same as in the alveolus. Thus, in the lungs, the
face area of the respiratory membrane to one-third or one-fourth of blood gains O2 and loses CO2 (figure 15.13, step 2).
normal can significantly restrict gas exchange. During strenuous During breathing, atmospheric air mixes with alveolar air.
exercise, even small decreases in the surface area of the respiratory The air entering and leaving the alveoli keeps the Po2 higher in the
membrane can adversely affect gas exchange. Possible reasons for alveoli than in the pulmonary capillaries. Increasing the breath-
having a decreased surface area include the surgical removal of lung ing rate makes the Po2 even higher in the alveoli than it is during
tissue, the destruction of lung tissue by cancer, and the degenera- slow breathing. During labored breathing, the rate of O2 diffusion
tion of the alveolar walls by emphysema. Collapse of the lung—as
occurs in pneumothorax—dramatically reduces the volume of the
alveoli, as well as the surface area for gas exchange.
Table 15.1 Partial Pressures of Gases at Sea Level
Gases Dry Air Humidified Air Alveolar Air Expired Air
Nitrogen mm HG % mm HG % mm HG % mm HG %
Oxygen
Carbon dioxide 600.2 78.98 563.4 74.09 569.0 74.9 566.0 74.5
Water vapor
159.5 20.98 149.3 19.67 104.0 13.6 120.0 15.7
0.3 0.04 0.3 0.04 40.0 5.3 27.0 3.6
0.0 0.0 47.0 6.20 47.0 6.2 47.0 6.2
428 Chapter 15
into the pulmonary capillaries increases because the difference in Increasing the rate of breathing also makes the Pco2 lower in
partial pressure between the alveoli and the pulmonary capillaries the alveoli than it is during normal, quiet breathing. Because the
has increased. There is a slight decrease in Po2 in the pulmonary alveolar Pco2 decreases, the difference in partial pressure between
veins due to mixing with deoxygenated blood from veins draining the alveoli and the pulmonary capillaries increases, which increas-
the bronchi and bronchioles; however, the Po2 in the blood is still es the rate of CO2 diffusion from the pulmonary capillaries into
higher than that in the tissues (figure 15.13, step 3). the alveoli.
PROCESS Figure 15.13 Gas Exchange Inspired air Expired air
PPCOO2 2==106.03 PPCOO2 2==12270
Differences in partial pressure are responsible for the
Alveolus Alveolus
exchange of O2 and CO2 that occurs between the alveoli
and the pulmonary capillaries and between the tissues
and the tissue capillaries.
1 Oxygen diffuses into the PO2 = 104 PCO2 = 40 PO2 = 104 PCO2 = 40
arterial ends of pulmonary 1 2
capillaries, and CO2
diffuses into the alveoli PO2 = 40 PCO2 = 45 PO2 = 104 PCO2 = 40
because of differences
in partial pressures. Pulmonary capillary
2 As a result of diffusion 3
at the venous ends of
pulmonary capillaries, the PO2 = 95 Blood in
PO2 in the blood is equal PCO2 = 40 pulmonary veins
to the PO2 in the alveoli,
and the PCO2 in the Right Left
blood is equal to the
PCO2 in the alveoli. Heart
3 The PO2 of blood in
the pulmonary veins
is less than in the
pulmonary capillaries
because of mixing with
deoxygenated blood from
veins draining the bronchi
and bronchioles.
Respiratory
4 Oxygen diffuses out of the Tissue capillary
arterial ends of tissue
capillaries, and CO2 PO2 = 40 PCO2 = 45 PO2 = 95 PCO2 = 40
diffuses out of the tissue
because of differences in 5 Interstitial 4
partial pressures. PO2 = 40 fluid
5 As a result of diffusion at PCO2 = 45 PO2 = 40 PCO2 = 45
the venous ends of tissue
capillaries, the PO2 in the PO2 = 20 PCO2 = 46
blood is equal to the PO2 Tissue cells
in the tissue, and the PCO2
in the blood is equal to the
PCO2 in the tissue.
Go back to step 1.
Respiratory System 429
Predict 8 in three ways: (1) About 7% is transported as CO2 dissolved in
How would inadequate ventilation affect the difference in Po2 and Pco2 the plasma; (2) 23% is transported in combination with blood
across the respiratory membrane? How would the rate of O2 and CO2 proteins, primarily hemoglobin; and (3) 70% is transported in the
diffusion across the membrane change? form of bicarbonate ions.
Carbon dioxide (CO2) reacts with water to form carbonic acid
Diffusion of Gases in the Tissues (H2CO3), which then dissociates to form H+ and bicarbonate ions
Blood flows from the lungs through the left side of the heart to the (HCO3−):
tissue capillaries. Figure 15.13 illustrates the partial pressure differ-
ences for O2 and CO2 across the wall of a tissue capillary. Oxygen Carbonic
diffuses from the capillary into the interstitial fluid because the Po2 anhydrase
is lower in the interstitial fluid than in the capillary. Oxygen diffuses
from the interstitial fluid into cells, in which the Po2 is less than in CO2 + H2O H2CO3 H+ + HCO3−
the interstitial fluid (figure 15.13, step 4). Within the cells, O2 is Carbon Water Carbonic Hydrogen Bicarbonate
used in cellular respiration. There is a constant difference in Po2 dioxide acid ion ion
between the tissue capillaries and the cells because the cells con-
tinuously use O2. There is also a constant diffusion gradient for CO2 An enzyme called carbonic anhydrase (kar-bon′ ik an-hı̄′ drās)
from the cells. Carbon dioxide therefore diffuses from cells into the is located inside red blood cells and on the surface of capillary epi-
interstitial fluid and from the interstitial fluid into the tissue capil- thelial cells. Carbonic anhydrase increases the rate at which CO2
laries, and an equilibrium between the blood and tissues is achieved reacts with water to form H+ and HCO3− in the tissue capillaries
(figure 15.13, step 5). (figure 15.14a). Thus, carbonic anhydrase promotes the uptake of
CO2 by red blood cells.
Predict 9 In the capillaries of the lungs, the process is reversed, so that
During exercise, more O2 moves into skeletal muscle cells, and more the HCO3− and H+ combine to produce H2CO3, which then forms
CO2 moves out of skeletal muscle cells. Explain how this happens. CO2 and H2O (figure 15.14b). The CO2 diffuses into the alveoli
and is expired.
15.5 Gas Transport in the Blood Carbon dioxide has an important effect on the pH of blood.
As CO2 levels increase, the blood pH decreases (becomes more
Learning Outcome After reading this section, you should be able to acidic) because CO2 reacts with H2O to form H2CO3. The H+
that results from the dissociation of H2CO3 is responsible for the
A. Explain how O2 and CO2 are transported in the blood. decrease in pH. Conversely, as blood levels of CO2 decline, the
blood pH increases (becomes less acidic, or more basic).
Oxygen Transport
Predict 10
After O2 diffuses through the respiratory membrane into the
blood, about 98.5% of the O2 transported in the blood combines What effect does rapid breathing have on blood pH? What effect
reversibly with the iron-containing heme groups of hemoglobin does holding your breath have on blood pH? Explain.
(see chapter 11). About 1.5% of the O2 remains dissolved in the
plasma. Hemoglobin with O2 bound to its heme groups is called 15.6 Rhythmic Breathing Respiratory
oxyhemoglobin (ok′ sē-hē-mō-glō′ bin).
The ability of hemoglobin to bind to O2 depends on the Po2. Learning Outcomes After reading this section, you should be able to
At high Po2, hemoglobin binds to O2, and at low Po2, hemoglobin
releases O2. In the lungs, Po2 normally is sufficiently high so that A. Describe the respiratory areas of the brainstem and how
hemoglobin holds as much O2 as it can. In the tissues, Po2 is lower they produce a rhythmic pattern of ventilation.
because the tissues are using O2. Consequently, hemoglobin releases
O2 in the tissues. Oxygen then diffuses into the cells, which use it in B. Name the neural mechanisms that can modify the normal
cellular respiration. At rest, approximately 23% of the O2 picked up rhythmic pattern of ventilation.
by hemoglobin in the lungs is released to the tissues.
The amount of O2 released from oxyhemoglobin is influ- C. Explain how blood pH, CO2, and O2 levels affect ventilation.
enced by four factors. More O2 is released from hemoglobin if
(1) the Po2 is low, (2) the Pco2 is high, (3) the pH is low, and The normal rate of breathing in adults is between 12 and 20 breaths
(4) the temperature is high. Increased muscular activity results in a per minute. In children, the rates are higher and may vary from
decreased Po2, an increased Pco2, a reduced pH, and an increased 20 to 40 per minute. The rate of breathing is determined by
temperature. Consequently, during physical exercise, as much as the number of times respiratory muscles are stimulated. The
73% of the O2 picked up by hemoglobin in the lungs is released basic rhythm of breathing is controlled by neurons within the
into skeletal muscles. medulla oblongata that stimulate the muscles of respiration. An
increased depth of breathing results from stronger contractions
Carbon Dioxide Transport and Blood pH of the respiratory muscles caused by recruitment of muscle
fibers and increased frequency of stimulation of muscle fibers.
Carbon dioxide diffuses from cells, where it is produced, into
the tissue capillaries. After CO2 enters the blood, it is transported Respiratory Areas in the Brainstem
Neurons involved with respiration are located in the brainstem.
The neurons that are active during inspiration and those active
during expiration are intermingled in these areas.
430 Chapter 15
1 In the tissues, carbon dioxide (CO2) Capillary wall
diffuses into the plasma and into red blood
cells. Some of the carbon dioxide remains Plasma
in the plasma.
1
2 In red blood cells, carbon dioxide reacts CO2 CO2 CO2
with water (H2O) to form carbonic acid
(H2CO3) in a reaction catalyzed by the
enzyme carbonic anhydrase (CA).
3 Carbonic acid dissociates to form 2 CA H2O
bicarbonate ions (HCO3– ) and hydrogen
ions (H+).
H2CO3
4 In the chloride shift, an antiporter allows 3 Red blood cell
HCO3– to diffuse out of the red blood cells Tissue cells HCO3– HCO3– + H+
and chloride ions (Cl –) to diffuse into them, O2 Cl– Cl– 6
which maintains their electrical neutrality.
O2 4
5 Oxygen (O2) is released from hemoglobin H
(Hb). Oxygen diffuses out of red blood cells
and plasma into the tissue. O2 Hb CO2 7 CO2
6 Hydrogen ions combine with hemoglobin, 5
which promotes the release of oxygen from
hemoglobin (Bohr effect).
7 Carbon dioxide combines with hemoglobin.
Hemoglobin that has released oxygen
readily combines with carbon dioxide
(Haldane effect).
(a) Gas exchange in the tissues
1 In the lungs, carbon dioxide (CO2) Alveolus Capillary wall CO2
diffuses from red blood cells and CO2 Plasma
plasma into the alveoli. 1 2 CA H2O
O2 CO2
Respiratory 2 Carbonic anhydrase catalyzes the H2CO3 Red blood cell
formation of CO2 and H2O from H2CO3. HCO3– 3
Cl–
3 Bicarbonate ions and H+ combine to O2 4 HCO3– + H+
replace H2CO3. Cl– 6
H
4 In the chloride shift, an antiporter allows
HanCdOc3h–lotoriddeiffuiosnesin(Ctol–t)hteordeidffubsloeooduct eolfls 5 O2 Hb CO2 7 CO2
them, which maintains their electrical
neutrality.
5 Oxygen diffuses into the plasma and into
red blood cells. Some of the oxygen
remains in the plasma. Oxygen binds to
hemoglobin.
6 Hydrogen ions are released from
hemoglobin, which promotes the uptake
of oxygen by hemoglobin (Bohr effect).
7 Carbon dioxide is released from
hemoglobin. Hemoglobin that is bound to
oxygen readily releases carbon dioxide
(Haldane effect).
(b) Gas exchange in the lungs
PROCESS Figure 15.14 Gas Exchange in the Tissues and in the Lungs
(a) in the tissues, Co2 diffuses into red blood cells, where the enzyme carbonic anhydrase (CA) is located. CA catalyzes the reaction of Co2 with h2o to form
carbonic acid (h2Co3). h2Co3 dissociates to form bicarbonate ions (hCo3–) and hydrogen ions (h+). oxygen is released from hemoglobin (hb) and diffuses into
tissue cells. (b) in the lungs, Co2 diffuses from red blood cells into the alveoli. CA catalyzes the formation of Co2 and h2o from h2Co3. h+ and hCo3– combine
to replace h2Co3. oxygen diffuses into red blood cells and binds to hemoglobin.
Respiratory System 431
The medullary respiratory center consists of two dorsal Pons
respiratory groups, each forming a longitudinal column of cells Pontine respiratory
located bilaterally in the dorsal part of the medulla oblongata, and group
two ventral respiratory groups, each forming a longitudinal col-
umn of cells located bilaterally in the ventral part of the medulla Dorsal Medullary
oblongata (figure 15.15). The dorsal respiratory group is primarily respiratory group respiratory
responsible for stimulating contraction of the diaphragm. The ven- Ventral center
tral respiratory group is primarily responsible for stimulating the respiratory group
external intercostal, internal intercostal, and abdominal muscles. A
part of the ventral respiratory group, the pre-Bötzinger complex, Medulla oblongata
is now known to establish the basic rhythm of breathing.
The pontine respiratory group is a collection of neurons Phrenic Medial view
in the pons (figure 15.15). It has connections with the medullary nerve Spinal cord
respiratory center and appears to play a role in switching between
inspiration and expiration. Intercostal
nerves
Generation of Rhythmic Breathing
Internal intercostal muscles
The medullary respiratory center generates the basic pattern of (involved in expiration)
spontaneous, rhythmic breathing. Although the precise mecha- External intercostal muscles
nism is not well understood, the generation of rhythmic breathing (involved in inspiration)
involves the integration of stimuli that start and stop inspiration.
Diaphragm Respiratory
1. Starting inspiration. The neurons in the medullary respiratory (involved in inspiration)
center that promote inspiration are continuously active. The
medullary respiratory center constantly receives stimulation Anterior view
from many sources, such as receptors that monitor blood gas
levels and the movements of muscles and joints. In addition, Figure 15.15 Respiratory Structures in the Brainstem
stimulation can come from parts of the brain concerned with
voluntary respiratory movements and emotions. When the Specific structures in the brainstem correlate with the nerves that innervate
inputs from all these sources reach a threshold level, somatic the muscles of respiration.
nervous system neurons stimulate respiratory muscles via
action potentials, and inspiration starts. lack of O2 in the brain. Children have used this strategy to encour-
age parents to give them what they want. However, as soon as
2. Increasing inspiration. Once inspiration begins, more and conscious control of respiration is lost, automatic control resumes,
more neurons are activated. The result is progressively and the person starts to breathe again.
stronger stimulation of the respiratory muscles, which lasts Several reflexes, such as sneeze and cough reflexes, can mod-
for approximately 2 seconds (s). ify breathing. The Hering-Breuer (her′ ing broy′ er) reflex supports
3. Stopping inspiration. The neurons stimulating the muscles
of respiration also stimulate the neurons in the medullary
respiratory center that are responsible for stopping inspiration.
The neurons responsible for stopping inspiration also receive
input from the pontine respiratory neurons, stretch receptors
in the lungs, and probably other sources. When the inputs to
these neurons exceed a threshold level, they cause the neurons
stimulating respiratory muscles to be inhibited. Relaxation
of respiratory muscles results in expiration, which lasts
approximately 3 s. The next inspiration begins with step 1.
Although the medullary neurons establish the basic rate and
depth of breathing, their activities can be influenced by input from
other parts of the brain and from peripherally located receptors.
Nervous Control of Breathing
Higher brain centers can modify the activity of the respiratory
center (figure 15.16a). For example, controlling air movements out
of the lungs makes speech possible, and emotions can make us sob
or gasp. In addition, breathing can be consciously controlled—that
is, it is possible to breathe or to stop breathing voluntarily. Some
people can hold their breath until they lose consciousness due to
432 Chapter 15
CLINICAL IMPACT Effects of High Altitude and Emphysema
Air is composed of 21% o2 at high altitudes, the respiratory system’s At first, arterial pco2 levels may be unaf-
both low and high altitudes. At sea level, fected by the reduced surface area of
ability to eliminate Co2 is not adversely
the atmospheric pressure is 760 mm hg, affected by the low atmospheric pressure. the respiratory membrane, because Co2
diffuses across the respiratory membrane
and po2 is about 160 mm hg (760 mm hg thus, the blood Co2 levels become lower
× 0.21 = 160 mm hg). At higher altitudes, than normal because of the increased rate 20 times more readily than does o2.
however, if alveolar ventilation increases
the atmospheric pressure is lower, and po2 and depth of breathing stimulated by the
is decreased. for example, at 10,000 ft to the point that Co2 exchange increases
low blood o2 levels. the decreased blood above normal, arterial Co2 becomes lower
above sea level, the atmospheric pressure is Co2 levels cause the blood ph to become than normal. more severe emphysema,
abnormally high.
523 mm hg. Consequently, po2 is 110 mm hg in which the respiratory membrane sur-
(523 mm hg × 0.21 = 110 mm hg). Because A similar situation can exist in peo-
face area is reduced to a minimum, can
po2 is lower at high altitudes, the blood lev- ple with emphysema; the destruction of
els of o2 can decline enough to stimulate decrease Co2 exchange to the point that
the carotid and aortic bodies. oxygen then the respiratory membrane allows less arterial Co2 becomes elevated.
becomes an important stimulus for elevat- o2 to move into the blood. the result-
ing low arterial po2 levels stimulate an
ing the rate and depth of breathing. At increased rate and depth of respiration.
Respiratory rhythmic respiratory movements by limiting the extent of inspiration Although O2 levels are not the major driving force of breath-
(figure 15.16d). As the muscles of inspiration contract, the lungs fill ing, there are O2-sensitive chemoreceptors in the carotid and aortic
with air. Sensory receptors that respond to stretch are located in the bodies (figure 15.16c). When blood O2 levels decline to a low level
lungs, and as the lungs fill with air, the stretch receptors are stimu- (hypoxia) such as during exposure to high altitude, emphysema,
lated. Action potentials from the lung stretch receptors are then sent shock, and asphyxiation, the aortic and carotid bodies are strongly
to the medulla oblongata, where they inhibit the respiratory center stimulated. They send action potentials to the respiratory center
neurons and cause expiration. In infants, the Hering-Breuer reflex and produce an increase in the rate and depth of breathing, which
plays an important role in regulating the basic rhythm of breathing increases O2 diffusion from the alveoli into the blood.
and in preventing overinflation of the lungs. In adults, however,
the reflex is important only when the tidal volume is large, as Because CO2 levels affect blood pH, the medullary chemore-
occurs during heavy exercise. ceptors are important for more than just regulating breathing rate;
the medullary chemoreceptors play a crucial role in maintaining
Touch, thermal, and pain receptors in the skin also stimulate blood pH. Figure 15.17 depicts the role breathing rate has on
the respiratory center, which explains why we gasp in response to blood pH. If blood CO2 levels decrease, such as during more rapid
being splashed with cold water or being pinched (figure 15.16e,f ). breathing, blood pH will increase (become more basic). Thus,
the homeostatic mechanism is that the medullary chemoreceptors
Chemical Control of Breathing signal a decreased breathing rate, which retains CO2 in the blood.
More CO2 in the blood causes H+ levels to increase, which causes
During cellular respiration, the body’s cells consume O2 and pro- blood pH to decrease to normal levels. Alternatively, if blood CO2
duce CO2 (see chapter 17). The primary function of the respiratory levels increase, such as during increased physical activity when
system is to add O2 to the blood and to remove CO2 from the blood. the body’s cells are producing more CO2 as waste, blood pH will
decrease (become more acidic). The medullary chemoreceptors
Surprisingly, the level of CO2, not O2, in the blood is the major will detect the elevated H+ and signal a faster breathing rate. As
driving force regulating breathing. Even a small increase in the CO2 breathing rate goes up, more CO2 will diffuse out of the blood and
level (hypercapnia), such as when holding your breath, results in blood pH will return to normal. Thus, CO2 levels are very influ-
a powerful urge to breathe. The mechanism by which CO2 in the ential on breathing rate. The opposite is also true, which is why
blood stimulates breathing involves the change in pH that accompa- hyperventilation without accompanying increases in CO2 levels
nies an increase in CO2 levels. Receptors in the medulla oblongata due to physical exercise can cause someone to pass out.
called chemoreceptors are sensitive to small changes in H+ con-
centration. Recall from the section “Carbon Dioxide Transport and effect of exercise on Breathing
Blood pH” that blood CO2 combines with water, which increases
H+ concentration. Thus, it is the H+ that is detected by the medul- The mechanisms by which breathing is regulated during exercise are
lary chemoreceptors (figure 15.16b). controversial, and no single factor can account for all the observed
responses. Breathing during exercise can be divided into two phases:
Predict 11
1. Breathing increases abruptly. At the onset of exercise,
Explain why a person who breathes rapidly and deeply the rate of breathing immediately increases. This initial
(hyperventilates) for several seconds experiences a short increase can be as much as 50% of the total increase that
period of time during which breathing does not occur (apnea)
before normal breathing resumes.
Respiratory System 433
(a) Higher centers of
the brain (speech, +
emotions, voluntary –
control of breathing,
and action potentials
in motor pathways)
(b) Medullary (chemosensitive
area) chemoreceptors
pH, CO2
(c) Carotid and Carotid
aortic body body
chemoreceptors Aortic
pH, CO2, O2 body
Input to respiratory
centers in the
medulla oblongata
and pons modifies
respiration.
(d) Hering-Breuer reflex
(stretch receptors
in lungs)
(e) Proprioceptors
in muscles
and joints
(f) Receptors for Brainstem
touch, temperature,
and pain stimuli
Figure 15.16 Nervous and Chemical Mechanisms of Breathing Respiratory
Several regulatory mechanisms affect the rate and depth of breathing. A plus sign indicates that the mechanism increases breathing and a minus sign
indicates that it results in a decrease in breathing.
will occur. The immediate increase occurs too quickly to “learns” to match breathing with the intensity of the exercise.
be explained by changes in metabolism or blood gases. As Well-trained athletes match their respiratory movements
axons pass from the motor cortex of the cerebrum through more efficiently with their level of physical activity than do
the motor pathways, numerous collateral fibers project to untrained individuals. Thus, centers in the brain involved in
the respiratory center. During exercise, action potentials in learning have an indirect influence on the respiratory center,
the motor pathways stimulate skeletal muscle contractions, but the exact mechanism is unclear.
and action potentials in the collateral fibers stimulate the 2. Breathing increases gradually. After the immediate increase
respiratory center (see figure 15.16). in breathing, breathing continues to increase gradually and
Furthermore, during exercise, body movements stimulate then levels off within 4–6 minutes after the onset of exercise.
proprioceptors in the joints of the limbs. Nerve fibers from Factors responsible for the immediate increase in breathing
these proprioceptors extend to the spinal cord to connect with may play a role in the gradual increase as well.
sensory nerve tracts ascending to the brain. Collateral fibers
from these nerve tracts connect to the respiratory center; Despite large changes in O2 consumption and CO2 produc-
therefore, movement of the limbs has a strong stimulatory tion during exercise, the average arterial O2, CO2, and pH levels
influence on the respiratory center (see figure 15.16e). remain constant and close to resting levels as long as the exercise
There may also be a learned component in the breathing is aerobic (see chapter 7). This suggests that changes in blood
response during exercise. After a period of training, the brain gases and pH do not play an important role in regulating breathing
434 Chapter 15
3 4
Actions Reactions
Medullary chemoreceptors detect an Effectors Respond:
increase in blood pH (often caused Decreased breathing increases
by a decrease in blood CO2), blood CO2.
causing a decrease in breathing.
Homeostasis Disturbed: 5 Homeostasis Restored:
Blood pH increases. Blood pH decreases.
2
1 Start hereBlood pH 6 Blood pH
(normal range) (normal range)
Homeostasis Disturbed: Homeostasis Restored:
Blood pH decreases. Blood pH increases.
Respiratory Actions Reactions
Medullary chemoreceptors detect a
decrease in blood pH (often caused Effectors Respond:
by an increase in blood CO2), Increased breathing decreases
causing an increase in breathing. blood CO2.
Homeostasis Figure 15.17 Regulation of Blood pH
(1) Blood ph is in its normal range. (2) Blood ph increases outside its normal range, which disturbs homeostasis. (3) the control centers for blood ph,
the medullary chemoreceptors, detect an increase in blood ph (blood becomes more basic) and respond to the increased ph by signaling a decreased
breathing rate. (4) the effectors, the diaphragm and other respiratory muscles, respond by slowing their contraction rate, which lowers the rate of
breathing. (5) As a result, more Co2 is retained, which causes ph to drop (blood becomes more acidic). (6) Blood ph returns to its normal range and
homeostasis is maintained. observe the responses to a decrease in blood ph by following the red arrows.
Respiratory System 435
DISEASES AND DISORDERS: Respiratory System
CONDITION DESCRIPTION Respiratory
Bronchi and Lungs Respiratory Disorders
Bronchitis (brong-k¯ı ′tis)
Inflammation of the bronchi caused by irritants, such as cigarette smoke or infections; swelling impairs breathing;
Emphysema (em-fi-se¯′ma˘) bronchitis can progress to emphysema
Destruction of alveolar walls; increased coughing increases pressure on the alveoli, causing rupture and destruction;
Adult respiratory distress loss of alveoli decreases surface area for gas exchange and decreases the lungs’ ability to expel air; progression
syndrome (ARDS) can be slowed, but there is no cure; alone or in combination with bronchitis, the condition is known as chronic
obstructive pulmonary disease (COPD)
Cystic fibrosis (fi-bro¯′sis) Caused by damage to the respiratory membrane, which promotes inflammation; amount of surfactant is reduced,
and fluid fills the alveoli, lessening gas exchange; ARDS usually develops after an injurious event, such as inhaling
Pulmonary fibrosis smoke from a fire or breathing toxic fumes
Genetic disorder that affects mucus secretions throughout the body due to an abnormal transport protein; mucus is
Lung cancer much more viscous and accumulates in ducts and tubes, such as the bronchioles; airflow is restricted, and infections
are more likely
Circulatory System Replacement of lung tissue with fibrous connective tissue, making the lungs less elastic; exposure to asbestos or coal
Thrombosis of the dust are common causes
pulmonary arteries Occurs in the epithelium of the respiratory tract; can easily spread to other parts of the body because of the rich
Anemia blood and lymphatic supply to the lungs
Carbon monoxide poisoning
Nervous System Blood clot in lung blood vessels; inadequate blood flow through the pulmonary capillaries, affecting respiratory
Sudden infant death syndrome function
(SIDS) Reduced hemoglobin lowers oxygen-carrying capacity of blood
Paralysis of the respiratory Carbon monoxide binds more strongly to hemoglobin than does O2 and prevents already-bound O2 from entering tissues
muscles
Thoracic Wall Most frequent cause of death of infants between 2 weeks and 1 year of age; cause is still unknown, but at-risk
babies can be placed on monitors that warn if breathing stops
Upper Respiratory Tract Damage to the spinal cord in the cervical or thoracic region interrupts nervous signals to the muscles of respiration
Strep throat
Diphtheria (dif-the¯r′e¯-a˘) Decreased elasticity of the thoracic wall prevents it from expanding to full capacity and reduces air movement; two
spine curvature conditions that reduce elasticity of the thoracic wall are scoliosis (sko¯-le¯-o¯′sis) and kyphosis (k¯ı -fo¯′sis)
Common cold Infectious Diseases of the Respiratory System
Lower Respiratory Tract
Whooping cough Caused by streptococcal bacteria (Streptococcus pyogenes); characterized by inflammation of the pharynx and fever
(pertussis; per-t˘us′is) Caused by the bacterium Corynebacterium diphtheriae; a grayish membrane forms in the throat and can completely
Tuberculosis (t¯u-ber′ky¯u-lo¯′sis) block respiratory passages; DPT immunization for children partially targets diphtheria
Results from a viral infection
Pneumonia (noo-mo¯′ne¯-a˘)
Caused by the bacterium Bordetella pertussis, which destroys cilia lining the respiratory epithelium, allowing mucus
Flu (influenza; in-fl¯u-en′za˘) to accumulate; leads to a very severe cough; DPT immunization for children partially targets pertussis
Caused by the bacterium Clostridium tuberculosis, which forms small, lumplike lesions called tubercles; immune
Fungal diseases system targets tubercles and causes larger lesions; certain strains of tuberculosis are resistant to antibiotics
Many bacterial or viral infections of the lungs that cause fever, difficulty in breathing, and chest pain; edema in the
lungs reduces their inflation ability and reduces gas exchange
Viral infection of the respiratory system; does not affect the digestive system, as is commonly misunderstood; causes
chills, fever, headache, and muscle aches
Fungal spores enter the respiratory tract attached to dust particles, usually resulting in minor respiratory infections
that in some cases can spread to other parts of the body
during aerobic exercise. However, during exercise, the values of The highest level of exercise that can be performed without
arterial O2, CO2, and pH levels rise and fall more than they do at causing a significant change in blood pH is the anaerobic thresh-
rest. Thus, even though their average values do not change, their old. If the exercise intensity becomes high enough to exceed the
oscillations may be a signal for helping control breathing.
anaerobic threshold, skeletal muscles produce lactate through the
SyStemS PaTHOLOGy
NGaemndee: r: Will asthma
age: male
18 background Information
oacnsrotcfeatoeuohsptpwmltbnehhdiepddraiotoemeslhihtytrtmndancoiihaconaoriesejnihdnunocnrrgoltgodgedymwedpemgiwswtagioneilieniasooevchtrtgettfhnyenogo,m.eeeoerhrhlxoCznteinWidetseesoogoncirtsrmrcamueroyutmcfuicoulocngooflpigwrlkfnmsrodhndoire,cetansoftooa,erew,ahisiviatmanWnnfnWieWihhupnrgnutga.lnilsoegedicli1elliebyloft8ltldoh,lv.lreahur’w-coAefB.srr.tydetimnaobeelWneDoldlupcnyttarteybsoanaithirted,hsulro-guhlopejeooAlsskosaysiumalielsnelgitdtgg.hpadteihigmhAhinhhhinamtiaegtiednfaplshtelglreyet.ndoorovaetedd asthma (az′ma˘; difficult breathing) is characterized by abnormally
increased constriction of the trachea and bronchi in response to
Respiratory various stimuli, which results in narrowed air passageways and
decreased ventilation efficiency. Symptoms include rapid and
shallow breathing, wheezing, coughing, and shortness of breath
(figure 15A). in contrast to many other respiratory disorders, the
symptoms of asthma typically reverse either spontaneously or
with therapy.
there is no definitive pathological feature or diagnostic test
for asthma, but three important characteristics of the disease
are chronic airway inflammation, airway hyperreactivity, and
airflow obstruction. the inflammation results in tissue dam-
age, edema, and mucus buildup, which can block airflow
through the bronchi. Airway hyperreactivity means that the
smooth muscle in the trachea and bronchi contracts greatly
in response to a stimulus, thus decreasing the diameter of the
airway and increasing resistance to airflow. the effects of
inflammation and airway hyperreactivity combine to cause
airflow obstruction (figure 15B).
many cases of asthma appear to be associated with a
chronic inflammatory response by the immune system. the
number of immune cells in the bronchi, including mast
cells, eosinophils, neutrophils, macrophages, and lympho-
cytes, increases. inflammation appears to be linked to air-
way hyperreactivity by some chemical mediators released
by immune cells (e.g., leukotrienes, prostaglandins, and
interleukins), which increase the airway’s sensitivity to stimulation
and cause smooth muscle contraction.
the stimuli that prompt airflow obstruction in asthma vary
from one individual to another. Some asthmatics react to particular
allergens, which are foreign substances that evoke an inappropriate
immune system response (see chapter 14). examples include inhaled
pollen, animal dander, and dust mites. many cases of asthma are
LM 1000x LM 400x
Asthmatic bronchiole: Normal bronchiole:
Note how constricted and mucus-filled it is. Note how clear it is.
Figure 15A Figure 15B
Strenuous exercise is one of the many factors
that can bring on an asthma attack.
436
INTeGumeNTaRy SKeLeTaL muSCuLaR
Cyanosis, a bluish skin color, results many of the immune cells Skeletal muscles are necessary for respiratory
from decreased blood o2 content. responsible for the inflammatory movements and the cough reflex. increased
response of asthma are produced in muscular work during a severe asthma attack can
the red bone marrow. the thoracic cause metabolic acidosis because of anaerobic
cage is necessary for respiration. respiration and excessive lactate production.
uRINaRy asthma NeRVOuS
modifying hydrogen ion secretion into Symptoms emotional upset or stress can provoke
the urine helps compensate for acid- • Rapid and shallow an asthma attack. peripheral and
base imbalances caused by asthma. breathing central chemoreceptor reflexes
• Wheezing affect ventilation. the cough reflex
DIGeSTIVe • Coughing helps remove mucus from respiratory
• Shortness of breath passages. pain, anxiety, and death from
ingested substances, such as aspirin, asphyxiation can result from the altered
sulfiting agents (preservatives), Treatment gas exchange caused by asthma. one
• Avoiding the causative theory of the cause of asthma is an
tartrazine, certain foods, and reflux of agent imbalance in the autonomic nervous
stomach acid into the esophagus can • Taking anti-inflammatory system (AnS) control of bronchiolar
medication smooth muscle, and drugs that
provoke an asthma attack. • Using bronchodilators enhance the sympathetic effects or
block the parasympathetic effects
LymPHaTIC aND ImmuNe CaRDIOVaSCuLaR are used to treat asthma.
immune cells release chemical mediators increased vascular permeability of lung blood eNDOCRINe
that promote inflammation, increase vessels results in edema. Blood carries ingested
substances that provoke an asthma attack to Steroids from the adrenal gland help
mucus production, and cause bronchiolar the lungs. Blood also carries immune cells from regulate inflammation and are used in
constriction, which is believed to be red bone marrow to the lungs. tachycardia asthma therapy.
a major factor in asthma. ingested commonly occurs during an asthma attack,
and the normal effects of respiration on
allergens, such as aspirin or sulfites in venous return are exaggerated, resulting
food, can provoke an asthma attack. in large fluctuations in blood pressure.
caused by an allergic reaction to substances in the droppings and infections, emotional upset, stress, air pollution, and even reflux of Respiratory
carcasses of cockroaches, which may explain the higher rate of stomach acid into the esophagus are known to elicit asthma attacks.
asthma in poor, urban areas. however, other inhaled substances,
such as chemicals in the workplace or cigarette smoke, can provoke treatment of asthma involves avoiding the causative stimulus
an asthma attack without stimulating an allergic reaction. over and taking medications. Steroids and mast cell–stabilizing agents,
200 substances have been associated with occupational asthma. which prevent the release of chemical mediators from mast cells,
An asthma attack can also be stimulated by ingested substances, can reduce airway inflammation. Bronchodilators are used to
such as aspirin; nonsteroidal anti-inflammatory compounds, such as increase airflow.
ibuprofen (¯ı -b¯u′pro¯-fen); sulfites in food preservatives; and tartrazine
(tar′tra˘-ze¯n) in food colorings. Asthmatics can substitute acetamino- Predict 12
phen (as-et-a˘-me¯′no¯-fen, a-set-a˘-min′o¯-fen) (e.g., tylenol) for aspirin.
It is not usually necessary to assess arterial blood gases when
other stimuli, such as strenuous exercise (especially in cold diagnosing and treating asthma. However, this information can
weather) can precipitate an asthma attack. Such episodes can sometimes be useful in severe asthma attacks. Suppose that Will had
often be avoided by using a bronchodilator prior to exercise. Viral a Po2 of 60 mm Hg and a Pco2 of 30 mm Hg when he first went to the
emergency room. Explain how that could happen.
437
438 Chapter 15
anaerobic process of anaerobic respiration (see figure 17.5). Lactate tion of 120 liters per minute (L/min) can increase his or her minute
released into the blood contributes to a decrease in blood pH, which ventilation to 150 L/min after training. Increases to 180 L/min are
stimulates the carotid bodies, resulting in increased breathing. In typical of highly trained athletes.
fact, ventilation can increase so much that arterial CO2 levels fall
below resting levels, and arterial O2 levels rise above resting levels. 15.8 Effects of Aging on the
Respiratory System
15.7 Respiratory Adaptations
to Exercise Learning Outcome After reading this section, you should be able to
Learning Outcome After reading this section, you should be able to A. Describe the effects of aging on the respiratory system.
A. Describe the regulation of ventilation during exercise and Aging affects most aspects of the respiratory system. But even
the changes in the respiratory system that result from though vital capacity, maximum ventilation rates, and gas exchange
exercise training. decrease with age, the elderly can engage in light to moderate
exercise because the respiratory system has a large reserve capacity.
In response to training, athletic performance increases because the With age, mucus accumulates within the respiratory pas-
cardiovascular and respiratory systems become more efficient at sageways. The mucus-cilia escalator is less efficient because the
delivering O2 and picking up CO2. In most individuals, breathing mucus becomes more viscous and the number of cilia and their
does not limit performance because breathing can increase to a rate of movement decrease. As a consequence, the elderly are
greater extent than can cardiovascular function. more susceptible to respiratory infections and bronchitis.
After training, vital capacity increases slightly, and residual Vital capacity decreases with age because of reduced ability to
volume decreases slightly. Tidal volume at rest and during stan- fill the lungs (decreased inspiratory reserve volume) and to empty the
dardized, submaximal exercise (activities normally encountered lungs (decreased expiratory reserve volume). As a result, maximum
in everyday life) does not change. At maximal exercise, however, minute ventilation rates decrease, which in turn decreases the ability
the tidal volume increases. Increased efficiency of the respiratory to perform intense exercise. These changes are related to the weaken-
system in response to training is evident because the respiratory rate ing of respiratory muscles and the stiffening of cartilage and ribs.
at rest or during standardized submaximal exercise in trained Residual volume increases with age as the alveolar ducts and
individuals is slightly lower; however, at maximal exercise, their many of the larger bronchioles increase in diameter. This increases
respiratory rate is usually increased. the dead space, which decreases the amount of air available for gas
Minute ventilation is affected by changes in tidal volume and exchange. In addition, gas exchange across the respiratory membrane
breathing rate. After training, minute ventilation is essentially declines because parts of the alveolar walls are lost, which decreases
unchanged or slightly reduced at rest, slightly reduced during stan- the surface area available for gas exchange, and the remaining walls
dardized submaximal exercise, and greatly increased at maximal thicken, which decreases the diffusion of gases. A gradual increase
exercise. For example, an untrained person with a minute ventila- in resting tidal volume with age compensates for these changes.
Respiratory ANSWER TO Learn to Predict This chapter explained that a pneumothorax, or the introduc-
tion of air into the pleural cavity through an opening in the thoracic
Mr. Theron suffers from emphysema, a respiratory disorder that wall or lung, can cause a lung to collapse. Due to his emphysema,
Mr. Theron’s lung collapsed when alveoli near the surface of the
results in the destruction of alveoli. This chapter explained that the lung ruptured, allowing air to enter the pleural space. The physi-
cian was able to diagnose Mr. Theron’s collapsed lung by listening
alveoli form the respiratory membrane, the site of gas exchange for respiratory sounds with a stethoscope. He detected respiratory
sounds in the right lung but not in the left lung. We would expect
between the atmosphere and the blood. Alveolar destruction would Mr. Theron’s respiratory movements to be even more exaggerated,
since the respiratory membrane was reduced by one-half when
directly reduce the respiratory membrane surface and, therefore, his left lung collapsed. The reduction in the respiratory membrane
would cause a drop in Po2 and an increase in Pco2, both of which
gas exchange. As a consequence, Mr. Theron has exaggerated would stimulate respiratory centers to increase ventilation. While
in the hospital, Mr. Theron’s breathing was assisted by a ventilator.
respiratory movements to compensate for the reduction in surface If his emphysema continues to severely worsen, he may require a
ventilator for the remainder of his life.
area. Blood Po2 is an important stimulus for the respiratory center,
and the increased respiratory movements keep the ventilation just Answers to the rest of this chapter’s Predict questions are in Appendix E.
adequate to maintain blood Po2 in the low normal range. Because
CO2 diffuses across the respiratory membrane at a faster rate than
O2, the elevated respiration required to maintain blood Po2 causes
too much CO2 to be expired, with the result that his blood Pco2
level has dropped below normal.
Respiratory System 439
Summary 3. The epithelium from the trachea to the terminal bronchioles is Respiratory
ciliated to facilitate removal of debris. Cartilage helps hold the tube
Respiration includes ventilation, the movement of air into and out of the system open (from the trachea to the bronchioles). Smooth muscle
lungs; the exchange of gases between the air and the blood; the transport controls the diameter of the tubes (especially the bronchioles).
of gases in the blood; and the exchange of gases between the blood and The alveoli are formed by simple squamous epithelium, and they
the tissues. facilitate diffusion of gases.
15.1 Functions of the Respiratory System (p. 412) 4. The respiratory membrane has six layers, including a film of water,
the walls of the alveolus and the capillary, and an interstitial space.
The respiratory system exchanges O2 and CO2 between the air and the The respiratory membrane is thin and has a large surface area that
blood, regulates blood pH, helps produce sounds, moves air over the sensory facilitates gas exchange.
receptors that detect smell, and protects against some pathogens.
Pleural Cavities
15.2 Anatomy of the Respiratory System (p. 413) The pleural membranes surround the lungs and provide protection
against friction.
Nose
1. The nose consists of the external nose and the nasal cavity. Lymphatic Supply
2. The bridge of the nose is bone, and most of the external nose Superficial and deep lymphatic vessels drain the lungs.
is cartilage. 15.3 Ventilation and Respiratory Volumes (p. 421)
3. The nasal cavity warms, humidifies, and cleans the air. The nares
Changing Thoracic Volume
open to the outside, and the choanae lead to the pharynx. The nasal 1. Inspiration occurs when the diaphragm contracts and the external
cavity is divided by the nasal septum into right and left parts. The
paranasal sinuses and the nasolacrimal duct open into the nasal intercostal muscles lift the rib cage, thus increasing the volume of
cavity. Hairs just inside the nares trap debris. The nasal cavity is the thoracic cavity. During labored breathing, additional muscles of
lined with pseudostratified epithelium containing cilia that trap inspiration increase rib movement.
debris and move it to the pharynx. 2. Expiration can be passive or active. Passive expiration during quiet
breathing occurs when the muscles of inspiration relax. Active
Pharynx expiration during labored breathing occurs when the diaphragm
1. The nasopharynx joins the nasal cavity through the choanae and relaxes and the internal intercostal and abdominal muscles depress
the rib cage to forcefully decrease the volume of the thoracic cavity.
contains the opening to the auditory tube and the pharyngeal tonsils.
2. The oropharynx joins the oral cavity and contains the palatine and Pressure Changes and Airflow
1. Respiratory muscles cause changes in thoracic volume, which in
lingual tonsils.
3. The laryngopharynx opens into the larynx and the esophagus. turn cause changes in alveolar volume and pressure.
2. During inspiration, air flows into the alveoli because atmospheric
Larynx
1. The larynx consists of three unpaired cartilages and six paired pressure is greater than alveolar pressure.
3. During expiration, air flows out of the alveoli because alveolar
ones. The thyroid cartilage and cricoid cartilage form most of the
larynx. The epiglottis covers the opening of the larynx pressure is greater than atmospheric pressure.
during swallowing.
2. The vestibular folds can prevent air, food, and liquids from passing Lung Recoil
into the larynx. 1. The lungs tend to collapse because of the elastic recoil of the
3. The vocal folds (true vocal cords) vibrate and produce sounds when
air passes through the larynx. The force of air movement controls connective tissue and the surface tension of the fluid lining the alveoli.
loudness, and changes in the length and tension of the vocal folds 2. The lungs normally do not collapse because surfactant reduces the
determine pitch.
surface tension of the fluid lining the alveoli, and pleural pressure
Trachea is lower than alveolar pressure.
The trachea connects the larynx to the main bronchi.
Changing Alveolar Volume
Bronchi 1. Increasing thoracic volume results in decreased pleural pressure,
The main bronchi extend from the trachea to each lung.
increased alveolar volume, decreased alveolar pressure, and air
Lungs movement into the lungs.
1. There are two lungs. 2. Decreasing thoracic volume results in increased pleural pressure,
2. The airway passages of the lungs branch and decrease in size. The decreased alveolar volume, increased alveolar pressure, and air
movement out of the lungs.
main bronchi form the lobar bronchi, which go to each lobe of the
lungs. The lobar bronchi form the segmental bronchi, which extend to Respiratory Volumes and Capacities
each bronchopulmonary segment of the lungs. The segmental bronchi 1. There are four measurements of respiratory volume: tidal,
branch many times to form the bronchioles. The bronchioles branch
to form the terminal bronchioles, which give rise to the respiratory inspiratory reserve, expiratory reserve, and residual.
bronchioles, from which alveolar ducts branch. Alveoli are air sacs 2. Respiratory capacities are the sum of two or more respiratory
connected to the alveolar ducts and respiratory bronchioles.
volumes; they include vital capacity and total lung capacity.
440 Chapter 15
Respiratory 3. The forced expiratory vital capacity measures the rate at which 15.6 Rhythmic Breathing (p. 429)
air can be expelled from the lungs.
Respiratory Areas in the Brainstem
15.4 Gas Exchange (p. 427) 1. The medullary respiratory center, specifically the pre-Bötzinger
1. Gas exchange between air and blood occurs in the respiratory complex, establishes rhythmic breathing.
membrane. 2. The pontine respiratory group appears to be involved with the
2. The parts of the respiratory passageways where gas exchange switch between inspiration and expiration.
between air and blood does not occur constitute the dead space.
Generation of Rhythmic Breathing
Respiratory Membrane Thickness 1. Inspiration begins when stimuli from many sources, such as
Increases in the thickness of the respiratory membrane result in
decreased gas exchange. receptors that monitor blood gases, reach a threshold.
2. Expiration begins when the neurons causing inspiration are inhibited.
Surface Area
Small decreases in surface area adversely affect gas exchange during Nervous Control of Breathing
strenuous exercise. When the surface area is decreased to one-third to 1. Higher brain centers allow voluntary control of breathing. Emotions
one-fourth of normal, gas exchange is inadequate under resting conditions.
and speech production affect breathing.
Partial Pressure 2. The Hering-Breuer reflex inhibits the respiratory center when the
1. The pressure exerted by a specific gas in a mixture of gases is
lungs are stretched during inspiration.
reported as the partial pressure of that gas. 3. Touch, thermal, and pain receptors can stimulate breathing.
2. Oxygen diffuses from a higher partial pressure in the alveoli to a
Chemical Control of Breathing
lower partial pressure in the pulmonary capillaries. Oxygen diffuses 1. Carbon dioxide is the major chemical regulator of breathing. An
from a higher partial pressure in the tissue capillaries to a lower
partial pressure in the tissue spaces. increase in blood CO2 causes a decrease in blood pH, resulting in
3. Carbon dioxide diffuses from a higher partial pressure in the tissues increased breathing.
to a lower partial pressure in the tissue capillaries. Carbon dioxide 2. Low blood levels of O2 can stimulate chemoreceptors in the carotid
diffuses from a higher partial pressure in the pulmonary capillaries and aortic bodies, also resulting in increased breathing.
to a lower partial pressure in the alveoli. 3. Chemoreceptors in the medulla oblongata respond to changes in
blood pH. Usually, changes in blood pH are produced by changes
15.5 Gas Transport in the Blood (p. 429) in blood CO2.
Oxygen Transport Effect of Exercise on Breathing
1. Most (98.5%) O2 is transported bound to hemoglobin. Some (1.5%) Input from higher brain centers and from proprioceptors stimulates the
respiratory center during exercise.
O2 is transported dissolved in plasma.
2. Oxygen is released from hemoglobin in tissues when the partial 15.7 Respiratory Adaptations to Exercise (p. 438)
pressure for O2 is low, the partial pressure for CO2 is high, pH is Training results in increased minute volume at maximal exercise because
low, and temperature is high. of increased tidal volume and respiratory rate.
Carbon Dioxide Transport and Blood pH 15.8 Effects of Aging on the Respiratory System
1. Carbon dioxide is transported in solution as plasma (7%), in
(p. 438)
combination with blood proteins (23%), and as bicarbonate ions (70%).
2. In tissue capillaries, CO2 combines with water inside the red blood 1. Vital capacity and maximum minute ventilation decrease with age
because of weakening of the respiratory muscles and stiffening of
cells to form carbonic acid that dissociates to form HCO3− and H+. the thoracic cage.
This reaction promotes the transport of CO2.
3. In lung capillaries, HCO3− combines with H+ to form carbonic acid. 2. Residual volume and dead space increase because the diameter of
The carbonic acid dissociates to form CO2, which diffuses out of the respiratory passageways increases.
red blood cells.
4. As blood CO2 levels increase, blood pH decreases; as blood CO2 3. An increase in resting tidal volume compensates for increased
levels decrease, blood pH increases. Changes in breathing change dead space, loss of alveolar walls (surface area), and thickening
blood CO2 levels and pH. of alveolar walls.
4. The ability to remove mucus from the respiratory passageways
decreases with age.
Review and Comprehension
1. Define respiration. 6. What are the functions of the vestibular and vocal folds? How are
2. What are the functions of the respiratory system? sounds of different loudness and pitch produced?
3. Describe the structures of the nasal cavity and their functions.
4. Name the three parts of the pharynx. With what structures does each 7. Starting at the larynx, name in order all the tubes air passes through
to reach an alveolus.
part communicate?
5. Name and give the functions of the three unpaired cartilages of 8. What is the function of the C-shaped cartilages in the trachea?
What happens to the amount of cartilage in the tube system of
the larynx. the respiratory system as the tubes become smaller? Explain why
breathing becomes more difficult during an asthma attack.
Respiratory System 441
9. What is the function of the ciliated epithelium in the 20. What is the partial pressure of a gas? Describe the diffusion of
tracheobronchial tree? Oan2dabnedtwCOee2nbtehtewteiessnutehecaaplivlelaorliieasnadndthtehpeutlimssounesariyn
partial pressures. capillaries
1 0. Distinguish among the lungs, a lobe of the lung, and a terms of
bronchopulmonary segment.
21. List the ways in which blood transports O2. What factors promote
1 1. List the components of the respiratory membrane. the release of O2 in tissues?
12. Describe the pleurae of the lungs. What is their function?
13. Describe the lymphatic supply of the lungs. What is its function? 22. List the ways in which blood transports CO2.
14. Explain how the muscles of inspiration and expiration change
2 3. How does the level of blood CblOoo2 dafpfeHc?t blood pH? How can
thoracic volume. changes in ventilation affect
15. Describe the pressure changes that cause air to move into and out of
24. Name the respiratory areas of the brainstem, and explain how
the lungs. What causes these pressure changes? rhythmic breathing is generated.
1 6. Give two reasons the lungs tend to recoil. What two factors keep the
25. Describe how higher brain centers and the Hering-Breuer reflex
lungs from collapsing? can modify breathing.
1 7. Explain how changes in thoracic volume result in changes in pleural
26. Explain the role of blood pH, CO2, and O2 in modifying breathing.
pressure, alveolar volume, alveolar pressure, and airflow during 2 7. How is breathing regulated during exercise?
inspiration and expiration.
1 8. Define tidal volume, inspiratory reserve volume, expiratory reserve 28. What effect does exercise training have on the respiratory system?
volume, and residual volume. Define vital capacity, total lung
capacity, and forced expiratory vital capacity. 2 9. Why do vital capacity, alveolar ventilation, and diffusion of
1 9. Describe the factors that affect the diffusion of gases across the gases across the respiratory membrane decrease with age?
respiratory membrane. Give some examples of diseases that Why are the elderly more likely to develop respiratory infections
decrease diffusion by altering these factors. and bronchitis?
Critical Thinking
1. Cardiopulmonary resuscitation (CPR) has replaced former methods 7. Patients with diabetes mellitus who are not being treated with
of sustaining ventilation. The back pressure/arm lift method is insulin therapy rapidly metabolize lipids, and acidic by-products
one such technique that is no longer used. This procedure must be of lipid metabolism accumulate in the bloodstream. How does this
performed with the victim lying face down. The rescuer presses affect ventilation? Why is the change in ventilation beneficial?
firmly on the base of the scapulae for several seconds, then grasps
the arms and lifts them. The sequence is then repeated. Although 8. Ima Anxious was hysterical and hyperventilating. The doctor made
this procedure is less efficient than CPR, it does result in ventilation her breathe into a paper bag. Because you are an especially astute
of the lungs. Explain why. student, you say to the doctor, “When Ima was hyperventilating,
isptnhhacueprseewrcaaasbsueadsrgie,,ndtCghueOchieu2nrrwggbaeblsoltoootordabdpCreCpOaeOtd2h2leielnevsvhetheloselustl;bodawighnh,acevarnneeadsihsnseech.rbeAerawsesaaebtsdlho.eroeIdndbisrnCteetaOoatdh2t,ihlnesegvheeilts,
2. Another technique for artificial respiration is mouth-to-mouth began to breathe more slowly. Please explain.” How do you think
resuscitation. The rescuer takes a deep breath, blows air into the the doctor would respond? (Hint: Recall that the effect of decreased
victim’s mouth, and then lets air flow out of the victim’s lungs. bsulododdenCdOe2corenastheeinvabsloomodotporrescseunrtee.r)results in vasodilation and a
The process is repeated. Explain the following:
a. Why do the victim’s lungs expand? 9. Hyperventilating before swimming underwater can increase the
b. Why does air move out of the victim’s lungs? time spent under water. Explain how that can happen. Sometimes
a person who has hyperventilated before swimming under water
3. A person’s vital capacity was measured while she was standing passes out while still under water and drowns. Explain. Respiratory
and while she was lying down. What difference, if any, in the
measurement would you predict, and why? 1 0. The blood pH of a runner was monitored during a race. Shortly
after the beginning of the race, her blood pH increased for a short
4. If water vapor forms 10% of the gases in air at sea level, what is the time. Propose an explanation to account for the increased pH values
partial pressure of water? following the start of the race.
5. A patient has pneumonia, and fluids accumulate within the alveoli. Answers in Appendix D
Explain why this results in an increased rate of respiration. How
can O2 therapy return this rate to normal?
6. A patient has severe emphysema that has extensively damaged
the alveoli and reduced the surface area of the respiratory
membrane. Although the ptaaktieenatbirseraethce(iiv.ein.,ghOe 2dtoheesranpoyt,fheeelsatisll
has a tremendous urge to
if he is getting enough air). Why does this occur?
16C H A P t e R Digestive System
leARn TO PReDiCt
Demondre, the 10-year-old African-American boy in the
picture, complained often of cramps, gas, and diarrhea
after eating. lately, he had been increasingly uncom-
fortable after lunch at school, where he ate cheese
pizza nearly every day. His mother took him to see his
doctor, who explained that Demondre should avoid
dairy products (which contain the sugar lactose), espe-
cially cheese pizza!
After reading the sections “Secretions of the Small
intestine” and “Digestion, Absorption, and transport,”
explain why Demondre can no longer eat lactose with-
out side effects.
16.1 fUnCtionS of tHe Module 12 Digestive System
DigeStive SyStem
2. Digestion of food. During the process of digestion, food is
Learning Outcome After reading this section, you should be able to broken down from complex particles to smaller molecules
that can be absorbed.
A. list the major functions of the digestive system.
3. Absorption of nutrients. The epithelial cells that line the lumen
Every cell of the body requires nutrients, yet most cells can- of the small intestine absorb the small molecules of nutrients
not leave their position in the body and travel to a food source. (amino acids, monosaccharides, fatty acids, vitamins,
Therefore, the digestive system must help deliver food to them. minerals, and water) that result from the digestive process.
The digestive system (figure 16.1), with the assistance of the
circulatory system, is like a gigantic “meals on wheels,” serving 4. Elimination of wastes. Undigested material, such as fiber
over 100 trillion customers the nutrients they need. It also has its from food, plus waste products excreted into the digestive
own quality control and waste disposal methods. Food is taken tract are eliminated in the feces.
into the digestive system, where it is broken down into smaller
and smaller particles. Enzymes in the digestive system break the
particles down into very small molecules, which are absorbed
into the circulation and transported all over the body. There,
those molecules are broken down by other enzymes to release
energy or are assembled into new molecules to build tissues and
organs. This chapter describes the structure and function of the
digestive organs and their accessory glands.
The functions of the digestive system include the following:
1. Ingestion of food. Food and water enter the body through
the mouth.
442
Digestive System 443
Pharynx Salivary 2. The submucosa lies just outside the mucosa. It is a thick
(throat) glands layer of loose connective tissue containing nerves, blood
Oral cavity vessels, and small glands. An extensive network of nerve
(mouth) Esophagus cell processes forms a plexus (network). Autonomic nerves
Stomach innervate this plexus.
Liver Pancreas
Gallbladder Small 3. The next tunic is the muscularis. In most parts of the
intestine digestive tract it consists of an inner layer of circular
Large smooth muscle and an outer layer of longitudinal
intestine smooth muscle. Another nerve plexus, also innervated
by autonomic nerves, lies between the two muscle layers.
Appendix Together, the nerve plexuses of the submucosa and
muscularis compose the enteric (en-tĕr′ ik) nervous
Rectum system. This nervous system, which is a division of the
Anus autonomic nervous system, is extremely important in
controlling movement and secretion within the tract
Figure 16.1 Digestive System (see chapter 8).
16.2 Anatomy and Histology 4. The fourth, or outermost, layer of the digestive tract is Digestive
of the Digestive System either a serosa or an adventitia. The serosa consists of
the peritoneum, which is a smooth epithelial layer, and its
Learning Outcome After reading this section, you should be able to underlying connective tissue. Regions of the digestive tract
not covered by peritoneum are covered by a connective
A. Describe the general histology of the digestive tract. tissue layer called the adventitia (ad′ ven-tish′ ă; foreign,
coming from outside), which is continuous with the
The digestive system consists of the digestive tract, or gastro- surrounding connective tissue.
intestinal (GI; gas′ trō-in-tes′ tin-ăl) tract, plus specific associated
organs. Because the digestive tract is open at the mouth and anus, Peritoneum
the inside of the tract is continuous with the outside environment,
and food entering the digestive tract may contain not only useful The body wall of the abdominal cavity and the abdominal organs
nutrients but also indigestible components such as fiber, or harmful is covered with serous membranes (figure 16.3). The serous mem-
materials such as bacteria. Therefore, the inner lining of the diges- brane that covers the organs is the serosa, or visceral peritoneum
tive tract serves as a protective barrier to those indigestible and (per′ i-tō-nē′ ŭm; to stretch over). The serous membrane that lines
harmful materials and nutrients must be specifically transported the wall of the abdominal cavity is the parietal peritoneum.
across the wall of the digestive tract. Once across the wall of the Many of the organs of the abdominal cavity are held in place
digestive tract, the nutrients enter the circulation to access tissues by connective tissue sheets called mesenteries (mes′ en-ter-ēz).
of the body. Mesentery is a general term referring to the serous membranes
The digestive tract consists of the oral cavity, pharynx, attached to the abdominal organs. The mesenteries consist of two
esophagus, stomach, small intestine, large intestine, and anus. layers of serous membranes with a thin layer of loose connective
Accessory glands are associated with the digestive tract (fig- tissue between them. The mesentery connecting the lesser curva-
ure 16.1). The salivary glands empty into the oral cavity, and the liver ture of the stomach to the liver and diaphragm is called the lesser
and pancreas are connected to the small intestine. omentum (ō-men′ tŭm), and the mesentery connecting the greater
Various parts of the digestive tract are specialized for differ- curvature of the stomach to the transverse colon and posterior body
ent functions. Nearly all segments of the digestive tract consist of wall is called the greater omentum. The greater omentum is unusual
four layers, called tunics. These are the mucosa, the submucosa, in that it is a long, double fold of mesentery that extends inferiorly
the muscularis, and a serosa or an adventitia (figure 16.2): from the stomach before looping back to the transverse colon
to create a cavity, or pocket, called the omental bursa (ber′ să).
1. The innermost tunic, the mucosa (mū-kō′ să), consists of Adipose tissue accumulates in the greater omentum, giving it the
mucous epithelium, a loose connective tissue called the appearance of a fat-filled apron that covers the anterior surface of the
lamina propria, and a thin smooth muscle layer, the abdominal viscera. The mesentery that attaches the small intestine
muscularis mucosae. The epithelium in the mouth, to the posterior abdominal wall is called the mesentery proper.
esophagus, and anus resists abrasion, and the epithelium
in the stomach and intestine absorbs and secretes. Predict 2
If you placed a pin completely through both folds of the greater
omentum, through how many layers of simple squamous epithelium
would the pin pass?
Other abdominal organs lie against the abdominal wall, have
no mesenteries, and are described as retroperitoneal (re′ trō-
per′ i-tō-nē′ ăl; behind the peritoneum). The retroperitoneal organs
include the duodenum, pancreas, ascending colon, descending
colon, rectum, kidneys, adrenal glands, and urinary bladder.
444 Chapter 16
CLINICAL IMPACT Peritonitis
Peritonitis (per′i-to¯-n¯ı ′tis) is irritation by substances, such as bile, that when an infected appendix ruptures. the
a potentially life-threatening inflamma- have escaped from the digestive tract. main symptoms of peritonitis are acute
tion of the peritoneal membranes. the or it can result from infection originat- abdominal pain and tenderness that are
inflammation can result from chemical ing in the digestive tract, as may occur worsened by movement.
Enteric Myenteric Blood vessels
plexus plexus Lymphatic vessel
Submucosal Nerve
plexus
Mesentery
Gland in
submucosa
Ducts from
glands
Lymphatic
nodule
Intestinal
gland
Mucosa Submucosa Muscularis Serosa
(mucous Circular (serous
membrane) muscle membrane;
layer visceral
Epithelium Longitudinal peritoneum)
Lamina muscle Connective
propria layer tissue
layer
Muscularis Simple
mucosae squamous
epithelium
Figure 16.2 Digestive Tract Histology
the four tunics are the mucosa, the submucosa, the muscularis, and a serosa or an adventitia. glands may exist along the digestive tract as part of the
epithelium, within the submucosa, or as large glands outside the digestive tract.
Digestive 16.3 oRAl CAvity, PHARynX, and tongue. The lips are muscular structures, formed mostly by the
AnD eSoPHAgUS orbicularis oris (ōr-bik′ū-lā′ris ōr′is) muscle (see figure 7.16). The
outer surfaces of the lips are covered by skin. The keratinized strati-
Learning Outcomes After reading this section, you should be able to
fied epithelium of the skin becomes thin at the margin of the lips. The
A. Describe the structure of a tooth.
B. Describe the major salivary glands. Compare their color from the underlying blood vessels can be seen through the thin,
structures and functions. transparent epithelium, giving the lips a reddish-pink appearance. At
C. Describe mastication and swallowing.
the internal margin of the lips, the epithelium is continuous with the
Anatomy of the oral Cavity
moist stratified squamous epithelium of the mucosa in the oral cavity.
The oral cavity (figure 16.4), or mouth, is the first part of the diges- The cheeks form the lateral walls of the oral cavity. The bucci-
tive tract. It is bounded by the lips and cheeks and contains the teeth
nator (bŭk′si-nā-tōr) muscles (see figure 7.16), located within the
cheeks, flatten the cheeks against the teeth. The lips and cheeks
are important in the process of mastication (mas-ti-kā′shŭn), or
Digestive System 445
Visceral peritoneum Liver
Lesser omentum
Peritoneal cavity Stomach
containing peritoneal Pancreas (retroperitoneal)
fluid Kidney (retroperitoneal)
Parietal peritoneum Duodenum (retroperitoneal)
Greater omentum Transverse colon
Omental bursa Mesentery proper
Small intestine
Urinary bladder Rectum (retroperitoneal)
(retroperitoneal)
Medial view
Figure 16.3 Peritoneum and Mesenteries
The parietal peritoneum lines the abdominal cavity (blue), and the visceral peritoneum covers abdominal organs (red). Retroperitoneal organs are behind the
parietal peritoneum. The mesenteries are membranes that connect abdominal organs to each other and to the body wall.
chewing. They help manipulate the food within the oral cavity and right upper, left upper, right lower, and left lower. In adults, each Digestive
hold the food in place while the teeth crush or tear it. Mastication quadrant contains one central and one lateral incisor (in-sı̄′ zŏr; to
begins the process of mechanical digestion, which breaks down cut); one canine (kā′ nı̄n; dog); first and second premolars (prē-
large food particles into smaller ones. The cheeks also help form mō′ lărz; molaris, a millstone); and first, second, and third molars
words during the speech process. (mō′ lărz). The third molars are called wisdom teeth because they
The tongue is a large, muscular organ that occupies most of the usually appear in the late teens or early twenties, when the person
oral cavity. The major attachment of the tongue is in the posterior
part of the oral cavity. The anterior part of the tongue is relatively is old enough to have acquired some degree of wisdom.
free, except for an anterior attachment to the floor of the mouth by The teeth of adults are called permanent teeth, or second-
a thin fold of tissue called the frenulum (fren′ ū-lŭm) (figure 16.4).
The muscles associated with the tongue are described in chap- ary teeth (figure 16.5a). Most of them are replacements for the
ter 7. The anterior two-thirds of the tongue is covered by papillae, 20 primary teeth, or deciduous (dē-sid′ ū-ŭs) teeth, also called
some of which contain taste buds (see chapter 9). The posterior milk or baby teeth, which are lost during childhood (figure 16.5b).
one-third of the tongue is devoid of papillae and has only a few Each tooth consists of a crown with one or more cusps
scattered taste buds. In addition, the posterior portion does contain (points), a neck, and a root (figure 16.6). The center of the tooth
a large amount of lymphatic tissue, which helps form the lingual is a pulp cavity, which is filled with blood vessels, nerves, and
tonsil (see chapter 14). connective tissue, called pulp. The pulp cavity is surrounded by
The tongue moves food in the mouth and, in cooperation with a living, cellular, bonelike tissue called dentin (den′ tin; dens,
the lips and cheeks, holds the food in place during mastication. It tooth). The dentin of the tooth crown is covered by an extremely
also plays a major role in the process of swallowing. In addition, hard, acellular substance called enamel, which protects the tooth
the tongue is a major sensory organ for taste, as well as one of the
major organs of speech. against abrasion and acids produced by bacteria in the mouth. The
surface of the dentin in the root is covered with cementum (se-
Teeth
men′ tŭm), which helps anchor the tooth in the jaw.
There are 32 teeth in the normal adult mouth, located in the The teeth are rooted within alveoli (al-vē′ ō-lı̄; sockets) along
mandible and maxillae. The teeth can be divided into quadrants: the alveolar processes of the mandible and maxillae. The alveo-
lar processes are covered by dense fibrous connective tissue and
moist stratified squamous epithelium, referred to as the gingiva
(jin′ ji-vă), or gums. The teeth are held in place by periodontal
446 Chapter 16
Upper lip
Gingiva covering the
maxillary alveolar
process
Hard palate Tongue
Soft palate Frenulum of the tongue
Uvula Openings of the
Cheek submandibular ducts
Molars Gingiva covering the
mandibular alveolar
Premolars process
Canine
Incisors Lower lip
Figure 16.4 Oral Cavity
Central incisor 67 8 9 10 11 Central incisor
Lateral incisor 5 12 (erupts at 6–8 months;
Canine 4 lost at 5–7 years)
First premolar 13 Lateral incisor
Second premolar (erupts at 8–11 months;
First molar 3 14 lost at 6–8 years) CD E FG
Second molar Maxillary Canine H
Third molar teeth (erupts at 16–20 months;
(wisdom tooth) 2 15 lost at 8–11 years) B Maxillary I
First molar teeth J
(a) 1 16 (erupts at 10–16 months;
lost at 9–11 years) A
Second molar
Digestive 32 17 (erupts at 20–24 months; TK
lost at 9–11 years) Mandibular
31 18
Mandibular (b) S teeth L
teeth RM
30 19 QPO N
29 20
28 21
2726 25 24 22 Figure 16.5 Teeth
23
(a) Permanent teeth. (b) Deciduous teeth. Dental professionals have
developed a “universal” numbering and lettering system for convenience
in identifying individual teeth.
Digestive System 447
Cusp
Enamel Crown Palate and Tonsils
Gingiva Neck
Dentin The palate (pal′ ăt), or roof of the oral cavity, separates the oral
Pulp cavity Root cavity from the nasal cavity and prevents food from passing into the
with nerves nasal cavity during chewing and swallowing. The palate consists of
and vessels two parts. The anterior part contains bone and is called the hard
palate, whereas the posterior portion consists of skeletal muscle and
Cementum connective tissue and is called the soft palate (see figure 16.4). The
Periodontal uvula (ū′ vū-lă; a grape) is a posterior extension of the soft palate.
ligaments The tonsils (ton′ silz) are located in the lateral posterior walls
Alveolar bone of the oral cavity, in the nasopharynx, and in the posterior surface
of the tongue. The tonsils are described in chapter 14.
Figure 16.6 Molar Tooth in Place in the Alveolar Bone Salivary Glands
A tooth consists of a crown, a neck, and a root. The root is covered with There are three major pairs of salivary (sal′ i-vār-ē) glands: the
cementum, and the tooth is held in the socket by periodontal ligaments. parotid, submandibular, and sublingual glands (figure 16.7). A con-
Nerves and vessels enter and exit the tooth through a foramen in the part siderable number of other salivary glands are scattered throughout
of the root deepest in the alveolus. the oral cavity, including on the tongue. Salivary glands produce
saliva, which is a mixture of serous (watery) and mucous fluids.
(per′ ē-ō-don′ tăl; around the teeth) ligaments, which are con- The salivary glands are compound alveolar glands. They have
nective tissue fibers that extend from the alveolar walls and are branching ducts with clusters of alveoli, resembling grapes, at the
embedded into the cementum. ends of the ducts (see chapter 4).
Formation of dental caries (kār′ ēz), or tooth decay, is the The largest of the salivary glands, the parotid (pă-rot′ id;
result of the breakdown of enamel by acids produced by bacteria beside the ear) glands, are serous glands located just anterior to
on the tooth surface. Enamel is nonliving and cannot repair itself. each ear. Parotid ducts enter the oral cavity adjacent to the second
Consequently, a dental filling is necessary to prevent further dam- upper molars.
age. Periodontal disease is inflammation and degeneration of the Mumps (mŭmpz) is an inflammation of the parotid gland
periodontal ligaments, gingiva, and alveolar bone. This disease is caused by a viral infection. The inflamed parotid glands become
the most common cause of tooth loss in adults. swollen, often making the cheeks quite large. The virus causing
mumps can also infect other structures. Mumps in an adult male
A Case in Point may involve the testes and can result in sterility.
The submandibular (sŭb-man-dib′ ū-lăr; below the mandible)
Lymph Nodes and a Toothache glands produce more serous than mucous secretions. Each gland
can be felt as a soft lump along the inferior border of the mandible.
Tu Thake went to his dentist for a checkup because he had The submandibular ducts open into the oral cavity on each side of
experienced pain in his right mandible for several days. During the the frenulum of the tongue (see figure 16.4).
routine exam, the dentist felt along the anterior and posterior edges
of his sternocleidomastoid muscle on each side and along the inferior Parotid duct
edges of the mandible on each side. Just inferior to the angle of the
right mandible, the dentist noted a lump, suggesting an enlargement Parotid Buccinator Digestive
of the superior cervical lymph nodes. Enlargement of the superior gland muscle
cervical lymph nodes indicates a problem in the face, because Masseter Mucous membrane
all lymphatic drainage from the face goes through these nodes. muscle (cut)
Additional swellings along the inferior edge of the anterior or central Ducts of the
part of the body of the mandible suggest a problem in the mandible. sublingual gland
The “problem” could be an infection or a cancerous growth, or it
could be idiopathic (of unknown origin). An infection may occur in Sublingual
a tooth, in the bone, or in the soft tissues of the area. In Tu’s case, gland
further examination revealed a small abscess near his right, first Submandibular
mandibular molar. The dentist opened the abscess and treated it duct
with topical and systemic antibiotics. The infection disappeared, as Submandibular
did the swelling in the superior cervical lymph nodes. gland
Figure 16.7 Salivary Glands
The large salivary glands are the parotid glands, the submandibular glands,
and the sublingual glands.
448 Chapter 16
The sublingual (sŭb-ling′ gwăl; below the tongue) glands, In addition to its role in digestion, saliva protects the mouth
the smallest of the three paired salivary glands, produce p rimarily from bacterial infection by washing the oral cavity with lysozyme
mucous secretions. They lie immediately below the mucous mem- (lı̄′ sō-zı̄m), a mildly antibacterial enzyme. Saliva also neutralizes
brane in the floor of the oral cavity. Each sublingual gland has the pH in the mouth, which reduces the harmful effects of bacterial
10–12 small ducts opening onto the floor of the oral cavity. acids on tooth enamel. Lack of salivary gland secretion (which can
result from radiation therapy) increases the chance of ulceration
Saliva and infection of the oral mucosa and caries (cavities) formation in
the teeth.
Saliva (să-lı̄′ vă) helps keep the oral cavity moist and contains The serous part of saliva dissolves molecules, which must be
enzymes that begin the process of digestion. Saliva is secreted at in solution to stimulate taste receptors. The mucous secretions of
the rate of approximately 1 liter (L) per day. The serous part of the submandibular and sublingual glands contain a large amount
saliva, produced mainly by the parotid and submandibular glands, of mucin (mū′ sin), a proteoglycan that gives a lubricating quality
contains a digestive enzyme called salivary amylase (am′ il-ās) to the secretions of the salivary glands.
(table 16.1), which breaks the covalent bonds between glucose Salivary gland secretion is regulated primarily by the auto-
molecules in starch and other polysaccharides to produce the nomic nervous system, with parasympathetic stimulation being
disaccharides maltose and isomaltose. Maltose and isomaltose the most important. Salivary secretions increase in response to
have a sweet taste; thus, the digestion of polysaccharides by salivary a variety of stimuli, such as tactile stimulation in the oral cav-
amylase enhances the sweet taste of food. ity and certain tastes, especially sour. Higher brain centers can
Food spends very little time in the mouth. Consequently, only stimulate parasympathetic activity and thus increase the activity
about 5% of the total carbohydrates humans absorb are digested in of the salivary glands in response to the thought of food, to odors,
the mouth. Also, most starches are contained in plant cells, which are or to the sensation of hunger. Sympathetic stimulation increases
surrounded by cell walls composed primarily of the polysaccharide the mucous content of saliva. When a person becomes frightened
cellulose (sel′ ū-lōs). Humans lack the necessary enzymes to digest and the sympathetic division of the autonomic nervous system is
cellulose. Cooking and thorough chewing of food disrupt the cel- stimulated, the person may have a dry mouth with thick mucus.
lulose covering and increase the efficiency of the digestive process.
Table 16.1 Functions of Digestive Secretions
Fluid or Enzyme Source Function
Mouth Salivary glands Moistens and lubricates food, neutralizes bacterial acids, flushes
Saliva (water, bicarbonate ions, mucus) bacteria from oral cavity
Digests starch
Salivary amylase Salivary glands Has weak antibacterial action
Lysozyme Salivary glands
Kills bacteria, converts pepsinogen to pepsin
Stomach Digests protein
Protects stomach lining
Hydrochloric acid Gastric glands Binds to vitamin B12, aids in its absorption
Pepsin* Gastric glands
Emulsify fats
Mucus Mucous cells Neutralize stomach acid
Digest protein
Intrinsic factor Gastric glands Digests starch
Digests lipid (triglycerides)
Small Intestine and Associated Glands Digest nucleic acid (DNA or RNA)
Protects duodenum from stomach acid and digestive enzymes
Bile salts Liver Digest polypeptide
Digests sucrose
Bicarbonate ions Pancreas Digests lactose
Digests maltose
Digestive Trypsin*, chymotrypsin*, carboxypeptidase* Pancreas
Pancreatic amylase Pancreas
Lipase Pancreas
Nucleases Pancreas
Mucus Duodenal glands and goblet cells
Peptidases** Small intestine
Sucrase** Small intestine
Lactase** Small intestine
Maltase** Small intestine
*These enzymes are secreted as inactive forms, then activated.
**These enzymes remain in the microvilli.
Digestive System 449
CLINICAL IMPACT Dietary Fiber
even though humans cannot eventually all the nutrients we need could Dietary fiber is classified as either soluble
digest cellulose, it is important to nor- be reduced into a single tablet and that or insoluble fiber. Soluble fiber binds fat
mal digestive function. Cellulose provides we no longer would have to eat food. it is and cholesterol in the intestines, lowering
bulk, or fiber, in the diet. the presence of now known that indigestible bulk is very low-density lipoprotein concentrations in
this bulk facilitates movement of material important to the normal function of the the blood, and slows the absorption of
through the digestive tract by providing digestive tract. Adults should consume glucose, preventing a glucose surge and
mass against which the muscular wall fiber in the form of cellulose, hemicellu- insulin spike. insoluble fiber pushes food
of the digestive tract can push. in the lose, pectin, vegetable gums, muscilage, through the intestinal tract, preventing
1950s, some nutritionists speculated that lignin, and beta-glucan, among others. constipation, and dilutes carcinogens.
mastication a bolus, or mass of food, is formed in the mouth. The bolus is Digestive
pushed by the tongue against the hard palate, forcing the bolus
Food taken into the mouth is chewed, or masticated, by the teeth. toward the posterior part of the mouth and into the oropharynx.
The incisors and canines primarily cut and tear food, whereas the
premolars and molars primarily crush and grind it. Mastication The pharyngeal phase of swallowing is a reflex that is initi-
breaks large food particles into many small ones, which have a ated when a bolus of food stimulates receptors in the oropharynx.
much larger total surface area than a few large particles would This phase of swallowing begins with the elevation of the soft
have. Because digestive enzymes act on molecules only at the palate, which closes the passage between the nasopharynx and
surface of the food particles, mastication increases the efficiency oropharynx. The pharynx elevates to receive the bolus of food
of digestion. from the mouth. The three pharyngeal constrictor muscles then
contract in succession, forcing the food through the pharynx.
Pharynx At the same time, the upper esophageal sphincter relaxes, and
food is pushed into the esophagus. As food passes through the
The pharynx (far′ingks), or throat, which connects the mouth with pharynx, the vestibular and vocal folds close, and the epiglottis
the esophagus, consists of three parts: the nasopharynx, the oro- (ep-i-glot′is; upon the glottis, opening of the larynx) is tipped
pharynx, and the laryngopharynx (see chapter 15). Normally, only posteriorly, so that the opening into the larynx is covered. These
the oropharynx and laryngopharynx transmit food. The posterior movements prevent food from passing into the larynx.
walls of the oropharynx and laryngopharynx are formed by the
superior, middle, and inferior pharyngeal constrictor muscles. Predict 3
esophagus What would happen if a person had a cleft of the soft palate, so
that the soft palate did not completely close the passage between
The esophagus (ē-sof′ă-gŭs; gullet) is a muscular tube, lined the nasopharynx and the oropharynx during swallowing? What
with moist stratified squamous epithelium, that extends from the happens if a person has an explosive burst of laughter while trying
pharynx to the stomach. It is about 25 centimeters (cm) long and to swallow a liquid? What happens if a person tries to swallow
lies anterior to the vertebrae and posterior to the trachea within the and speak at the same time?
mediastinum. The upper two-thirds of the esophagus has skeletal
muscle in its wall, while the lower one-third has smooth muscle in The esophageal phase of swallowing is responsible for
its wall. It passes through the diaphragm and ends at the stomach. moving food from the pharynx to the stomach. Muscular con-
The esophagus transports food from the pharynx to the stomach. tractions of the esophagus occur in peristaltic (per-i-stal′tik;
Upper and lower esophageal sphincters, located at the upper and peri, around + stalsis, constriction) waves (figure 16.9). A wave
lower ends of the esophagus, respectively, regulate the movement of relaxation of the esophageal muscles precedes the bolus of
of food into and out of the esophagus. The lower esophageal food down the esophagus, and a wave of strong contraction
sphincter is sometimes called the cardiac sphincter. Numerous of the circular muscles follows and propels the bolus through
mucous glands produce a thick, lubricating mucus that coats the the esophagus. Gravity assists the movement of material, espe-
inner surface of the esophagus. cially liquids, through the esophagus. However, the peristaltic
contractions that move material through the esophagus are suf-
Swallowing ficiently forceful to allow a person to swallow even while doing
a headstand or floating in the zero-gravity environment of space.
Swallowing, or deglutition (dē-gloo-tish′ŭn), can be divided into The peristaltic contractions cause relaxation of the lower esopha-
three phases: the voluntary phase, the pharyngeal phase, and the geal sphincter in the esophagus as the peristaltic waves approach
esophageal phase (figure 16.8). During the voluntary phase, the stomach.
450 Chapter 16
Tongue Hard palate Soft palate
Nasopharynx Soft palate
1 2 Superior pharyngeal
constrictor
Bolus Larynx Middle pharyngeal
Oropharynx constrictor
Epiglottis
Inferior pharyngeal
constrictor
Upper esophageal
sphincter
Esophagus
11. During the voluntary phase, a bolus of food 22. During the pharyngeal phase, the soft
(yellow) is pushed by the tongue against the palate is elevated, closing off the
hard and soft palates and posteriorly toward nasopharynx. The pharynx and larynx are
the oropharynx (blue arrow indicates tongue elevated (blue arrows indicate muscle
movement; black arrow indicates movement movement; green arrow indicates
of the bolus). Tan: bone; purple: cartilage; elevation of the larynx).
red: muscle.
3
3
Superior Middle Epiglottis
pharyngeal pharyngeal Opening of larynx
constrictor constrictor
Vestibular fold
Vocal fold
33. Successive constriction of the pharyngeal constrictors from superior to inferior (blue arrows)
forces the bolus through the pharynx and into the esophagus. As this occurs, the vestibular and
vocal folds expand medially to close the passage of the larynx. The epiglottis (green arrow) is
bent down over the opening of the larynx largely by the force of the bolus pressing against it.
Digestive Inferior pharyngeal
constrictor
Upper esophageal 4 Esophagus 5
sphincter
Esophagus
44. As the inferior pharyngeal constrictor 55. During the esophageal phase, the bolus is
contracts, the upper esophageal sphincter moved by peristaltic contractions of the
relaxes (outwardly directed blue arrows), esophagus toward the stomach (inwardly
allowing the bolus to enter the esophagus. directed blue arrows).
PROCESS Figure 16.8 Events During the Three Phases of Swallowing
Digestive System 451
Digestive tract Wave of
Bolus relaxation
1 A wave of smooth muscle relaxation moves
ahead of the bolus, allowing the digestive 1
tract to expand.
2 A wave of contraction of the smooth muscle Bolus
behind the bolus propels it through the moves.
digestive tract.
2 Wave of
PROCESS Figure 16.9 Peristalsis contraction
16.4 Stomach 3. endocrine cells, which produce regulatory chemicals; and Digestive
4. chief cells, which produce pepsinogen (pep-sin′ ō-jen), a
Learning Outcomes After reading this section, you should be able to
precursor of the protein-digesting enzyme pepsin (pep′ sin;
A. Outline the anatomical and physiological characteristics of pepsis, digestion).
the stomach.
Secretions of the Stomach
B. Describe the stomach secretions, their functions, and
their regulation. The stomach functions primarily as a storage and mixing chamber
for ingested food. As food enters the stomach, it is mixed with
C. Describe gastric movements and stomach emptying and stomach secretions to become a semifluid mixture called chyme
how they are regulated. (kı̄m; juice). Although some digestion occurs in the stomach, that
is not its principal function.
Anatomy of the Stomach Stomach secretions from the gastric glands include hydrochloric
acid, pepsin, mucus, and intrinsic factor (table 16.1). Hydrochloric
The stomach (figure 16.10) is an enlarged segment of the digestive acid produces a pH of about 2.0 in the stomach. The acid kills micro-
tract in the left superior part of the abdomen. The opening from the organisms and activates pepsin from its inactive form, called pep-
esophagus into the stomach is called the gastroesophageal opening. sinogen. Pepsin breaks covalent bonds of proteins to form smaller
The region of the stomach around the gastroesophageal opening is peptide chains. Pepsin exhibits optimum enzymatic activity at a
called the cardiac region because it is near the heart. The most supe- pH of about 2.0. A thick layer of mucus lubricates the epithelial
rior part of the stomach is the fundus (fŭn′ dŭs). The largest part of cells of the stomach wall and protects them from the damaging effect
the stomach is the body, which turns to the right, forming a greater of the acidic chyme and pepsin. Irritation of the stomach mucosa
curvature on the left and a lesser curvature on the right. The open- stimulates the secretion of a greater volume of mucus. Intrinsic (in-
ing from the stomach into the small intestine is the pyloric (pı̄-lōr′ ik; trin′ sik) factor binds with vitamin B12 and makes it more readily
gatekeeper) opening, which is surrounded by a relatively thick absorbed in the small intestine. Vitamin B12 is important in deoxyri-
ring of smooth muscle called the pyloric sphincter. The region of bonucleic acid (DNA) synthesis and in red blood cell production.
the stomach near the pyloric opening is the pyloric region.
The muscular layer of the stomach is different from other Regulation of Stomach Secretions
regions of the digestive tract in that it consists of three layers: an
outer longitudinal layer, a middle circular layer, and an inner oblique Approximately 2 L of gastric secretions (gastric juice) are pro-
layer. These muscular layers produce a churning action in the stom- duced each day. Both nervous and hormonal mechanisms regulate
ach, important in the digestive process. The submucosa and mucosa gastric secretions. The neural mechanisms involve central nervous
of the stomach are thrown into large folds called rugae (roo′ gē; system (CNS) reflexes integrated within the medulla oblongata.
wrinkles) (figure 16.10a) when the stomach is empty. These folds Higher brain centers can influence these reflexes. Local reflexes
allow the mucosa and submucosa to stretch, and the folds disappear are integrated within the enteric plexus in the wall of the diges-
as the stomach is filled. tive tract and do not involve the CNS. Hormones produced by the
The stomach is lined with simple columnar epithelium. stomach and intestine help regulate stomach secretions.
The mucosal surface forms numerous tubelike gastric pits (fig- Regulation of stomach secretions can be divided into three
ure 16.10b), which are the openings for the gastric glands. The phases: the cephalic, gastric, and intestinal phases. The cephalic
epithelial cells of the stomach can be divided into five groups. The phase can be viewed as the “get started” phase, when the stomach
first group consists of surface mucous cells on the inner surface of secretions are increased in anticipation of incoming food. This
the stomach and lining the gastric pits. Those cells produce mucus, is followed by the gastric, “go for it,” phase, when most of the
which coats and protects the stomach lining. The remaining four stimulation of secretion occurs. Finally, the intestinal phase is the
cell types are in the gastric glands. They are “slow down” phase, during which stomach secretion decreases.
In the cephalic (se-fal′ ik; kephale, head) phase (figure 16.11a),
1. mucous neck cells, which produce mucus; sensations of taste, the smell of food, stimulation of tactile
2. parietal cells, which produce hydrochloric acid and
intrinsic factor;
452 Chapter 16
Esophagus Fundus
Location of lower Body
esophageal sphincter
Gastroesophageal opening Serosa
Cardiac part
ture Longitudinal muscle layer Muscularis
r curvature Circular muscle layer
Pyloric sphincter Lesser curva
Pyloric orifice Oblique muscle layer
Submucosa
Pyloric canal Mucosa
Pyloric antrum
Pyloric
part
Duodenum Greate
Rugae
(a)
Gastric
pit
Gastric Surface Surface Goblet
glands mucous mucous cell
cell cell
Lamina Mucosa Mucous
propria neck cell
Gastric pit
Mucous
neck cells
Parietal
cells
Chief
cells
Endocrine
cells
Submucosa
Blood vessels Muscularis
Oblique muscle
Digestive layer Connective Serosa
Circular muscle tissue layer visceral
layer Simple peritoneum
Longitudinal squamous
muscle layer epithelium
LM 30x
(b) (c)
Figure 16.10 Anatomy and Histology of the Stomach
(a) Cutaway section reveals muscular layers and internal anatomy. (b) A section of the stomach wall illustrates its histology, including several gastric pits
and glands. (c) Photomicrograph of gastric glands.
Digestive System 453
CLINICAL IMPACT Hypertrophic Pyloric Stenosis
hypertrophic pyloric stenosis normal stomach emptying. infants with this intestine, and the infant fails to gain weight.
is a common defect of the stomach in defect develop symptoms 3 to 10 weeks the defect can be corrected with surgery to
infants. it occurs in 1 in 150 males and after birth. the infants exhibit projectile enlarge the pyloric opening.
1 in 750 females. the pyloric sphincter is (forceful) vomiting. Because the pyloric
greatly thickened and thus interferes with opening is blocked, little food enters the
receptors during the process of chewing and swallowing, and the duodenum drops to 2.0 or below, the inhibitory influence of
pleasant thoughts of food stimulate centers within the medulla the intestinal phase is greatest. The hormone secretin (se-krē′tin),
oblongata that influence gastric secretions. Action potentials are which inhibits gastric secretions, is released from the duodenum in
sent from the medulla oblongata along parasympathetic axons response to low pH (table 16.2). Fatty acids and peptides in the duo-
within the vagus nerves to the stomach. Within the stomach wall, denum initiate the release of the hormone cholecystokinin (kō′lē-
the preganglionic neurons stimulate postganglionic neurons in the sis-tō-kı̄′nin), which also inhibits gastric secretions (table 16.2).
enteric plexus. The postganglionic neurons stimulate secretory Acidic chyme (pH < 2.0) in the duodenum also inhibits CNS stim-
activity in the cells of the stomach mucosa, causing the release of ulation and initiates local reflexes that inhibit gastric secretion.
hydrochloric acid, pepsin, mucus, and intrinsic factor. The neurons
also stimulate the release of gastrin and histamine from endocrine A CASe in Point
cells. Gastrin (gas′trin) is a hormone that enters the circulation
and is carried back to the stomach, where it stimulates additional Heartburn
secretory activity (table 16.2). Histamine is both a paracrine chem-
ical signal that acts locally and a hormone that enters the blood to Hart Burne had a large meat-lover’s pizza delivered to his
stimulate gastric gland secretory activity. Histamine is the most apartment. He had been eating chips and drinking beer before the
potent stimulator of hydrochloric acid secretion. Drugs that block pizza came. Hart consumed the whole pizza and two more bottles
the actions of histamine can lower acid levels. of beer as he watched the last half of a game on tv. By the end of
the game, he was feeling uncomfortably full, so he lay down on the
The gastric phase is the period during which the greatest couch. Within half an hour, Hart felt a severe pain in his chest and
volume of gastric secretion occurs (figure 16.11b). The gastric headed for the bathroom to find an antacid.
phase is activated by the presence of food in the stomach. During
the gastric phase, the food in the stomach is mixed with gastric heartburn, or gastritis, is a painful or burning sensation in the
secretions. Distention of the stomach stimulates stretch recep- chest usually associated with an increase in gastric acid secretion
tors. Action potentials generated by these receptors activate CNS and/or a backflush of acidic chyme into the esophagus. overeating,
reflexes and local reflexes, resulting in the cascade of events that eating fatty foods, lying down immediately after a meal, consuming
increases secretion, as in the cephalic phase. Peptides, produced by too much alcohol or caffeine, smoking, and wearing extremely
the action of pepsin on proteins, stimulate the secretion of gastrin, tight clothing can all cause heartburn. medications can relieve
which in turn stimulates additional hydrochloric acid secretion. the symptoms of heartburn by blocking gastric acid secretion.
Cimetidine (si-met′i-de¯n; tagamet) and ranitidine (ra˘-n¯ı ′ti-de¯n;
The intestinal phase of gastric secretion primarily inhibits Zantac) block histamine stimulation of acid release from parietal
gastric secretions (figure 16.11c). It is controlled by the entrance cells, and esomeprazole (eh-so¯-meh′pra-zo¯l; nexium) blocks the
of acidic chyme into the duodenum, which initiates both neural proton pump in parietal cells that generates gastric acid. Antacids
and hormonal mechanisms. When the pH of the chyme entering (e.g., tums) neutralize acids already secreted into the stomach.
TabLe 16.2 Functions of the Major Digestive System hormones Digestive
hormone Source Function
gastrin gastric glands increases gastric secretions
Secretin Duodenum
Decreases gastric secretions
Cholecystokinin Duodenum increases pancreatic and bile secretions high in bicarbonate ions
Decreases gastric motility
Decreases gastric secretions
Strongly decreases gastric motility
increases gallbladder contraction
increases pancreatic enzyme secretion
454 Chapter 16
Cephalic Phase
1 The taste, smell, or thought of food or tactile Taste, smell, or thought of food
sensations of food in the mouth stimulate the (chemoreceptors)
medulla oblongata (green arrows). 1 Tactile sensation in mouth
2 Vagus nerves carry parasympathetic action Hypothalamus
potentials to the stomach (pink arrow), where
enteric plexus neurons are activated. Medulla oblongata
3 Postganglionic neurons stimulate secretion by Secretions
parietal and chief cells and stimulate gastrin and stimulated
histamine secretion by endocrine cells.
4
4 Gastrin is carried through the circulation back to the
stomach (purple arrow), where, along with Vagus nerves
histamine, it stimulates secretion.
(a) 2
3
Circulation
Histamine
Gastrin
Stomach
Gastric Phase Vagus nerves
1 Distention of the stomach stimulates Medulla
mechanoreceptors (stretch receptors) and activates oblongata
a parasympathetic reflex. Action potentials
generated by the mechanoreceptors are carried by Secretions 1 Secretions
the vagus nerves to the medulla oblongata (green stimulated stimulated
arrow). Distention
2 3 4
2 The medulla oblongata increases action potentials
in the vagus nerves that stimulate secretions by Histamine Circulation
parietal and chief cells and stimulate gastrin and Gastrin
histamine secretion by endocrine cells (pink arrow).
3 Distention of the stomach also activates local
reflexes that increase stomach secretions (orange
arrow).
4 Gastrin is carried through the circulation back to the
stomach (purple arrow), where, along with
histamine, it stimulates secretion.
(b)
Intestinal Phase Stomach
1 Chyme in the duodenum with a pH less than 2 or
Vagus
containing fat digestion products (lipids) inhibits nerves
gastric secretions by three mechanisms (2–4).
2 Chemoreceptors in the duodenum are stimulated Medulla oblongata
by H+ (low pH) or lipids. Action potentials generated
Digestive by the chemoreceptors are carried by the vagus Vagus Decreased Secretions
nerves to the medulla oblongata (green arrow), nerves gastric inhibited
where they inhibit parasympathetic action potentials secretions
(pink arrow), thereby decreasing gastric secretions. 4
3 Local reflexes activated by H+ or lipids also inhibit
gastric secretion (orange arrows). 2 Local
4 Secretin and cholecystokinin produced by the reflexes
duodenum (brown arrows) decrease gastric 1 3
secretions in the stomach. pH <2
(c) or lipids
Circulation
Secretin and cholecystokinin
PROCESS Figure 16.11 Regulation of Stomach Secretions
Digestive System 455
CLINICAL IMPACT Peptic Ulcers
Approximately 10% of people Helicobacter pylori, which is also linked Antibiotic treatment to eradicate
in the United States will develop a peptic to gastritis and gastric cancer. Because H. pylori is the best therapy for ulcers.
ulcer during their lifetime. Peptic ulcers diet, stress, smoking and alcohol cause other treatments involve drugs that pre-
are caused when the gastric juices (acid excess acid secretion in the stomach, vent histamine-stimulated acid secretion
and pepsin) digest the mucosal lining of these lifestyle patterns were deemed or that directly inhibit the proton pumps
the digestive tract. Peptic ulcers can occur responsible for ulcers for many years. that secrete the acid, but these are only
in the duodenum, stomach, or esophagus. Although these factors can contribute to temporary.
ulcers, it is now clear that the root cause
nearly all peptic ulcers are due is H. pylori.
to infection by a specific bacterium,
To summarize, once gastric acid secretion begins, further secre- In an empty stomach, peristaltic contractions that approach Digestive
tion is controlled by negative-feedback loops involving nerves and tetanic contractions can occur for about 2 to 3 minutes. The
hormones. First, during the gastric phase, high acid levels in the contractions are increased by low blood glucose levels and are
stomach trigger a decrease in additional acid secretion. Second, dur- sufficiently strong to create an uncomfortable sensation called a
ing the intestinal phase, acidic chyme entering the duodenum trig- “hunger pang.” Hunger pangs usually begin 12 to 24 hours after
gers a decrease in gastric acid secretion. These negative-feedback the previous meal; in less time for some people. If nothing is
loops ensure that the acidic chyme entering the duodenum is neu- ingested, hunger pangs reach their maximum intensity within 3 or
tralized, which is required for the digestion of food by pancreatic 4 days and then become progressively weaker.
enzymes and for the prevention of peptic ulcer formation.
16.5 SmAll inteStine
movement in the Stomach
Learning Outcomes After reading this section, you should be able to
Two types of stomach movement aid digestion and help move chyme
through the digestive tract: mixing waves and peristaltic waves A. list the characteristics of the small intestine that account
(figure 16.12). Both types of movement result from smooth muscle for its large surface area.
contractions in the stomach wall. The contractions occur about every
20 seconds and proceed from the body of the stomach toward the B. Describe the secretions and movements that occur in the
pyloric sphincter. Relatively weak contractions result in mixing small intestine.
waves, which thoroughly mix ingested food with stomach secretions
to form chyme. The more fluid part of the chyme is pushed toward Anatomy of the Small intestine
the pyloric sphincter, whereas the more solid center moves back
toward the body of the stomach. Stronger contractions result in peri- The small intestine is about 6 meters (m) long and consists of three
staltic waves, which force the chyme toward and through the pyloric parts: the duodenum, the jejunum, and the ileum (figure 16.13). The
sphincter. The pyloric sphincter usually remains closed because of duodenum (doo-od′ĕ-nŭm, doo-ō-dĕ′nŭm) is about 25 cm long
mild tonic contraction. Each peristaltic contraction is sufficiently (the term duodenum means 12, suggesting that it is 12 in. long).
strong to cause partial relaxation of the pyloric sphincter and to The jejunum (jĕ-joo′nŭm) is about 2.5 m long and makes up two-
pump a few milliliters of chyme through the pyloric opening and fifths of the total length of the small intestine. The ileum (il′ē-ŭm)
into the duodenum. Increased motility leads to increased emptying. is about 3.5 m long and makes up three-fifths of the small intestine.
If the stomach empties too fast, the efficiency of digestion and The duodenum nearly completes a 180-degree arc as it
absorption in the small intestine is reduced. However, if the rate curves within the abdominal cavity. Part of the pancreas lies
of emptying is too slow, the highly acidic contents of the stomach within this arc. The common bile duct from the liver and the
may damage the stomach wall. To prevent these two extremes, pancreatic duct from the pancreas join and empty into the duo-
stomach emptying is regulated. The hormonal and neural mecha- denum (see figure 16.17).
nisms that stimulate stomach secretions also increase stomach
motility, so that the increased secretions are effectively mixed with The small intestine is the major site of digestion and absorp-
the stomach contents. The major stimulus of gastric motility and tion of food, which are accomplished due to the presence of a
emptying is distension of the stomach wall. Inhibition of gastric large surface area. The small intestine has three modifications that
motility and emptying is accomplished by the same negative- increase its surface area about 600-fold: circular folds, villi, and
feedback loops associated with the intestinal phase of gastric microvilli. The mucosa and submucosa form a series of circular
secretion. In particular, cholecystokinin is a major inhibitor of folds that run perpendicular to the long axis of the digestive tract
motility and emptying (table 16.2). Hence, stomach emptying is (figure 16.14a). Tiny, fingerlike projections of the mucosa form
slower after a fatty meal due to the release of cholecystokinin. numerous villi (vil′i; sing. villus), which are 0.5–1.5 mm long
(figure 16.14b). Most of the cells composing the surface of the
villi have numerous cytoplasmic extensions, called microvilli
456 Chapter 16
1 A mixing wave initiated in the body Esophagus
of the stomach progresses toward
the pyloric sphincter (pink arrows Mixing 1 Chyme
directed inward ). wave Body of
Pyloric stomach
2 The more fluid part of the chyme sphincter
is pushed toward the pyloric
sphincter (blue arrows), whereas Duodenum
the more solid center of the chyme
squeezes past the peristaltic 2
constriction back toward the body
of the stomach (orange arrow). Pyloric More fluid More solid
region chyme chyme
3 Peristaltic waves (purple
arrows) move in the same
direction and in the same way as
the mixing waves but are stronger.
4 Again, the more fluid part of the 3 4
chyme is pushed toward the 5
pyloric region (blue arrows),
whereas the more solid center of
the chyme squeezes past the
peristaltic constriction back
toward the body of the stomach
(orange arrow).
5 Peristaltic contractions force a
few milliliters of the most fluid
chyme through the pyloric opening
into the duodenum (small red arrows).
Most of the chyme, including the more
solid portion, is forced back toward
the body of the stomach for further
mixing (yellow arrow).
Digestive PROCESS Figure 16.12 Movements in the Stomach (mı̄′ krō-vil′ ı̄) (figure 16.14c,d). Each villus is covered by simple
columnar epithelium. Within the loose connective tissue core of
Stomach
each villus are a blood capillary network and a lymphatic capil-
Duodenum lary called a lacteal (lak′ tē-ăl; resembling milk) (figure 16.14c).
The blood capillary network and the lacteal are very important in
Ascending
colon transporting absorbed nutrients.
Jejunum
Mesentery The mucosa of the small intestine is simple columnar epi-
thelium with four major cell types: (1) absorptive cells, which
Ileocecal
junction have microvilli, produce digestive enzymes, and absorb digest-
Ileum ed food; (2) goblet cells, which produce a protective mucus;
Cecum (3) granular cells, which may help protect the intestinal epi-
Appendix thelium from bacteria; and (4) endocrine cells, which produce
Figure 16.13 Small Intestine regulatory hormones.
The epithelial cells are located within tubular glands of the
mucosa, called intestinal glands or crypts of Lieberkühn, at the
base of the villi. Granular and endocrine cells are located in the
bottom of the glands. The submucosa of the duodenum contains
mucous glands, called duodenal glands, which open into the base
of the intestinal glands.
The duodenum, jejunum, and ileum are similar in structure.
However, progressing from the duodenum through the ileum,
Digestive System 457
Villi
Blood capillary
network
Lacteal
Circular folds Epithelium
Intestinal
Epithelium gland
Submucosa
Circular muscle
Longitudinal muscle
(a) Serosa (b) Duodenal
gland
Top of
circular fold
Microvilli of epithelial cell surface
Villus Epithelial
cell
Capillary
(blood)
Lacteal
(lymph)
(d)
(c)
Figure 16.14 Anatomy and Histology of the Duodenum
(a) Wall of the duodenum, showing the circular folds. (b) Villi on a circular fold. (c) A single villus, showing the lacteal and capillary network.
(d) Transmission electron micrograph of microvilli on the surface of a villus.
there are gradual decreases in the diameter of the small intestine, Secretions of the Small Intestine Digestive
in the thickness of the intestinal wall, in the number of circular
folds, and in the number of villi. Lymphatic nodules are common Secretions from the mucosa of the small intestine contain mainly
along the entire length of the digestive tract, and clusters of lym- mucus, ions, and water. Intestinal secretions lubricate and p rotect
phatic nodules, called Peyer patches, are numerous in the ileum. the intestinal wall from the acidic chyme and the action of diges-
These lymphatic tissues help protect the intestinal tract from tive enzymes. They also keep the chyme in the small intestine in a
harmful pathogens. liquid form to facilitate the digestive process. Most of the secretions
The site where the ileum connects to the large intestine is entering the small intestine are produced by the intestinal mucosa,
called the ileocecal (il′ ē-ō-sē′ kăl) junction. It has a ring of smooth but the secretions of the liver and the pancreas also enter the small
muscle, the ileocecal sphincter, and an ileocecal valve (see figure intestine and play important roles in digestion.
16.21a), which allow the intestinal contents to move from the ileum The epithelial cells in the walls of the small intestine have
to the large intestine, but not in the opposite direction. enzymes, bound to their free surfaces, that are significant in the final
steps of digestion. Peptidases (pep′ ti-dās-ez) break the peptide bonds
458 Chapter 16
in proteins to form amino acids. Disaccharidases (dı̄-sak′ ă-rid-ās-ez) Two large accessory glands, the liver and the pancreas, produce
break down disaccharides, such as maltose, into monosaccharides, secretions that empty into the duodenum.
such as glucose. The amino acids and monosaccharides can be
absorbed by the intestinal epithelium (see table 16.1). Anatomy of the Liver
Mucus is produced by duodenal glands and by goblet cells,
which are dispersed throughout the epithelial lining of the entire The liver (figure 16.16a,b; see figure 16.1) weighs about 1.36
small intestine and within intestinal glands. Hormones released kilograms (kg) (3 lb) and is located in the right upper quadrant of
from the intestinal mucosa stimulate liver and pancreatic secretions. the abdomen, tucked against the inferior surface of the diaphragm.
Secretion by duodenal glands is stimulated by the vagus nerve, secre- The posterior surface of the liver is in contact with the right ribs
tin release, and chemical or tactile irritation of the duodenal mucosa. 5–12. It is divided into two major lobes, the right lobe and the
left lobe, which are separated by a connective tissue septum, the
Movement in the Small Intestine falciform (fal′ si-fōrm) ligament. Two smaller lobes, the caudate
(kaw′ dāt; having a tail) lobe and the quadrate (kwah′ drāt; square)
Mixing and propulsion of chyme are the primary mechanical lobe, can be seen from an inferior view. Also seen from the infe-
events that occur in the small intestine. Peristaltic contractions rior view is the porta, which is the “gate” through which blood
proceed along the length of the intestine for variable distances vessels, ducts, and nerves enter or exit the liver.
and cause the chyme to move along the small intestine (see fig- The liver receives blood from two sources (see chapter 13).
ure 16.9). Segmental contractions are propagated for only short The hepatic (he-pa′ tik; associated with the liver) artery takes
distances and mix intestinal contents (figure 16.15). o xygen-rich blood to the liver, which supplies liver cells with
The ileocecal sphincter at the juncture of the ileum and the oxygen. The hepatic portal vein carries blood that is oxygen-
large intestine remains mildly contracted most of the time, but poor but rich in absorbed nutrients and other substances from
peristaltic contractions reaching the ileocecal sphincter from the the digestive tract to the liver. Liver cells process nutrients and
small intestine cause the sphincter to relax and allow chyme to detoxify harmful substances from the blood. Blood exits the liver
move from the small intestine into the cecum. The ileocecal valve through hepatic veins, which empty into the inferior vena cava.
prevents movement from the large intestine back into the ileum. Many delicate connective tissue septa divide the liver into
lobules with portal triads at their corners. The portal triads
Absorption in the Small Intestine contain three structures: the hepatic artery, the hepatic portal
vein, and the hepatic duct (figure 16.16c). Hepatic (he-pa′ tik)
A major function of the small intestine is the absorption of cords, formed by platelike groups of cells called hepatocytes
nutrients. Most absorption occurs in the duodenum and jejunum, (hep′ ă-tō-sı̄ts), are located between the center and the margins
although some absorption also occurs in the ileum (see “Digestion, of each lobule. The hepatic cords are separated from one another
Absorption, and Transport” later in this chapter). by blood channels called hepatic sinusoids (si′ nŭ-soydz, sı̄′ nū-
soydz; resembling cavities). The sinusoid epithelium contains
16.6 Liver and Pancreas phagocytic cells that help remove foreign particles from the
blood. Blood from the hepatic portal vein and the hepatic artery
Learning Outcomes After reading this section, you should be able to flows into the sinusoids and mixes together. The mixed blood
flows toward the center of each lobule into a central vein. The
A. Describe the anatomy, histology, and ducts of the liver central veins from all the lobes unite to form the hepatic veins,
and pancreas. which carry blood out of the liver to the inferior vena cava.
B. Describe the major functions of the liver and pancreas,
and explain how they are regulated.
Digestive 1 A secretion introduced into the Secretion or food
digestive tract or into food within 1
the tract begins in one location.
2 Contraction waves
2 Segments of the digestive tract
alternate between contraction
and relaxation.
3 Material (brown) in the intestine is 3 Contraction waves
spread out in both directions from 4
the site of introduction.
4 The secretion or food is spread out
in the digestive tract and becomes more
diffuse (lighter color) through time.
PROCESS Figure 16.15 Segmental Contractions in the Small Intestine
Digestive System 459
Liver
Inferior vena cava Left lobe
Right lobe
Falciform Round ligament
ligament Gallbladder
(a) Hepatic ducts Portal
Hepatic portal vein triad
Gallbladder Hepatic artery
Quadrate lobe
Right lobe Left lobe
Caudate lobe
Lesser
Inferior vena cava omentum
(b)
To hepatic vein and
inferior vena cava
Liver lobule
Hepatic cords Central vein Digestive
Bile canaliculi Hepatic
phagocytic cells
Hepatocyte Hepatic duct
Hepatic portal vein Portal triad
Hepatic artery
Hepatic From digestive tract
To common bile duct sinusoid
From aorta
(c)
Figure 16.16 Liver
(a) Anterior view. (b) Inferior view. (c) Histology.
460 Chapter 16
A system of ducts from the liver to the duodenum serves as (bil-i-roo′ bin), a bile pigment that results from the breakdown of
a pathway for bile and other secretions (figure 16.17). A cleftlike hemoglobin (see chapter 11). Gallstones may form if the amount
lumen, the bile canaliculus (kan′ ă-lik′ ū-lŭs; pl. kan′ ă lik′ ū-lı̄, of cholesterol secreted by the liver becomes excessive and is not
little canals), is between the cells of each hepatic cord. Bile, able to be dissolved by the bile salts.
produced by the hepatocytes, flows through the bile canaliculi to Neural and hormonal stimuli regulate the secretion and release
the hepatic ducts in the portal triads. The hepatic ducts converge of bile (figure 16.18). Bile secretion by the liver is stimulated by
and empty into the right and left hepatic ducts, which transport parasympathetic stimulation through the vagus nerve. Secretin,
bile out of the liver. The right and left hepatic ducts unite to which is released from the duodenum, also stimulates bile secre-
form a single common hepatic duct. The common hepatic duct tion and release. Cholecystokinin stimulates the gallbladder to
is joined by the cystic (sis′ tik; kystis, bladder) duct from the contract and release bile into the duodenum. In addition, most
gallbladder to form the common bile duct. The gallbladder (90% of) bile salts are reabsorbed in the ileum. The blood carries
is a small sac on the inferior surface of the liver that stores and the bile salts back to the liver, where they stimulate additional bile
concentrates bile (see figure 16.16a,b). The common bile duct salt secretion and are once again secreted into the bile. The loss of
joins the pancreatic duct and opens into the duodenum at the bile salts in the feces is reduced by this recycling process.
duodenal papilla (pă-pil′ ă) (figure 16.17). The opening into the The liver can remove sugar from the blood and store it in the
duodenum is regulated by a sphincter. form of glycogen (table 16.3). It can also store fat, vitamins, copper,
and iron. This storage function is usually short-term.
Functions of the Liver Foods are not always ingested in the proportion needed by
the tissues. If this is the case, the liver can convert some nutrients
The liver performs important digestive and excretory functions, into others (table 16.3). For example, if a person eats a meal that
stores and processes nutrients, detoxifies harmful chemicals, and is very high in protein, a large amount of amino acids and only a
synthesizes new molecules (table 16.3). small amount of lipids and carbohydrates are delivered to the liver.
The liver produces and secretes about 600–1000 mL of bile The liver can break down the amino acids and cycle many of them
each day. Bile (bı̄l) contains no digestive enzymes, but it plays an through metabolic pathways to produce ATP and to synthesize
important role in digestion by diluting and neutralizing stomach lipids and glucose (see chapter 17).
acid and by dramatically increasing the efficiency of fat digestion The liver also transforms some nutrients into more readily
and absorption. Digestive enzymes cannot act efficiently on large usable substances. Ingested fats, for example, can be combined
fat globules. Bile salts emulsify fats, breaking the fat globules into with choline and phosphorus in the liver to produce phospholipids,
smaller droplets, much like the action of detergents in dishwater which are essential components of cell membranes.
(see tables 16.1 and 16.3). The small droplets are more easily Many ingested substances are harmful to body cells. In addi-
digested by digestive enzymes. Bile also contains excretory products, tion, the body itself produces many by-products of metabolism
such as cholesterol, fats, and bile pigments, including bilirubin
1 The hepatic ducts from the liver lobes Gallbladder Liver
combine to form the common hepatic 1
duct. 2 Hepatic ducts
Common
3 hepatic duct
2 The common hepatic duct combines Cystic duct Spleen
with the cystic duct from the gallbladder Hepatic portal vein
to form the common bile duct.
Digestive 3 The common bile duct joins the Common bile duct 5
pancreatic duct. Accessory pancreatic 4
duct
4 The combined duct empties into the
duodenum at the duodenal papilla. Duodenal Pancreatic
papilla duct
5 Pancreatic secretions may also enter Pancreas
the duodenum through an accessory
pancreatic duct, which also empties
into the duodenum.
Duodenum
(cutaway view)
PROCESS Figure 16.17 Flow of Bile and Pancreatic Secretions Through the Duct System of the Liver, Gallbladder, and Pancreas
Digestive System 461
Table 16.3 Functions of the Liver
Function Explanation
Digestion Bile neutralizes stomach acid and emulsifies fats, which facilitates fat digestion.
Excretion
Bile contains excretory products, such as cholesterol, fats, and bile pigments (e.g., bilirubin), that result from
Nutrient storage hemoglobin breakdown.
Nutrient conversion Liver cells remove sugar from the blood and store it in the form of glycogen; they also store fat, vitamins (A, B12, D,
E, and K), copper, and iron.
Detoxification of harmful chemicals
Liver cells convert some nutrients into others; for example, amino acids can be converted to lipids or glucose, fats
Synthesis of new molecules can be converted to phospholipids, and vitamin D is converted to its active form.
Liver cells remove ammonia from the circulation and convert it to urea, which is eliminated in the urine; other
substances are detoxified and secreted in the bile or excreted in the urine.
The liver synthesizes blood proteins, such as albumin, fibrinogen, globulins, and clotting factors.
that, if accumulated, are toxic. The liver is an important line of eliminated by the kidneys in the urine. The liver also removes other
defense against many of those harmful substances. It detoxifies substances from the circulation and excretes them into the bile.
them by altering their structure, which makes their excretion easier The liver can also produce unique new compounds (table 16.3).
(table 16.3). For example, the liver removes ammonia, a toxic Many of the blood proteins, such as albumins, fibrinogen, globu-
by-product of amino acid metabolism, from the circulation and lins, and clotting factors, are synthesized in the liver and released
converts it to urea, which is then secreted into the circulation and into the circulation.
Brain Anatomy of the Pancreas
1 Vagus nerve stimulation (red arrow) The pancreas is located retroperitoneal, posterior to the stomach in
causes the gallbladder to contract, the inferior part of the left upper quadrant (see figure 16.1). It has a
thereby releasing bile into the head near the midline of the body and a tail that extends to the left,
duodenum. where it touches the spleen (figure 16.19; see figure 16.17). It is a
complex organ composed of both endocrine and exocrine tissues
2 Secretin, produced by the that perform several functions. The endocrine part of the pancreas
duodenum (purple arrows) and consists of pancreatic islets, or islets of Langerhans. The islet cells
carried through the circulation to the produce the hormones insulin and glucagon, which enter the blood.
liver, stimulates bile secretion by the These hormones are very important in controlling blood levels of
liver (green arrows inside the liver). nutrients, such as glucose and amino acids (see chapter 10).
3 Cholecystokinin, produced by the 1
duodenum (pink arrows) and
carried through the circulation to the Vagus nerves
gallbladder, stimulates the
gallbladder to contract and the Bile Bile Liver
sphincters to relax, thereby Bile
releasing bile into the duodenum
(green arrow outside the liver). 2 Gallbladder Digestive
Secretin
4 Bile salts also stimulate bile Cholecystokinin Bile Stomach
secretion. Over 90% of bile salts Pancreas
are reabsorbed in the ileum and 3 CholeSceycsrteotkininin
returned to the liver (green Bile salts
arrows), where they stimulate
additional secretion of bile salts. 4
Circulation
PROCESS Figure 16.18 Control of Bile Secretion and Release Duodenum
462 Chapter 16
The exocrine part of the pancreas is a compound acinar gland continues the polysaccharide digestion that began in the oral cavity.
(see chapter 4). The acini (as′ i-nı̄; grapes) produce digestive The pancreatic enzymes also include lipase (lip′ ās), a lipid-digesting
enzymes. Clusters of acini are connected by small ducts, which join enzyme, and nucleases (noo′ klē-ās-ez), which are enzymes that
to form larger ducts, and the larger ducts join to form the pancreatic degrade DNA and RNA to their component nucleotides.
duct. The pancreatic duct joins the common bile duct and empties The exocrine secretory activity of the pancreas is controlled by
into the duodenum. both hormonal and neural mechanisms (figure 16.20; see table 16.2).
Secretin initiates the release of a watery pancreatic solution that
Functions of the Pancreas contains a large amount of HCO3−. The primary stimulus for
secretin release is the presence of acidic chyme in the duodenum.
The exocrine secretions of the pancreas include bicarbonate ions Cholecystokinin stimulates the pancreas to release an enzyme-rich
(HCO3−), which neutralize the acidic chyme that enters the small solution. The primary stimulus for cholecystokinin release is the
intestine from the stomach. The increased pH resulting from the presence of fatty acids and amino acids in the duodenum. In turn,
secretion of HCO3− stops pepsin digestion but provides the proper enzymes secreted by the pancreas act to digest these fatty acids and
environment for the function of pancreatic enzymes. Pancreatic amino acids. Parasympathetic stimulation through the vagus nerves
enzymes are also present in the exocrine secretions and are impor- also stimulates the secretion of pancreatic juices rich in pancreatic
tant in digesting all major classes of food (see table 16.1). Without enzymes. Sympathetic action potentials inhibit pancreatic secretion.
the enzymes produced by the pancreas, lipids, proteins, and carbo-
hydrates cannot be adequately digested. Predict 4
The major proteolytic (protein-digesting) enzymes are trypsin
(trip′ sin), chymotrypsin (kı̄-mō-trip′ sin), and carboxypeptidase Explain how secretin production in response to acidic chyme and
(kar-box′ ē-pep′ ti-dās). These enzymes continue the protein diges- bicarbonate ion secretion in response to secretin constitute a
tion that started in the stomach, and pancreatic amylase (am′ il-ās) negative-feedback mechanism.
Duodenum Common bile duct Jejunum
Accessory Body of pancreas Pancreatic duct
pancreatic
duct
Tail of pancreas
Duodenal Pancreatic
papilla islet
Cells producing
Head of glucagon
pancreas Cells producing
(a) insulin
Acini cells
(secrete enzymes)
Digestive
Vein
To
pancreatic
(b) duct To
bloodstream
Figure 16.19 Anatomy and Histology of the Duodenum and Pancreas
(a) The head of the pancreas lies within the duodenal curvature, with the pancreatic duct emptying into the duodenum. (b) Histology of the pancreas,
showing both the acini and the pancreatic duct system.
Digestive System 463
16.7 Large Intestine A Case in Point
Learning Outcomes After reading this section, you should be able to Appendicitis
A. List the parts of the large intestine, and describe its Lowe Payne had been feeling nauseated for a couple of days and
anatomy and histology. had lost his appetite. Suddenly, he felt a sharp pain in his lower right
abdomen. The pain was so intense that his mother, Lotta, took Lowe
B. Describe the major functions of the large intestine, and to the hospital. There, he was diagnosed as having appendicitis.
explain how movement is regulated. Part of that diagnosis includes applying slight pressure, such as by
pushing with the fingertips, to a specific point in the right lower
Anatomy of the Large Intestine quadrant of the abdomen. That point, called the McBurney point,
is midway between the umbilicus and the right anterior superior
The large intestine consists of the cecum, colon, rectum, and anal iliac spine of the coxal bone.
canal (figure 16.21; see figure 16.1). Appendicitis is an inflammation of the appendix that usually
occurs because of obstruction. Secretions from the appendix
Cecum cannot pass the obstruction; therefore, they accumulate, causing
The cecum (sē′ kŭm) is the proximal end of the large intestine enlargement and pain. Bacteria in the area can cause infection.
where it joins with the small intestine at the ileocecal junction. Symptoms include sudden abdominal pain, particularly in the
The cecum is located in the right lower quadrant of the abdomen right lower quadrant, along with a slight fever, loss of appetite,
near the iliac fossa. The cecum is a sac that extends inferiorly constipation or diarrhea, nausea, and vomiting. If the appendix
about 6 cm past the ileocecal junction. Attached to the cecum is a bursts, the infection can spread throughout the peritoneal cavity,
tube about 9 cm long called the appendix. causing peritonitis (see Clinical Impact earlier in this chapter),
with life-threatening results. Each year, 500,000 people in the
Colon United States suffer from appendicitis. The usual treatment is an
The colon (kō′ lon) is about 1.5–1.8 m long and consists of four parts: appendectomy, surgical removal of the appendix.
the ascending colon, the transverse colon, the descending colon,
and the sigmoid colon (figure 16.21). The ascending colon extends The mucosal lining of the colon contains numerous straight,
superiorly from the cecum to the right colic flexure, near the liver, tubular glands called crypts, which contain many mucus-producing
where it turns to the left. The transverse colon extends from the goblet cells. The longitudinal smooth muscle layer of the colon
right colic flexure to the left colic flexure near the spleen, where the does not completely envelop the intestinal wall but forms three
colon turns inferiorly; and the descending colon extends from the bands called teniae coli (tē′ nē-ē kō′ lı̄).
left colic flexure to the pelvis, where it becomes the sigmoid colon.
The sigmoid colon forms an S-shaped tube that extends medially and Rectum
then inferiorly into the pelvic cavity and ends at the rectum. The rectum is a straight, muscular tube that begins at the termina-
tion of the sigmoid colon and ends at the anal canal (figure 16.21).
Brain The muscular tunic is composed of smooth muscle and is relatively
thick in the rectum compared to the rest of the digestive tract.
Stomach
Anal Canal
The last 2–3 cm of the digestive tract is the anal canal. It begins at
the inferior end of the rectum and ends at the anus (external diges-
tive tract opening). The smooth muscle layer of the anal canal is
Vagus nerves 1 Pancreatic
juices
2 Secretin 1 Parasympathetic stimulation from the vagus Digestive
nerve (red arrow) causes the pancreas to
Cholecystokinin Pancreas release a secretion rich in digestive enzymes.
3 Duodenum
2 Secretin (purple arrows), released from
Circulation Cholecystokinin the duodenum, stimulates the pancreas
to release a watery secretion, rich in
bicarbonate ions.
3 Cholecystokinin (pink arrows), released from
the duodenum, causes the pancreas to
release a secretion rich in digestive enzymes.
Secretin
PROCESS Figure 16.20 Control of Pancreatic Secretion
464 Chapter 16
Transverse colon
Right colic flexure Left colic flexure
Ascending Descending colon
colon
Ileum Teniae coli
Ileocecal Sigmoid colon
valve
Cecum
Appendix
Rectum Internal anal sphincter (b)
External anal sphincter
Anal canal
(a)
Figure 16.21 Large Intestine
(a) The large intestine consists of the cecum, colon, rectum, and anal canal. The teniae coli are bands of smooth muscle along the length of
the colon. (b) Radiograph of the large intestine following a barium enema.
Digestive even thicker than that of the rectum and forms the internal anal Feces distend the rectal wall and stimulate the defecation
sphincter at its superior end. The external anal sphincter at the reflex, which involves local and parasympathetic reflexes. Local
inferior end of the anal canal is formed by skeletal muscle. reflexes cause weak contractions, whereas parasympathetic
Hemorrhoids are enlarged or inflamed rectal, or hemor- reflexes cause strong contractions and are normally responsible
rhoidal, veins that supply the anal canal. Hemorrhoids may cause for most of the defecation reflex. Action potentials produced in
pain, itching, and/or bleeding around the anus. Treatments include response to the distention travel along sensory nerve fibers to the
increasing bulk (indigestible fiber) in the diet, taking sitz baths, and sacral region of the spinal cord, where motor action potentials are
using hydrocortisone suppositories. Surgery may be necessary if initiated that reinforce peristaltic contractions in the lower colon
the condition is extreme and does not respond to other treatments. and the rectum. Action potentials from the spinal cord also cause
the internal anal sphincter to relax. The external anal sphincter,
Functions of the Large Intestine which is composed of skeletal muscle and is under conscious
cerebral control, prevents feces from moving out of the rectum
Normally, 18–24 hours are required for material to pass through and through the anal opening. If this sphincter is relaxed volun-
the large intestine, in contrast to the 3–5 hours required for chyme tarily, feces are expelled. The defecation reflex persists for only a
to move through the small intestine. While in the colon, chyme is few minutes and quickly subsides. Generally, the reflex is reini-
converted to feces (fē′ sēz). The formation of feces involves the tiated after a period that may be as long as several hours. Mass
absorption of water and salts, the secretion of mucus, and exten- movements in the colon are usually the reason for the reinitiation
sive action of microorganisms. The colon stores the feces until of the defecation reflex.
they are eliminated by the process of defecation (def-ĕ-kā′ shŭn). Defecation can be initiated by voluntary actions that stimulate
Numerous microorganisms inhabit the colon. They reproduce a defecation reflex. These actions include a large inspiration of air,
rapidly and ultimately constitute about 30% of the dry weight of followed by closure of the larynx and forceful contraction of the
the feces. Some bacteria in the intestine synthesize vitamin K and abdominal muscles. As a consequence, the pressure in the abdomi-
other vitamins, which are passively absorbed in the colon. nal cavity increases and forces feces into the rectum. Stretch of
Every 8–12 hours, large parts of the colon undergo several the rectum initiates a defecation reflex. The increased abdominal
strong contractions, called mass movements, which propel the pressure also helps push feces through the rectum.
colon contents a considerable distance toward the anus. Each mass
movement contraction extends over 20 or more centimeters of the Predict 5
large intestine, which is a much longer part of the digestive tract
than that covered by a peristaltic contraction. These mass movements Explain how an enema stimulates defecation.
are very common following some meals, especially breakfast.
Digestive System 465
16.8 Digestion, Absorption, an energy-storage molecule in plants. Cellulose forms the walls of
and Transport plant cells. Glycogen is an energy-storage molecule in animals and
is contained in muscle and in the liver. When uncooked meats are
Learning Outcomes After reading this section, you should be able to processed or stored, the glycogen is broken down to glucose, which
is further broken down, so that little, if any, glycogen remains.
A. Describe the digestion, absorption, and transport of Therefore, almost all dietary carbohydrates come from plants.
carbohydrates, proteins, vitamins, and minerals. Starch is broken down by enzymes. Cellulose is a polysaccharide
that is not digested but is important for providing fiber in the diet.
B. Describe the digestion, absorption, and transport of fats Salivary amylase begins the digestion of carbohydrates in
and lipids. the mouth (figure 16.23). The carbohydrates then pass to the
stomach, where digestion continues until the food is well mixed
C. Discuss water movement into and out of the digestive tract. with acid, which inactivates salivary amylase. In the duodenum,
pancreatic amylase continues the digestion of carbohydrates, and
Digestion is the breakdown of food to molecules that are small absorption begins. The amylases break down polysaccharides to
enough to be absorbed into the circulation. Mechanical digestion disaccharides (dı̄-sak′ ă-rı̄dz; two sugars chemically linked; see
breaks large food particles into smaller ones. Chemical digestion chapter 2). A group of enzymes called disaccharidases that are
involves the breaking of covalent chemical bonds in organic mol bound to the microvilli of the intestinal epithelium break down
ecules by digestive enzymes. Carbohydrates break down into mono- the disaccharides to monosaccharides. One disaccharidase is lac-
saccharides, lipids break down into fatty acids and monoglycerides, tase, which breaks down lactose (milk sugar); many people have
and proteins break down into amino acids (figure 16.22). heard of lactase, because lack of it leads to lactose intolerance.
Absorption begins in the stomach, where some small, lipid- Lactase is made at birth; however, in 5–15% of the European-
soluble molecules, such as alcohol and aspirin, can diffuse through American population and 80–90% of the African-American and
the stomach epithelium into the circulation. Most absorption Asian-American populations, lactase synthesis sharply declines
occurs in the duodenum and jejunum, although some occurs in 3–4 years after weaning. In the absence of lactase, ingestion of
the ileum. Some molecules can diffuse through the intestinal dairy products causes intestinal cramping, bloating, and diarrhea.
wall, whereas others must be transported across the intestinal wall. An increasing selection of foods for lactose-intolerant people can
Transport requires carrier molecules and includes facilitated dif- be found in the supermarket.
fusion, cotransport, and active transport. Cotransport and active The monosaccharides (mon-ō-sak′ ă-rı̄dz; single sugars) glu-
transport require energy to move the transported molecules across cose, galactose, and fructose are taken up through the intestinal
the intestinal wall. epithelial cells (figure 16.24; see chapter 3). Cotransport of the major
monosaccharide, glucose, and Na+ is driven by a Na+ concentration
Carbohydrates gradient that is established by the sodium-potassium pump. Diffusion
of Na+ down its concentration gradient provides the energy to trans-
Ingested carbohydrates (kar-bō-hı̄′ drāts) consist primarily of port glucose across the cell membrane. This mechanism is also used
starches, cellulose, sucrose (table sugar), and small amounts for galactose transport, while fructose is taken up by facilitated
of fructose (fruit sugar) and lactose (milk sugar). Starches, cel- diffusion (see chapter 3). Once inside the intestinal epithelial cell,
lulose, sucrose, and fructose are derived from plants, and lactose monosaccharides are transported into the capillaries of the intestinal
is derived from animals. Polysaccharides (pol-ē-sak′ ă-rı̄dz) are villi and are carried by the hepatic portal system to the liver. Liver
large carbohydrates, such as starches, cellulose, and glycogen, cells convert different types of monosaccharides to glucose, which
that consist of many sugars linked by chemical bonds. Starch is then leaves the liver via the circulation to be distributed throughout
the body. Glucose enters the cells by facilitated diffusion. The rate
Food of glucose transport into most types of cells is greatly influenced by
insulin and can increase tenfold in the presence of insulin. Without
Carbohydrates Lipids Proteins insulin, glucose enters most cells very slowly. Digestive
Monosaccharides Fatty Mono- Amino Lipids
acids glycerides acids
Lipid molecules are insoluble or only slightly soluble in water
Figure 16.22 Digestion (see chapter 2). They include triglycerides, phospholipids, ste-
roids, and fat-soluble vitamins. Triglycerides (trı̄-glis′ er-ı̄dz) are
Food consists primarily of carbohydrates, lipids, and proteins. Carbohydrates the most common type of lipid. They consist of three fatty acids
are broken down into monosaccharides, lipids into fatty acids and bound to glycerol. Triglycerides are often referred to as fats. Fats
monoglycerides, and proteins into amino acids. are saturated if their fatty acids have only single bonds between
carbons and unsaturated if they have one (monounsaturated)
or more (polyunsaturated) double bonds between carbons (see
chapter 2). Saturated fats are solid at room temperature, whereas
polyunsaturated fats are liquid at room temperature. Saturated fats
are found in meat, dairy products, eggs, nuts, coconut oil, and
palm oil. Unsaturated fats are found in fish and most plant oils.
466 Chapter 16
CLINICAL IMPACT High- and Low-Density Lipids
Cholesterol is a steroid mol- seek advice from their physician, reduce transport cholesterol from the tissues to
ecule that plays important roles in the their intake of foods rich in cholesterol the liver. in addition to assessing total
body (see chapter 2). However, cho- and other fats, and increase their level cholesterol levels, blood tests can reveal
lesterol levels in the blood are of great of exercise. Some people with very high the levels of HDls and lDls in a person’s
concern to many adults because people cholesterol levels may have to take medi- blood. lDl is commonly considered “bad”
with high blood cholesterol run a much cation to lower their cholesterol. because it deposits cholesterol in arterial
greater risk of heart disease and stroke walls, which leads to atherosclerosis. HDl
than do people with low cholesterol. A fats, including cholesterol, are not is considered “good” because it transports
total cholesterol level of less than 180 mil- soluble in water; thus, they are transported cholesterol to the liver for removal from the
ligrams (mg)/dl is considered low, which in the blood as lipid-protein complexes, or body by excretion in the bile. A high HDl/
is usually good, although an extremely low lipoproteins (see figure 16.26). Cholesterol lDl ratio in the bloodstream is related
cholesterol level can be harmful. A choles- is found primarily in two types of lipo- to a lower risk of heart disease. Aerobic
terol level above 200 mg/dl is considered proteins. low-density lipoproteins (lDls) exercise is one way to elevate HDl and
too high. People with high levels should carry cholesterol to the tissues for use by decrease lDl levels.
the cells. High-density lipoproteins (HDls)
Carbohydrates Lipids Proteins
Mouth
(salivary glands)
Salivary amylase
Polysaccharides, Disaccharides
Stomach
Pepsin
Duodenum Bile salts Polypeptides
(pancreas, liver) (liver) Trypsin, chymotrypsin,
Lipase carboxypeptidase
Pancreatic amylase (pancreas) (pancreas)
Digestive Disaccharides Peptides
Peptidases
Epithelium of
small intestine
Disaccharidases
Monosaccharides Fatty acids Amino acids
Monoglycerides
Figure 16.23 Digestion of Carbohydrates, Lipids, and Proteins
the enzymes involved in digesting carbohydrates, lipids, and proteins are depicted in relation to the region of the digestive tract where each functions.
Digestive System 467
Villus
Intestinal Capillary Lacteal
epithelial cell
Monosaccharide (glucose) transport
1 Glucose is absorbed by symport with Na+ Glucose 4
into intestinal epithelial cells. 1 3
2 eSsytmabploisrht eisddbryiveanNbay+–aKs+odpiuummpg.radient
3 Glucose moves out of the intestinal Na+ Na+ ATP ADP
Na+ K+ 2
epithelial cells by facilitated diffusion.
4 Glucose enters the capillaries of the
intestinal villi and is carried through the
hepatic portal vein to the liver.
PROCESS Figure 16.24 Transport of Glucose Across the Intestinal Epithelium To liver
Lipid transport Villus
1 Bile salts surround fatty acids and
Capillary Lacteal
monoglycerides to form micelles.
2 Micelles attach to the plasma membranes Intestinal
epithelial cell
of intestinal epithelial cells, and the fatty
acids and monoglycerides pass by simple Micelles contact Triglycerides Protein coat 4
diffusion into the intestinal epithelial cells. epithelial cell
3 Within the intestinal epithelial cell, the fatty membrane.
acids and monoglycerides are converted to 1
triglycerides; proteins coat the triglycerides
to form chylomicrons, which move out of 3
the intestinal epithelial cells by exocytosis.
4 The chylomicrons enter the lacteals of the Bile Fatty acids 2 Simple Exocytosis
intestinal villi and are carried through the diffusion
lymphatic system to the general circulation. salt and monoglycerides
Micelle
PROCESS Figure 16.25 Transport of Lipids Across the Intestinal Epithelium Chylomicron Lymphatic
system
The first step in lipid digestion is emulsification (ē-mŭl′ si-fi- In the intestine, bile salts aggregate around small droplets of Digestive
kā′ shŭn), by which large lipid droplets are transformed into much digested lipids to form micelles (mi-selz′ , mı̄-selz′ ; small morsels)
smaller droplets. Emulsification is accomplished by bile salts (figure 16.25). The hydrophobic (water-fearing) ends of the bile
secreted by the liver. The enzymes that digest lipids are soluble in salts are directed toward the lipid particles, and the hydrophilic
water and can digest the lipids only by acting at the surface of the (water-loving) ends are directed outward, toward the water envi-
droplets. The emulsification process increases the surface area of ronment. When a micelle comes in contact with the epithelial cells
the lipid droplets exposed to the digestive enzymes by increasing of the small intestine, the lipids, fatty acids, and monoglyceride
the number of lipid droplets and decreasing the size of each droplet. molecules pass, by simple diffusion, from the micelles through the
Lipase, secreted by the pancreas, digests lipid molecules (see cell membranes of the epithelial cells.
figure 16.23). The primary products of this digestive process are Once inside the intestinal epithelial cells, the fatty acids and
fatty acids and monoglycerides. monoglycerides are recombined to form triglycerides. These, and
468 Chapter 16
MICROBES IN YOUR BODY Fecal Transplants
Would you be shocked if your lead to death in some patients. Additionally, saline, filtered, and then introduced into the
doctor said that the one thing that could save a more virulent, resistant strain of C. diff has recipient’s gastrointestinal (gi) tract by one
your life was feces? Unfortunately, we are emerged. this strain is resistant to certain of two ways: the upper gi tract route or the
in the midst of a global, hospital-acquired antibiotics and makes a greater number of lower gi tract route. the upper gi tract route
diarrhea epidemic. the cause of this epi- spores and more of the toxins responsible for uses either a gastroscope or nasogastric
demic is a bacterium called Clostridium the diarrhea. tube to transfer the material to the recipi-
difficile (commonly referred to as C. diff ), ent’s intestine. of the two, this one is easier
a pathogen that is normally found in the So, where do feces come into play? and costs less. However, there is the possibil-
colon, but is controlled by the normal micro- Because antibiotic treatments are not effec- ity the donor microbiota may not reach the
biota. one of the most critical functions of tive (65% infection recurrence), physicians end of the colon or that the patient may
the normal gut microbiota is prevention of are considering an old treatment: fecal vomit the fecal material. the lower gi tract
infections through competition with patho- transplants. the first documented case of route uses a colonoscope or enema and is
gens. As a consequence, when a patient transplanting feces from a healthy donor into the preferred approach, but does run the
takes antibiotics, C. diff can flourish and a diseased recipient was in 1958. fecal trans- risk of perforating the colon. thus, as of
cause life-threatening diarrhea. treatment plantation has since been used successfully yet, there is no standardized method for
of C. diff infections with specific antibiot- in veterinary medicine for decades. However, transferring the donor feces. However,
ics will often stop the diarrhea initially. due to the unappealing nature of this treat- research is showing that more and more
However, C. diff are spore-forming bacteria. ment, it has only recently been considered patients may overcome their initial reluc-
Spores are very stable structures that allow an option in humans. now more commonly tance when presented with a predictable
bacteria to withstand harsh conditions until known as intestinal fecal transplantation success rate and greater reliability than
favorable conditions return and the bacteria (ift), it has been shown to effectively treat other protocols. in addition, the recent
can regrow. thus, antibiotics kill only the diarrhea in over 90% of C. diff infections. “RePooPulating” study shows promise that
C. diff cells, not the spores. Hence, it is very the idea is that a healthy donor—usually a doctors may soon be able to treat C. diff
common for patients to suffer multiple recur- close family household member such as a infections simply by prescribing a pill that
rences of diarrhea for months, which can spouse or significant partner—donates their contains normal microbiota.
feces. the feces are mixed with physiological
other lipids, are packaged inside a protein coat. The packaged Chylomicron
lipid-protein complexes, or lipoproteins, are called chylomicrons
(kı̄-lō-mi′kronz). Chylomicrons leave the epithelial cells and Phospholipid (4%)
enter the lacteals, lymphatic capillaries within the intestinal villi.
Lymph containing large amounts of absorbed lipid is called chyle Triglyceride (90%)
(kı̄l; milky lymph). The lymphatic system carries the chyle to the Cholesterol (5%)
bloodstream. Chylomicrons are transported to the liver, where Protein (1%)
the lipids are stored, converted into other molecules, or used as
energy. They are also transported to adipose tissue, where they are Low-density lipoprotein Phospholipid (20%)
stored until an energy source is needed elsewhere in the body. Other (LDL)
lipoproteins, called low-density lipoproteins (LDLs) and high- Triglyceride (10%)
Digestive density lipoproteins (HDLs), transport cholesterol and fats in the Cholesterol (45%)
blood (figure 16.26). Protein (25%)
Proteins High-density lipoprotein Phospholipid (30%)
(HDL) Triglyceride (5%)
Proteins are chains of amino acids. They are found in most of the
plant and animal products we eat. Pepsin is an enzyme secreted by Cholesterol (20%)
the stomach that breaks down proteins, producing shorter amino Protein (45%)
acid chains called polypeptides (see figure 16.23). Only about
10–20% of the total ingested protein is digested by pepsin. After the Figure 16.26 Lipoproteins
remaining proteins and polypeptide chains leave the stomach and
enter the small intestine, the enzymes trypsin, chymotrypsin, and
carboxypeptidase, produced by the pancreas in their inactive forms
and activated in the intestine, continue the digestive process. These
enzymes produce small peptides, which are further broken down
Digestive System 469
Villus
Amino acid transport Intestinal Capillary Lacteal
1 Acidic and most neutral amino acids are absorbed epithelial cell
by symport into intestinal epithelial cells. Amino acid
2 SbyymapNoart+i–sKd+rivpeunmbpy. a sodium gradient established
3 Amino acids move out of intestinal epithelial 1 3 4
cells. Na+ Na+ ATP ADP
4 Amino acids enter the capillaries of the intestinal Na+ K+ 2
villi and are carried through the hepatic portal
vein to the liver.
PROCESS Figure 16.27 Transport of Amino Acids Across the Intestinal Epithelium To liver
into tripeptides (three amino acids), dipeptides (two amino acids), Ingestion Salivary gland secretions
or single amino acids by digestive enzymes called peptidases. The (2 L) (1 L)
peptidases are bound to the microvilli of the small intestine.
Absorption of tripeptides, dipeptides, or individual amino acids Gastric
occurs through the intestinal epithelial cells by various cotransport secretions
mechanisms. Many, but not all, amino acids are taken up by cotrans- (2 L)
port with Na+ similar to glucose cotransport (figure 16.27). Within
the intestinal epithelial cells, tripeptides and dipeptides are broken Bile Pancreatic
down into amino acids. The amino acids then enter blood capillaries (0.7 L) secretions
in the villi and are carried by the hepatic portal vein to the liver. The Small intestine (1.2 L)
amino acids may be modified in the liver, or they may be released secretions 92%
into the bloodstream and distributed throughout the body. (2 L) absorbed in the
Amino acids are actively transported into the various cells small intestine
of the body. This transport is stimulated by growth hormone and 6%–7% Digestive
insulin. Most amino acids are used as building blocks to form absorbed in the
new proteins, but some may be metabolized, with a portion of large intestine
the released energy used to produce ATP. The body cannot store
excess amino acids. Instead, they are partially broken down and 1% in Ingestion or
used to synthesize glycogen or lipids, which can be stored. The feces secretion
body can store only small amounts of glycogen, so most of the (Water in feces ϭ Ingested ϩ Secreted Ϫ Absorbed) Absorption
excess amino acids are converted to lipids.
Figure 16.28 Fluid Volumes in the Digestive Tract
Water and Minerals
Approximately 9 L of water enter the digestive tract each day
(figure 16.28). We ingest about 2 L in food and drink, and the
remaining 7 L are from digestive secretions. Approximately
92% of that water is absorbed in the small intestine, about 7% is
absorbed in the large intestine, and about 1% leaves the body in
the feces. Water can move in either direction by osmosis across
the wall of the digestive tract. The direction of its movement is
determined by osmotic gradients across the epithelium. When the
chyme is diluted, water moves out of the intestine into the blood.
If the chyme is concentrated and contains little water, water moves
into the lumen of the small intestine.
Sodium, potassium, calcium, magnesium, and phosphate ions
are actively transported from the small intestine. Vitamin D is
SyStemS PaThOLOGY
NGaemndee: r: tom Diarrhea
age: male
38 background Information
tuoacrwbrgrfnewieeoteseomaenouaixfstltmttatolfewhibitlttnrudohtwawayeilete.dcseaodfHkastnsew.adnewecteHekaodaahnaeaeytrictslnsekelshwwoe,.,osecuhteatitainrvCioarsfotsarooamttooemahfpgomcsfyremmoetaeiotceolaaietrghiidtm,foeeolntnnxe,iieyauonlclsptxddet.sopognrcieionotoownvflao.ormtreemoeuniHeuseuhelcrmsctosnrrueeinloaeetisyftac,dscunraathadtfnwi.eypdlnrhnbfaDs,elbherqesodehdysoquheexutsioimrobasudartt.miimertrfeenrrrweeeptimonggtitnmerdinihu,tafkmedtaeeetanahdlpeylneitslahde.toneutygh Diarrhea is one of the most common complaints in clinical medicine.
Diarrhea affects more than half the tourists in developing countries
Figure 16A (figure 16A), where it may result from eating food to which the
digestive tract is not accustomed or from ingesting food or water
many tourists
develop diarrhea. contaminated with pathogens.
Diarrhea is any change in bowel habits involving increased stool
frequency or volume or increased stool fluidity. it is not a disease
in itself, but it can be a symptom of a wide variety of disorders.
Diarrhea that lasts less than 2–3 weeks is acute diarrhea; diarrhea
lasting longer is considered chronic. Acute diarrhea is usually self-
limiting, but some forms of diarrhea can be fatal if not treated.
Diarrhea results from either a decrease in fluid absorption in the
intestine or an increase in fluid secretion. it can also be caused
by increased bowel motor activity that moves chyme rapidly
through the small intestine, so that more water enters the
colon. normally, about 600 ml of fluid enter the colon each
day, and all but 150 ml are reabsorbed. the loss of more than
200 ml of fluid per day in the stool is considered abnormal.
mucus secretion by the colon increases dramatically in
response to diarrhea. this mucus contains large quantities
of bicarbonate ions, which come from the dissociation of
carbonic acid into bicarbonate ions and hydrogen ions
within the blood supply to the colon. the bicarbonate
ions enter the mucus secreted by the colon, whereas the
hydrogen ions remain in the circulation; as a result, the
blood pH decreases. thus, a condition called metabolic
acidosis can develop (see chapter 18).
Diarrhea is usually caused by bacteria, viruses, amoebic
parasites, or chemical toxins. Symptoms can begin from as
little as 1–2 hours after bacterial toxins are ingested to
as long as 24 hours or more for some strains of bacteria.
nearly any bacterial species is capable of causing diarrhea.
Some types of bacterial diarrhea are associated with severe
vomiting, whereas others are not. Some bacterial toxins
also induce fever. identifying the causal organism usually
requires laboratory analysis of the food or stool, but in cases
of acute diarrhea, the infectious agent is seldom identified.
Digestive
required for the transport of Ca2+. Negatively charged Cl− moves As a person ages, the connective tissue layers of the digestive
passively through the wall of the duodenum and jejunum with the tract—the submucosa and serosa—tend to thin. The blood sup-
positively charged Na+, but Cl− is actively transported from the ileum. ply to the digestive tract decreases. There is also a decrease in
the number of smooth muscle cells in the muscularis, resulting in
16.9 effeCtS of Aging on tHe decreased motility in the digestive tract. In addition, goblet cells
DigeStive SyStem within the mucosa secrete less mucus. Glands along the diges-
tive tract, such as the gastric glands, the liver, and the pancreas,
Learning Outcome After reading this section, you should be able to also tend to secrete less with age. However, these changes by
A. Describe the effects of aging on the digestive system. themselves don’t appreciably decrease the function of the diges-
tive system.
470
INTeGUMeNTaRY MUSCULaR NeRvOUS
Pallor is due to vasoconstriction of muscular weakness may result due to local reflexes in the colon respond to
blood vessels in the skin, resulting from ion loss, metabolic acidosis, fever, and increased colon fluid volume by stimulating
general malaise. the stimulus to defecate mass movements and the defecation reflex.
a decrease in blood volume. Pallor may become so strong that it overcomes Abdominal pain, much of which is felt as
and sweating increase in response to the voluntary control mechanisms. referred pain, can occur as the result of
inflammation and distention of the colon.
abdominal pain and anxiety. Diarrhea nervous system function decreases due to
ion loss. Reduced blood volume stimulates
URINaRY Symptoms a sensation of thirst in the CnS.
• Increased stool frequency
in the event of fluid loss from the • Increased stool volume eNDOCRINe
intestine, the kidney is activated to • Increased stool fluidity
compensate for metabolic acidosis by A decrease in extracellular fluid volume, due
increasing hydrogen ion secretion and Treatments to the loss of fluid in the feces, stimulates the
• Replacing lost fluid release of hormones (antidiuretic hormone
bicarbonate ion reabsorption. and ions from the posterior pituitary and aldosterone
• Eating a bland diet from the adrenal cortex) that increase water
ReSPIRaTORY without dairy retention and sodium reabsorption in the
kidney. in addition, decreased extracellular
increased HCo3– secretion and H+ fluid volume and anxiety result in increased
absorption reduce blood pH. As the release of epinephrine and norepinephrine
result of reduced blood pH, the rate from the adrenal medulla.
of respiration increases to eliminate
carbon dioxide, which helps
eliminate excess H+.
LYMPhaTIC aND IMMUNe CaRDIOvaSCULaR
White blood cells migrate to the colon in response to movement of extracellular fluid into the
infection and inflammation. in the case of bacterial diarrhea, colon results in decreased blood volume.
the reduced blood volume activates the
the immune response is initiated to begin production of baroreceptor reflex, which maintains
antibodies against bacteria and bacterial toxins. blood volume and blood pressure.
treatment of diarrhea involves replacing lost fluids and ions. milk products. Breads, rice, and baked fish or chicken can be added
the diet should be limited to clear fluids during at least the first day to the diet as the person’s condition improves. A normal diet can be
or so. medicines that may help combat diarrhea include bismuth resumed after 2–3 days.
subsalicylate (s˘ub-sa˘-lis′i-la¯t), which increases mucus and HCo3–
secretion and decreases pepsin activity, and loperamide (lo¯-per′a˘- Predict 6
m¯ıd), which slows intestinal motility. Patients should avoid milk and Predict the effects of prolonged diarrhea on the cardiovascular system.
Through the years, the digestive tract, like the skin and decline; the ability of the hepatic phagocytic cells to remove
lungs, is directly exposed to materials from the outside environ- particulate contaminants decreases; and the liver’s ability to
ment. Some of those substances can cause mechanical damage store glycogen decreases.
to the digestive tract, and others may be toxic to the tissues.
Because the connective tissue of the digestive tract becomes This overall decline in the defenses of the digestive tract
thin with age and because the protective mucous covering is leaves elderly people more susceptible to infections and to the
reduced, the digestive tract of elderly people becomes less and effects of toxic agents. Elderly people are more likely to develop
less protected from these outside influences. In addition, the ulcerations and cancers of the digestive tract. Colorectal cancers,
mucosa of elderly people tends to heal more slowly following for example, are the second leading cause of cancer deaths in the
injury. The liver’s ability to detoxify certain chemicals tends to United States, with an estimated 135,000 new cases and 57,000
deaths each year.
471
472 Chapter 16
Gastroesophageal reflux disorder (GERD) increases with The enamel on the surface of elderly people’s teeth becomes
advancing age. It is probably the main reason that elderly people take thinner with age and may expose the underlying dentin. In addition,
antacids and inhibitors of hydrochloric acid secretion. Disorders the gingiva covering the tooth root recedes, exposing additional
that are not necessarily age-induced, such as hiatal hernia and dentin. Exposed dentin may become painful and change the person’s
irregular or inadequate esophageal motility, may be worsened by eating habits. Many elderly people also lose teeth, which can have
the effects of aging, because of general decreased motility in the a marked effect on eating habits unless artificial teeth are provided.
digestive tract. The muscles of mastication tend to become weaker; as a result,
older people tend to chew their food less before swallowing.
DISEASES AND DISORDERS: Digestive System
Digestive CONDITION DESCRIPTION
Stomach Contraction of the diaphragm and abdominal muscles and relaxation of the esophageal sphincters to forcefully
Vomiting expel gastric contents; vomiting reflex is initiated by irritation of the stomach or small intestine
Peptic ulcer
Liver Lesions in the lining of the stomach or duodenum, usually due to infection by the bacterium Helicobacter pylori;
Cirrhosis (sir-o¯′sis) stress, diet, smoking, or alcohol may be predisposing factors; antibiotic therapy is the accepted treatment
Hepatitis (hep-a˘-ti′tis)
Characterized by damage to and death of hepatic cells and replacement by connective tissue; results in loss of
Hepatitis A normal liver function and interference with blood flow through the liver; a common consequence of alcoholism
Hepatitis B
Hepatitis C Inflammation of the liver that causes liver cell death and replacement by scar tissue; if not corrected, results in
Gallstones loss of liver function and eventually death; symptoms include nausea, abdominal pain, fever, chills, malaise, and
Intestine jaundice; caused by any of seven distinct viruses
Inflammatory bowel disease
(IBD) Infectious hepatitis; usually transmitted by poor sanitation practices or from mollusks living in contaminated waters
Irritable bowel syndrome (IBS) Serum hepatitis; usually transmitted through blood or other body fluids through either sexual contact or
Gluten enteropathy contaminated hypodermic needles
(celiac disease)
Constipation (kon-sti-pa¯′ sh˘un) Often a chronic disease leading to cirrhosis and possibly cancer of the liver
Food poisoning Most often due to excess cholesterol in the bile; gallstones can occasionally enter the cystic duct, where they block
Typhoid (t¯ı ′foyd) fever the release of bile and/or pancreatic enzymes, which interferes with digestion
Cholera (kol′er-a) Localized inflammatory degeneration that may occur anywhere along the digestive tract but most commonly
involves the distal ileum and proximal colon; the intestinal wall often becomes thickened, constricting the lumen,
Giardiasis (je¯-ar-d¯ı ′a-sis) with ulcers and fissures in the damaged areas; symptoms include diarrhea, abdominal pain, fever, fatigue, and
weight loss; cause is unknown; treatments involve anti-inflammatory drugs, avoidance of foods that produce
Intestinal parasites symptoms, and surgery in some cases; also called Crohn’s disease or ulcerative colitis
Diarrhea (d¯ı -a˘-re¯′a˘)
Dysentery (dis′ en-ta¯r-e¯) Disorder of unknown cause marked by alternating bouts of constipation and diarrhea; may be linked to stress or
depression; high familial incidence
Malabsorption in the small intestine due to the effects of gluten, a protein in certain grains, especially wheat; the
reaction can destroy newly formed epithelial cells, causing the intestinal villi to become blunted and decreasing the
intestinal surface, which reduces absorption of nutrients
Slow movement of feces through the large intestine, causing the feces to become dry and hard because of increased fluid
absorption while being retained; often results from inhibiting normal defecation reflexes; spasms of the sigmoid colon
resulting from irritation can also result in slow feces movement and constipation; high-fiber diet can be preventative
Infections of the Digestive Tract
Caused by ingesting bacteria or toxins, such as Staphylococcus aureus, Salmonella, or Escherichia coli; symptoms
include nausea, abdominal pain, vomiting, and diarrhea; in severe cases, death can occur
Caused by a virulent strain of the bacterium Salmonella typhi, which can cross the intestinal wall and invade other
tissues; symptoms include severe fever, headaches, and diarrhea; usually transmitted through poor sanitation
practices; leading cause of death in many underdeveloped countries
Caused by a bacterium, Vibrio cholerae, in contaminated water; bacteria produce a toxin that stimulates the secretion
of chloride, bicarbonate, and water into the intestine, resulting in severe diarrhea; the loss of as much as 12–20 L of
fluid per day causes shock, circulatory collapse, and even death; still a major health problem in parts of Asia
Caused by a protozoan, Giardia lamblia, that invades the intestine; symptoms include nausea, abdominal cramps,
weakness, weight loss, and malaise; bacteria are transmitted in the feces of humans and other animals, often by
drinking from contaminated wilderness streams
Common under conditions of poor sanitation; parasites include tapeworms, pinworms, hookworms, and roundworms
Intestinal mucosa secretes large amounts of water and ions due to irritation, inflammation, or infection; diarrhea
moves feces out of the intestine more rapidly, which speeds recovery
Severe form of diarrhea with blood or mucus in the feces; can be caused by bacteria, protozoa, or amoebae
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