Essentials of Anatomy and Physiology

Copyright © 2007 by F. A. Davis.

The Lymphatic System and Immunity 333

Table 14–1 CLASSES OF ANTIBODIES

Name Location Functions
IgG
IgA Blood • Crosses the placenta to provide passive immunity for newborns
IgM Extracellular fluid • Provides long-term immunity following recovery or a vaccine
External secretions (tears,
• Present in breast milk to provide passive immunity for breast-fed infants
saliva, etc.) • Found in secretions of all mucous membranes
Blood
• Produced first by the maturing immune system of infants
IgD B lymphocytes • Produced first during an infection (IgG production follows)
IgE Mast cells or basophils • Part of the ABO blood group

• Receptors on B lymphocytes

• Important in allergic reactions (mast cells release histamine)

produced is specific for only one antigen. Because be a need for them. The structure of antibodies is
there are so many different pathogens, you might shown in Fig. 14–8, and the five classes of antibodies
think that the immune system would have to be capa- are described in Table 14–1.
ble of producing many different antibodies, and in fact
this is so. It is estimated that millions of different anti- The antibodies produced will bond to the antigen,
gen-specific antibodies can be produced, should there forming an antigen–antibody complex. This complex
results in opsonization, which means that the antigen

A B

Antigen binding
site

Figure 14–8. Antibodies. Complement IgG IgD IgE
(A) Structure of one IgG mol- binding site
ecule. Notice how the many Disulfide
disulfide bonds maintain the Macrophage bonds
shape of the molecule. binding site
(B) Structure of the five classes IgA IgM
of antibodies. (C) Antibody Bacteria Virus
activity: Agglutination of bac- Toxin
teria and neutralization of
viruses or toxins.
QUESTION: In part C, why
does neutralization inactivate
a bacterial toxin?

Agglutination Neutralization
C

Copyright © 2007 by F. A. Davis.

334 The Lymphatic System and Immunity

BOX 14–3 DIAGNOSTIC TESTS Antibody Responses

Several important laboratory tests involve anti- The first exposure to a foreign antigen does stimulate
bodies and may be very useful to confirm a diag- antibody production, but antibodies are produced
nosis. slowly and in small amounts (see Fig. 14–9). Let us
take as a specific example the measles virus. On a per-
Complement fixation test—determines the son’s first exposure to this virus, antibody production is
presence of a particular antibody in the patient’s usually too slow to prevent the disease itself, and the
blood, but does not indicate when the infection person will have clinical measles. Most people who get
occurred. measles recover, and upon recovery have antibodies
Antibody titer—determines the level or and memory cells that are specific for the measles
amount of a specific antibody in the patient’s virus.
blood. If another titer is done 1 to several weeks
later, an increase in the antibody level shows the On a second exposure to this virus, the memory
infection to be current. cells initiate rapid production of large amounts of anti-
Fluorescent antibody test—uses antibodies bodies, enough to prevent a second case of measles.
tagged with fluorescent dyes, which are added This is the reason why we develop immunity to certain
to a clinical specimen such as blood, sputum, or diseases, and this is also the basis for the protection
a biopsy of tissue. If the suspected pathogen is given by vaccines (see Box 14–4: Vaccines).
present, the fluorescent antibodies will bond to it
and the antigen–antibody complex will “glow” As mentioned previously, antibodies label patho-
when examined with a fluorescent microscope. gens or other foreign antigens for phagocytosis or
complement fixation. More specifically, antibodies
Tests such as these are used in conjunction cause agglutination or neutralization of pathogens
with patient history and symptoms to arrive at a before their eventual destruction. Agglutination
diagnosis. means “clumping,” and this is what happens when
antibodies bind to bacterial cells. The bacteria that are
is now “labeled” for phagocytosis by macrophages or clumped together by attached antibodies are more eas-
neutrophils. The antigen–antibody complex also stim- ily phagocytized by macrophages (see Fig. 14–8).
ulates the process of complement fixation (see Box
14–3: Diagnostic Tests). The activity of viruses may be neutralized by anti-
bodies. A virus must get inside a living cell in order to
Some of the circulating complement proteins are reproduce itself. However, a virus with antibodies
activated, or fixed, by an antigen–antibody complex. attached to it is unable to enter a cell, cannot repro-
Complement fixation may be complete or partial. If duce, and will soon be phagocytized. Bacterial toxins
the foreign antigen is cellular, the complement pro- may also be neutralized by attached antibodies. The
teins bond to the antigen–antibody complex, then to antibodies change the shape of the toxin, prevent it
one another, forming an enzymatic ring that punches from exerting its harmful effects, and promote its
a hole in the cell to bring about the death of the cell. phagocytosis by macrophages.
This is complete (or entire) complement fixation and
is what happens to bacterial cells (it is also the cause of Allergies are also the result of antibody activity
hemolysis in a transfusion reaction). (see box 14–5: Allergies).

If the foreign antigen is not a cell—let’s say it’s a TYPES OF IMMUNITY
virus for example—partial complement fixation takes
place, in which some of the complement proteins bond If we consider the source of immunity, that is, where
to the antigen–antibody complex. This is a chemotac- it comes from, we can begin with two major cate-
tic factor. Chemotaxis means “chemical movement” and gories: genetic immunity and acquired immunity.
is actually another label that attracts macrophages to Genetic immunity is conferred by our DNA, and
engulf and destroy the foreign antigen. acquired immunity is developed or acquired by natu-
ral or artificial means.
In summary, adaptive immunity is very specific,
does create memory, and because it does, often be- Genetic immunity does not involve antibodies or
comes more efficient with repeated exposures. the immune system; it is the result of our genetic
makeup. What this means is that some pathogens

Copyright © 2007 by F. A. Davis.

The Lymphatic System and Immunity 335

Primary and secondary antibody responses

IgG

First Second
exposure exposure

to to
antigen antigen

IgG

Antibody level IgM IgM
10 days 20 days 10 days 20 days
some months some years

Time after exposure

Figure 14–9. Antibody responses to first and subsequent exposures to a pathogen. See
text for description.
QUESTION: State the two differences in IgG production after a first and a second exposure
to the same antigen.

BOX 14–4 VACCINES the inactivated toxins of these bacteria. Vaccines for
pneumococcal pneumonia and meningitis contain
The purpose of vaccines is to prevent disease. A vac- bacterial capsules. These vaccines cannot cause dis-
cine contains an antigen that the immune system ease because the capsules are non-toxic and non-
will respond to, just as it would to the actual living; there is nothing that can reproduce.
pathogen. The types of vaccine antigens are a killed Influenza and rabies vaccines contain killed viruses.
or weakened (attenuated) pathogen, part of a Measles and the oral polio vaccines contain attenu-
pathogen such as a bacterial capsule, or an inacti- ated (weakened) viruses.
vated bacterial toxin called a toxoid.
Although attenuated pathogens are usually
Because the vaccine itself does not cause disease strongly antigenic and stimulate a protective
(with very rare exceptions), the fact that antibody immune response, there is a very small chance that
production to it is slow is not detrimental to the the pathogen may regain its virulence and cause
person. The vaccine takes the place of the first the disease. The live-virus oral polio vaccine (still
exposure to the pathogen and stimulates produc- being used in the quest to eliminate polio through-
tion of antibodies and memory cells. On exposure out the world) has a risk of 1 in 500,000 of causing
to the pathogen itself, the memory cells initiate polio. The killed-virus injectable polio vaccine has
rapid production of large amounts of antibody, no such risk.
enough to prevent disease.

We now have vaccines for many diseases. The
tetanus and diphtheria vaccines contain toxoids,

Copyright © 2007 by F. A. Davis.

336 The Lymphatic System and Immunity

BOX 14–5 ALLERGIES In an allergic reaction, the effects of inflamma-
tory chemicals create symptoms such as watery
An allergy is a hypersensitivity to a particular for- eyes and runny nose (hay fever) or the more serious
eign antigen, called an allergen. Allergens include wheezing and difficult breathing that characterize
plant pollens, foods, chemicals in cosmetics, antibi- asthma. Several medications are available to coun-
otics such as penicillin, dust, and mold spores. Such teract these effects (see Chapter 15 for a description
allergens are not themselves harmful. Most people, of asthma).
for example, can inhale pollen, eat peanuts, or take
penicillin with no ill effects. Anaphylactic shock is an extreme allergic
response that may be elicited by exposure to peni-
Hypersensitivity means that the immune system cillin or insect venoms. On the first exposure, the
overresponds to the allergen, and produces tissue person becomes highly sensitized to the foreign
damage by doing so. Allergic responses are charac- antigen. On the second exposure, histamine is
terized by the production of IgE antibodies, which released from mast cells throughout the body and
bond to mast cells. Mast cells are specialized con- causes a drastic decrease in blood volume. The
nective tissue cells and are numerous in the con- resulting drop in blood pressure may be fatal in
nective tissue of the skin and mucous membranes. only a few minutes. People who know they are
Chemicals in mast cells include histamine and allergic to bee stings, for example, may obtain a
leukotrienes, which are released by the bonding of self-contained syringe of epinephrine to carry with
IgE antibodies or when tissue damage occurs. them. Epinephrine can delay the progression of
anaphylactic shock long enough for the person to
These chemicals contribute to the process of seek medical attention.
inflammation by increasing the permeability of cap-
illaries and venules. Tissue fluid collects and more
WBCs are brought to the damaged area.

cause disease in certain host species but not in others. by the injection of immune globulins (gamma globu-
Dogs and cats, for example, have genetic immunity lins or preformed antibodies) after presumed exposure
to the measles virus, which is a pathogen only for peo- to a particular pathogen. Such immune globulins are
ple. Mouse leukemia viruses affect only mice, not available for German measles, hepatitis A and B,
people; we have genetic immunity to them. This is tetanus and botulism (anti-toxins), and rabies. These
not because we have antibodies against these mouse are not vaccines; they do not stimulate immune mech-
viruses, but rather that we have genes that are the anisms, but rather provide immediate antibody pro-
codes for proteins that make it impossible for such tection. Passive immunity is always temporary, lasting
pathogens to reproduce in our cells and tissues. a few weeks to a few months, because antibodies from
Monkeys have similar protective genes and proteins another source eventually break down.
for the human AIDS virus; HIV does not cause disease
in these monkeys. Because this is a genetic character- Active immunity is the production of one’s own
istic programmed in DNA, genetic immunity always antibodies and may be stimulated by natural or artifi-
lasts a lifetime. cial means. Naturally acquired active immunity means
that a person has recovered from a disease and now
Acquired immunity does involve antibodies. has antibodies and memory cells specific for that
Passive immunity means that the antibodies are from pathogen. Artificially acquired active immunity is the
another source, whereas active immunity means that result of a vaccine that has stimulated production of
the individual produces his or her own antibodies. antibodies and memory cells (see Box 14–6: Vaccines
That Have Changed Our Lives). No general state-
One type of naturally acquired passive immunity is ment can be made about the duration of active immu-
the placental transmission of antibodies (IgG) from nity. Recovering from plague, for example, confers
maternal blood to fetal circulation. The baby will then lifelong immunity, but the plague vaccine does not.
be born temporarily immune to the diseases the Duration of active immunity, therefore, varies with
mother is immune to. Such passive immunity may be the particular disease or vaccine.
prolonged by breast-feeding, because breast milk also
contains maternal antibodies (IgA). The types of immunity are summarized in Table
14–2.
Artificially acquired passive immunity is obtained

Copyright © 2007 by F. A. Davis.

The Lymphatic System and Immunity 337

BOX 14–6 VACCINES THAT HAVE CHANGED OUR LIVES

In 1797, Edward Jenner (in England) published his they are no longer possible reservoirs or sources of
results on the use of the cowpox virus called vac- the pathogen for others, and the spread of disease
cinia as the first vaccine for smallpox, a closely may be greatly limited.
related virus. (He was unaware of the actual patho-
gens, because viruses had not yet been discovered, Other diseases that have been controlled by the
but he had noticed that milkmaids who got cow- use of vaccines are tetanus, mumps, influenza,
pox rarely got smallpox.) In 1980, the World Health measles, and German measles. Whooping cough
Organization declared that smallpox had been had been controlled until recently, when the vacci-
eradicated throughout the world. A disease that nation rate decreased; the annual number of cases
had killed or disfigured millions of people through- in the United States has more than doubled. The
out recorded history is now considered part of his- vaccine for hepatitis B has significantly decreased
tory (except for the possible use of the virus as a the number of cases of this disease among health-
biological weapon). care workers, and the vaccine is recommended for
all children. People who have been exposed to
In the 19th century in the northern United rabies, which is virtually always fatal, can be pro-
States, thousands of children died of diphtheria tected by a safe vaccine.
every winter. Today there are fewer than 10 cases
of diphtheria each year in the entire country. In Without such vaccines our lives would be very
the early 1950s, 50,000 cases of paralytic polio different. Infant mortality or death in childhood
were reported in the United States each year. would be much more frequent, and all of us would
Today, wild-type polio virus is not found in North have to be much more aware of infectious diseases.
America. In many parts of the world this is still true; many of
the developing countries in Africa and Asia still can-
Smallpox, diphtheria, and polio are no longer not afford extensive vaccination programs for their
the terrible diseases they once were, and this is children. Many of the diseases mentioned here,
because of the development and widespread use of which we may rarely think of, are still a very signif-
vaccines. When people are protected by a vaccine, icant part of the lives of millions of people.

Table 14–2 TYPES OF IMMUNITY AGING AND THE
LYMPHATIC SYSTEM
Type Description
Genetic The aging of the lymphatic system is apparent in the
• Does not involve antibodies; is decreased efficiency of immune responses. Elderly
Acquired programmed in DNA people are more likely than younger ones to develop
Passive shingles, when an aging immune system cannot keep
• Some pathogens affect certain the chickenpox virus dormant. They are also more
NATURAL host species but not others susceptible to infections such as influenza and to what
are called secondary infections, such as pneumonia
ARTIFICIAL • Does involve antibodies following a case of the flu. Vaccines for both of these
are available, and elderly people should be encouraged
Active • Antibodies from another source to get them. Elderly people should also be sure to get
NATURAL • Placental transmission of antibodies a tetanus-diphtheria booster every 10 years.

ARTIFICIAL from mother to fetus Autoimmune disorders are also more common
• Transmission of antibodies in among older people; the immune system mistakenly
perceives a body tissue as foreign and initiates its des-
breast milk truction. Rheumatoid arthritis and myasthenia gravis
• Injection of preformed antibodies are examples of autoimmune diseases. The incidence
of cancer is also higher. Malignant cells that once
(gamma globulins or immune might have been quickly destroyed remain alive and
globulins) after presumed exposure proliferate.

• Production of one’s own antibodies
• Recovery from a disease, with pro-

duction of antibodies and memory
cells
• A vaccine stimulates production of
antibodies and memory cells

Copyright © 2007 by F. A. Davis.

338 The Lymphatic System and Immunity

SUMMARY known that people under great stress have immune
systems that may not function as they did when stress
The preceding discussions of immunity will give you a was absent.
small idea of the complexity of the body’s defense sys-
tem. However, there is still much more to be learned, At present, much research is being done in this
especially about the effects of the nervous system and field. The goal is not to eliminate all disease, for that
endocrine system on immunity. For example, it is would not be possible. Rather, the aim is to enable
people to live healthier lives by preventing certain
diseases.

STUDY OUTLINE Lymph Nodes—encapsulated masses of lym-
phatic tissue
Functions of the Lymphatic System 1. Found in groups along the pathways of lymph ves-
1. To return tissue fluid to the blood to maintain
sels.
blood volume (see Fig. 14–1). 2. As lymph flows through the nodes:
2. To protect the body against pathogens and other
• foreign material is phagocytized by fixed macro-
foreign material. phages

Parts of the Lymphatic System • lymphocytes are activated and fixed plasma cells
1. Lymph and lymph vessels. produce antibodies to foreign antigens (see Fig.
2. Lymphatic tissue: lymph nodes and nodules, 14–4)

spleen, and thymus; lymphocytes mature and pro- 3. The major paired groups of lymph nodes are the
liferate. cervical, axillary, and inguinal groups. These are
at the junctions of the head and extremities with
Lymph—the tissue fluid that enters lymph the trunk; remove pathogens from the lymph from
capillaries the extremities before the lymph is returned to the
1. Similar to plasma, but more WBCs are present, blood.

and has less protein. Lymph Nodules—small unencapsulated
2. Must be returned to the blood to maintain blood masses of lymphatic tissue
1. Found beneath the epithelium of all mucous mem-
volume and blood pressure.
branes, that is, the tracts that have natural openings
Lymph Vessels to the environment.
1. Dead-end lymph capillaries are found in most tis- 2. Destroy pathogens that penetrate the epithelium of
the respiratory, digestive, urinary, or reproductive
sue spaces; collect tissue fluid and proteins (see Fig. tracts.
14–2). 3. Tonsils are the lymph nodules of the pharynx;
2. The structure of larger lymph vessels is like that of Peyer’s patches are those of the small intestine.
veins; valves prevent the backflow of lymph.
3. Lymph is kept moving in lymph vessels by: Spleen—located in the upper left abdominal
• constriction of the lymph vessels quadrant behind the stomach
• the skeletal muscle pump 1. The fetal spleen produces RBCs.
• the respiratory pump 2. Functions after birth:
4. Lymph from the lower body and upper left quad-
rant enters the thoracic duct and is returned to the • contains lymphocytes to be activated and fixed
blood in the left subclavian vein (see Fig. 14–3). plasma cells that produce antibodies
5. Lymph from the upper right quadrant enters the
right lymphatic duct and is returned to the blood in • contains fixed macrophages (RE cells) that
the right subclavian vein.

Copyright © 2007 by F. A. Davis.

The Lymphatic System and Immunity 339

phagocytize pathogens and old RBCs; bilirubin • Complement proteins lyse foreign cells, attract
is formed and sent to the liver for excretion in WBCs, and contribute to inflammation
bile
• stores platelets and destroys damaged platelets • Inflammation—the response to any kind of dam-
age; vasodilation and increased capillary perme-
Thymus—inferior to the thyroid gland; in ability bring tissue fluid and WBCs to the area.
the fetus and infant the thymus is large (see Purpose: to contain the damage, eliminate the
Fig. 14–5); with age the thymus shrinks cause, and make tissue repair possible.
1. Produces T lymphocytes (T cells). Signs: redness, heat, swelling, and pain
2. Produces thymic hormones that make T cells
Adaptive Immunity (see Fig. 14–7)
immunologically competent, that is, able to recog- 1. Is very specific, may involve antibodies, does create
nize foreign antigens and provide immunity.
memory, and responses become more efficient.
Immunity—the ability to destroy foreign Consists of cell-mediated and antibody-mediated
antigens and prevent future cases of certain immunity; is carried out by T cells, B cells, and
infectious diseases macrophages.
1. Antigens are chemical markers that identify cells. 2. T lymphocytes (T cells)—in the embryo are pro-
duced in the thymus and RBM; they require the
Human cells have “self ” antigens—the HLA types. hormones of the thymus for maturation; migrate to
2. Foreign antigens stimulate antibody production the spleen, lymph nodes, and nodules.
3. B lymphocytes (B cells)—in the embryo are pro-
or other immune responses, and include bacteria, duced in the RBM; migrate to the spleen, lymph
viruses, fungi, protozoa, and malignant cells. nodes, and nodules.
4. The antigen must first be recognized as foreign;
Innate Immunity (see Fig. 14–6) this is accomplished by B cells or by helper T cells
1. Is nonspecific, responses are always the same, does that compare the foreign antigen to “self ” antigens
present on macrophages.
not create memory, and does not become more 5. Helper T cells strongly initiate one or both of the
efficient. Consists of barriers, defensive cells, and immune mechanisms: cell-mediated immunity and
chemical defenses. antibody-mediated immunity.
2. Barriers
• Unbroken stratum corneum and sebum; living Cell-Mediated (cellular) Immunity (see Fig.
14–7)
epidermal cells secrete defensins 1. Does not involve antibodies; is effective against
• Subcutaneous tissue with WBCs
• Mucous membranes and areolar CT with WBCs; intracellular pathogens, malignant cells, and grafts
of foreign tissue.
upper respiratory epithelium is ciliated 2. Helper T cells recognize the foreign antigen, are
• HCl in gastric juice antigen specific, and begin to divide to form differ-
• Lysozyme in saliva and tears ent groups of T cells.
3. Defensive cells 3. Memory T cells will remember the specific foreign
• Phagocytes—macrophages, neutrophils, eosino- antigen.
4. Cytotoxic (killer) T cells chemically destroy for-
phils; macrophages also activate the lymphocytes eign cells and produce cytokines to attract macro-
of adaptive immunity phages.
• Langerhans cells and other dendritic cells—acti-
vate lymphocytes Antibody-Mediated (Humoral) Immunity
• Natural killer cells—destroy foreign cells by rup- (see Fig. 14–7)
turing their cell membranes 1. Does involve antibody production; is effective
• Basophils and mast cells—produce histamine and
leukotrienes (inflammation) against pathogens and foreign cells.
4. Chemical defenses
• Interferon blocks viral reproduction

Copyright © 2007 by F. A. Davis.

340 The Lymphatic System and Immunity

2. B cells and helper T cells recognize the foreign 3. Bond to the foreign antigen to label it for phagocy-
antigen; the B cells are antigen specific and begin tosis (opsonization).
to divide.
Antibody Responses and Functions (see Fig.
3. Memory B cells will remember the specific foreign 14–9)
antigen. 1. On the first exposure to a foreign antigen, antibod-

4. Other B cells become plasma cells that produce ies are produced slowly and in small amounts, and
antigen-specific antibodies. the person may develop clinical disease.
2. On the second exposure, the memory cells initiate
5. An antigen–antibody complex is formed, which rapid production of large amounts of antibodies,
attracts macrophages (opsonization). and a second case of the disease may be prevented.
This is the basis for the protection given by vac-
6. Complement fixation is stimulated by antigen– cines, which take the place of the first exposure.
antibody complexes. The complement proteins 3. Antibodies cause agglutination (clumping) of bac-
bind to the antigen–antibody complex and lyse cel- terial cells; clumped cells are easier for macro-
lular antigens or enhance the phagocytosis of non- phages to phagocytize (see Fig. 14–8).
cellular antigens. 4. Antibodies neutralize viruses by bonding to them
and preventing their entry into cells.
Antibodies—immune globulins (Ig) or 5. Antibodies neutralize bacterial toxins by bonding
gamma globulins (see Table 14–1 and to them and changing their shape.
Fig. 14–8)
1. Proteins produced by plasma cells in response to Types of Immunity (see Table 14–2)

foreign antigens.
2. Each antibody is specific for only one foreign anti-

gen.

REVIEW QUESTIONS 8. Innate immunity includes barriers, defensive
cells, and chemicals; give two examples of each.
1. Explain the relationships among plasma, tissue (p. 328)
fluid, and lymph, in terms of movement of water
throughout the body. (p. 322) 9. Explain how a foreign antigen is recognized as
foreign. Which mechanism of adaptive immunity
2. Describe the system of lymph vessels. Explain how involves antibody production? Explain what
lymph is kept moving in these vessels. Into which opsonization means. (pp. 330, 333)
veins is lymph emptied? (p. 322)
10. State the functions of helper T cells, cytotoxic
3. State the locations of the major groups of lymph T cells, and memory T cells. Plasma cells
nodes, and explain their functions. (pp. 322–323) differentiate from which type of lymphocyte?
State the function of plasma cells. What other
4. State the locations of lymph nodules, and explain type of cell comes from B lymphocytes? (pp. 330,
their functions. (pp. 324) 332)

5. Describe the location of the spleen and explain its 11. What is the stimulus for complement fixation?
functions. If the spleen is removed, what organs How does this process destroy cellular antigens
will compensate for its functions? (p. 324) and non-cellular antigens? (pp. 334)

6. Explain the function of the thymus, and state when 12. Explain the antibody reactions of agglutination
(age). this function is most important. (pp. 324, and neutralization. (p. 334)
326)
13. Explain how a vaccine provides protective immu-
7. Name the different kinds of foreign antigens to nity in terms of first and second exposures to a
which the immune system responds, and state three pathogen. (p. 334)
general differences between innate immunity and
adaptive immunity. (p. 327)

Copyright © 2007 by F. A. Davis.

The Lymphatic System and Immunity 341

14. Explain the difference between the following: (pp. c. Natural and artificial passive acquired immunity
336–337) d. Natural and artificial active acquired immunity
a. Genetic immunity and acquired immunity
b. Passive acquired immunity and active acquired
immunity

FOR FURTHER THOUGHT is to expose the individual to the pathogen, what
does this tell you about the polio viruses (there are
1. Bubonic plague, also called black plague, is a seri- three) and their usual site of infection?
ous disease caused by a bacterium and spread by
fleas from rats or rodents to people. It got its 4. Everyone should have a tetanus booster shot every
“black” name from “buboes,” dark swellings found 10 years. That is what we often call a “tetanus
in the groin or armpit of people with plague. shot.” Someone who sustains a soil-contaminated
Explain what buboes are, and why they were usu- injury should also receive a tetanus booster (if none
ally found in the groin and armpit. in the past 10 years). But someone who has symp-
toms of tetanus should get TIG, tetanus immune
2. In Rh disease of the newborn, maternal antibodies globulin. Explain the difference, and why TIG is so
enter fetal circulation and destroy the red blood important.
cells of the fetus. A mother with type O blood has
anti-A and anti-B antibodies, but may have a dozen 5. People with AIDS are susceptible to many other
type A children without any problem at all. Explain diseases. Which of these would be least likely:
why. (Look at Table 14–1 and Fig. 14–8.) pneumonia, rheumatoid arthritis, yeast infection of
the mouth, or protozoan infection of the intes-
3. Most vaccines are given by injection. The oral tines? Explain your answer.
polio vaccine (OPV), however, is not; it is given by
mouth. Remembering that the purpose of a vaccine

Copyright © 2007 by F. A. Davis.

Chapter Outline CHAPTER 15

Divisions of the Respiratory System Student Objectives
Nose and Nasal Cavities • State the general function of the respiratory sys-
Pharynx
Larynx tem.
Trachea and Bronchial Tree
Lungs and Pleural Membranes • Describe the structure and functions of the nasal

Alveoli cavities and pharynx.
Mechanism of Breathing
Inhalation • Describe the structure of the larynx and explain
Exhalation
Pulmonary Volumes the speaking mechanism.
Exchange of Gases
Diffusion of Gases—Partial Pressures • Describe the structure and functions of the trachea
Transport of Gases in the Blood
Regulation of Respiration and bronchial tree.
Nervous Regulation
Chemical Regulation • State the locations of the pleural membranes, and
Respiration and Acid–Base Balance
Respiratory Acidosis and Alkalosis explain the functions of serous fluid.
Respiratory Compensation
Aging and the Respiratory System • Describe the structure of the alveoli and pul-

BOX 15–1 ASTHMA monary capillaries, and explain the importance of
BOX 15–2 HYALINE MEMBRANE DISEASE surfactant.
BOX 15–3 PNEUMOTHORAX
BOX 15–4 EMPHYSEMA • Name and describe the important air pressures
BOX 15–5 THE HEIMLICH MANEUVER
BOX 15–6 PULMONARY EDEMA involved in breathing.
BOX 15–7 PNEUMONIA
BOX 15–8 CARBON MONOXIDE • Describe normal inhalation and exhalation and

forced exhalation.

• Name the pulmonary volumes and define each.
• Explain the diffusion of gases in external respira-

tion and internal respiration.

• Describe how oxygen and carbon dioxide are

transported in the blood.

• Explain the nervous and chemical mechanisms

that regulate respiration.

• Explain how respiration affects the pH of body

fluids.

342

Copyright © 2007 by F. A. Davis.

The Respiratory System

New Terminology Related Clinical Terminology

Alveoli (al-VEE-oh-lye) Cyanosis (SIGH-uh-NOH-sis)
Bronchial tree (BRONG-kee-uhl TREE) Dyspnea (DISP-nee-ah)
Epiglottis (ep-i-GLAH-tis) Emphysema (EM-fi-SEE-mah)
Glottis (GLAH-tis) Heimlich maneuver (HIGHM-lik ma-NEW-ver)
Intrapleural pressure (IN-trah-PLOOR-uhl PRES- Hyaline membrane disease (HIGH-e-lin MEM-

shur) brain dis-EEZ)
Intrapulmonic pressure (IN-trah-pull-MAHN-ik Pneumonia (new-MOH-nee-ah)
Pneumothorax (NEW-moh-THAW-raks)
PRES-shur) Pulmonary edema (PULL-muh-ner-ee
Larynx (LA-rinks)
Partial pressure (PAR-shul PRES-shur) uh-DEE-muh).
Phrenic nerves (FREN-ik NURVZ)
Pulmonary surfactant (PULL-muh-ner-ee sir-FAK-

tent)
Residual air (ree-ZID-yoo-al AYRE)
Respiratory acidosis (RES-pi-rah-TOR-ee ass-i-

DOH-sis)
Respiratory alkalosis (RES-pi-rah-TOR-ee al-kah-

LOH-sis)
Soft palate (SAWFT PAL-uht)
Tidal volume (TIGH-duhl VAHL-yoom)
Ventilation (VEN-ti-LAY-shun)
Vital capacity (VY-tuhl kuh-PASS-i-tee)

Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.

343

Copyright © 2007 by F. A. Davis.

344 The Respiratory System

Sometimes a person will describe a habit as being with skin. Just inside the nostrils are hairs, which help
block the entry of dust.
“as natural as breathing.” Indeed, what could be more
natural? We rarely think about breathing, and it isn’t The two nasal cavities are within the skull, sepa-
something we look forward to, as we might look for- rated by the nasal septum, which is a bony plate made
ward to a good dinner. We just breathe, usually at the of the ethmoid bone and vomer. The nasal mucosa
rate of 12 to 20 times per minute, and faster when (lining) is ciliated epithelium, with goblet cells that
necessary (such as during exercise). You may have produce mucus. Three shelf-like or scroll-like bones
heard of trained singers “learning how to breathe,” called conchae project from the lateral wall of each
but they are really learning how to make their breath- nasal cavity (Figs. 15–1 and 6–6). Just as shelves in a
ing more efficient. cabinet provide more flat space for storage, the con-
chae increase the surface area of the nasal mucosa. As
Most of the respiratory system is concerned with air passes through the nasal cavities it is warmed and
what we think of as breathing: moving air into and out humidified, so that air that reaches the lungs is warm
of the lungs. The lungs are the site of the exchanges of and moist. Bacteria and particles of air pollution are
oxygen and carbon dioxide between the air and the trapped on the mucus; the cilia continuously sweep
blood. Both of these exchanges are important. All the mucus toward the pharynx. Most of this mucus is
of our cells must obtain oxygen to carry out cell respi- eventually swallowed, and most bacteria present will
ration to produce ATP. Just as crucial is the elimina- be destroyed by the hydrochloric acid in the gastric
tion of the CO2 produced as a waste product of cell juice.
respiration, and, as you already know, the proper func-
tioning of the circulatory system is essential for the In the upper nasal cavities are the olfactory recep-
transport of these gases in the blood. tors, which detect vaporized chemicals that have been
inhaled. The olfactory nerves pass through the eth-
DIVISIONS OF THE moid bone to the brain.
RESPIRATORY SYSTEM
You may also recall our earlier discussion of the
The respiratory system may be divided into the upper paranasal sinuses, air cavities in the maxillae, frontal,
respiratory tract and the lower respiratory tract. The sphenoid, and ethmoid bones (see Figs. 15–1 and 6–9).
upper respiratory tract consists of the parts outside These sinuses are lined with ciliated epithelium, and
the chest cavity: the air passages of the nose, nasal cav- the mucus produced drains into the nasal cavities. The
ities, pharynx, larynx, and upper trachea. The lower functions of the paranasal sinuses are to lighten the
respiratory tract consists of the parts found within skull and provide resonance (more vibrating air) for
the chest cavity: the lower trachea and the lungs them- the voice.
selves, which include the bronchial tubes and alveoli.
Also part of the respiratory system are the pleural PHARYNX
membranes and the respiratory muscles that form the
chest cavity: the diaphragm and intercostal muscles. The pharynx is a muscular tube posterior to the nasal
and oral cavities and anterior to the cervical vertebrae.
Have you recognized some familiar organs and For descriptive purposes, the pharynx may be divided
structures thus far? There will be more, because this into three parts: the nasopharynx, oropharynx, and
chapter includes material from all of the previous laryngopharynx (see Fig. 15–1).
chapters. Even though we are discussing the body
system by system, the respiratory system is an excel- The uppermost portion is the nasopharynx, which
lent example of the interdependent functioning of all is behind the nasal cavities. The soft palate is elevated
the body systems. during swallowing to block the nasopharynx and pre-
vent food or saliva from going up rather than down.
NOSE AND NASAL CAVITIES The uvula is the part of the soft palate you can see at
the back of the throat. On the posterior wall of the
Air enters and leaves the respiratory system through nasopharynx is the adenoid or pharyngeal tonsil, a
the nose, which is made of bone and cartilage covered lymph nodule that contains macrophages. Opening
into the nasopharynx are the two eustachian tubes,
which extend to the middle ear cavities. The purpose

Copyright © 2007 by F. A. Davis.

The Respiratory System 345

Frontal sinus

Conchae Superior Ethmoid bone Olfactory receptors
Middle Sphenoid sinus
Inferior Opening of
eustachian tube

Pharyngeal tonsil

Nostril

Hard Maxilla Nasopharynx
palate
Palatine Soft palate
bone
Uvula
Palatine tonsil
Oropharynx
Lingual tonsil

Epiglottis

Laryngopharynx

Hyoid bone Esophagus
Larynx

Thyroid
cartilage

Cricoid
cartilage

Trachea

Figure 15–1. Midsagittal section of the head and neck showing the structures of the
upper respiratory tract.
QUESTION: Describe the shape of the conchae by using a familiar object. What is the func-
tion of the conchae?

of the eustachian tubes is to permit air to enter or food passageway, although not for both at the same
leave the middle ears, allowing the eardrums to vibrate time. The oropharynx is behind the mouth; its
properly. mucosa is stratified squamous epithelium, continuous
with that of the oral cavity. On its lateral walls are the
The nasopharynx is a passageway for air only, but palatine tonsils, also lymph nodules. Together with
the remainder of the pharynx serves as both an air and

Copyright © 2007 by F. A. Davis.

346 The Respiratory System

the adenoid and the lingual tonsils on the base of the like a trap door or hinged lid, to prevent the entry of
tongue, they form a ring of lymphatic tissue around saliva or food into the larynx.
the pharynx to destroy pathogens that penetrate the
mucosa. The mucosa of the larynx is ciliated epithelium,
except for the vocal cords (stratified squamous epithe-
The laryngopharynx is the most inferior portion lium). The cilia of the mucosa sweep upward to
of the pharynx. It opens anteriorly into the larynx and remove mucus and trapped dust and microorganisms.
posteriorly into the esophagus. Contraction of the
muscular wall of the oropharynx and laryngopharynx The vocal cords (or vocal folds) are on either side
is part of the swallowing reflex. of the glottis, the opening between them. During
breathing, the vocal cords are held at the sides of the
LARYNX glottis, so that air passes freely into and out of the tra-
chea (Fig. 15–3). During speaking, the intrinsic mus-
The larynx is often called the voice box, a name that cles of the larynx pull the vocal cords across the glottis,
indicates one of its functions, which is speaking. The and exhaled air vibrates the vocal cords to produce
other function of the larynx is to be an air passageway sounds that can be turned into speech. It is also phys-
between the pharynx and the trachea. Air passages ically possible to speak while inhaling, but this is not
must be kept open at all times, and so the larynx is what we are used to. The cranial nerves that are motor
made of nine pieces of cartilage connected by liga- nerves to the larynx for speaking are the vagus and
ments. Cartilage is a firm yet flexible tissue that pre- accessory nerves. You may also recall that for most
vents collapse of the larynx. In comparison, the people, the speech areas are in the left cerebral hemi-
esophagus is a collapsed tube except when food is pass- sphere.
ing through it.
TRACHEA AND BRONCHIAL TREE
The largest cartilage of the larynx is the thyroid
cartilage (Fig. 15–2), which you can feel on the ante- The trachea is about 4 to 5 inches (10 to 13 cm) long
rior surface of your neck. The epiglottis is the upper- and extends from the larynx to the primary bronchi.
most cartilage. During swallowing, the larynx is The wall of the trachea contains 16 to 20 C-shaped
elevated, and the epiglottis closes over the top, rather pieces of cartilage, which keep the trachea open. The
gaps in these incomplete cartilage rings are posterior,
Epiglottis to permit the expansion of the esophagus when food is
Hyoid swallowed. The mucosa of the trachea is ciliated
bone epithelium with goblet cells. As in the larynx, the cilia
sweep upward toward the pharynx.
Thyroid
cartilage The right and left primary bronchi (Fig. 15–4) are
the branches of the trachea that enter the lungs. Their
Vocal structure is just like that of the trachea, with C-shaped
cords cartilages and ciliated epithelium. Within the lungs,
each primary bronchus branches into secondary
Cricoid bronchi leading to the lobes of each lung (three right,
cartilage two left). The further branching of the bronchial tubes
is often called the bronchial tree. Imagine the trachea
Tracheal as the trunk of an upside-down tree with extensive
cartilages branches that become smaller and smaller; these
smaller branches are the bronchioles. No cartilage is
AB present in the walls of the bronchioles; this becomes
clinically important in asthma (see Box 15–1: Asthma).
Figure 15–2. Larynx. (A) Anterior view. (B) Midsagittal The smallest bronchioles terminate in clusters of alve-
section through the larynx, viewed from the left side. oli, the air sacs of the lungs.
QUESTION: What is the function of the epiglottis?
LUNGS AND PLEURAL MEMBRANES

The lungs are located on either side of the heart in
the chest cavity and are encircled and protected by the

Copyright © 2007 by F. A. Davis.

The Respiratory System 347

Epiglottis
Vocal cord

Trachea

Glottis

AB

Figure 15–3. Vocal cords and glottis, superior view. (A) Position of the vocal cords dur-
ing breathing. (B) Position of the vocal cords during speaking.
QUESTION: What makes the vocal cords vibrate?

rib cage. The base of each lung rests on the diaphragm monary capillaries, which permits efficient diffusion of
below; the apex (superior tip) is at the level of the clav- gases (Fig. 15–5).
icle. On the medial surface of each lung is an indenta-
tion called the hilus, where the primary bronchus and Each alveolus is lined with a thin layer of tissue
the pulmonary artery and veins enter the lung. fluid, which is essential for the diffusion of gases, be-
cause a gas must dissolve in a liquid in order to enter
The pleural membranes are the serous membranes or leave a cell (the earthworm principle—an earth-
of the thoracic cavity. The parietal pleura lines the worm breathes through its moist skin, and will suffo-
chest wall, and the visceral pleura is on the surface of cate if its skin dries out). Although this tissue fluid is
the lungs. Between the pleural membranes is serous necessary, it creates a potential problem in that it
fluid, which prevents friction and keeps the two mem- would make the walls of an alveolus stick together
branes together during breathing. internally. Imagine a plastic bag that is wet inside; its
walls would stick together because of the surface ten-
Alveoli sion of the water. This is just what would happen in
alveoli, and inflation would be very difficult.
The functional units of the lungs are the air sacs called
alveoli. The flat alveolar type I cells that form most of This problem is overcome by pulmonary surfac-
the alveolar walls are simple squamous epithelium. In tant, a lipoprotein secreted by alveolar type II cells,
the spaces between clusters of alveoli is elastic con- also called septal cells. Surfactant mixes with the tissue
nective tissue, which is important for exhalation. fluid within the alveoli and decreases its surface ten-
Within the alveoli are macrophages that phagocytize sion, permitting inflation of the alveoli (see Box 15–2:
pathogens or other foreign material that may not have Hyaline Membrane Disease). Normal inflation of the
been swept out by the ciliated epithelium of the alveoli in turn permits the exchange of gases, but
bronchial tree. There are millions of alveoli in each before we discuss this process, we will first see how air
lung, and their total surface area is estimated to be 700 gets into and out of the lungs.
to 800 square feet (picture a sidewalk two and a half
feet wide that is as long as an American football field, MECHANISM OF BREATHING
or a rectangle 25 feet by 30 feet). Each alveolus is sur-
rounded by a network of pulmonary capillaries (see Ventilation is the term for the movement of air to and
Fig 15–4). Recall that capillaries are also made of sim- from the alveoli. The two aspects of ventilation are
ple squamous epithelium, so there are only two cells inhalation and exhalation, which are brought about by
between the air in the alveoli and the blood in the pul-

Copyright © 2007 by F. A. Davis. Arteriole
Pulmonary capillaries
348 The Respiratory System Alveolar duct

Frontal sinuses Alveolus
Venule
Sphenoidal sinuses
Nasal cavity B

Nasopharynx Left lung
Left primary bronchus
Soft palate Superior lobe
Epiglottis

Larynx and vocal folds
Trachea

Superior lobe
Right lung
Right primary
bronchus

Middle lobe

Mediastinum Bronchioles
Inferior lobe
Inferior lobe Cardiac notch
Pleural membranes
Diaphragm Pleural space

A

Figure 15–4. Respiratory system. (A) Anterior view of the upper and lower respiratory
tracts. (B) Microscopic view of alveoli and pulmonary capillaries. (The colors represent the
vessels, not the oxygen content of the blood within the vessels.)
QUESTION: What are the first branches of the trachea, and how do they resemble the tra-
chea in structure?

the nervous system and the respiratory muscles. The when it contracts, the diaphragm flattens and moves
respiratory centers are located in the medulla and downward. The intercostal muscles are found between
pons. Their specific functions will be covered in a later the ribs. The external intercostal muscles pull the
section, but it is the medulla that generates impulses ribs upward and outward, and the internal inter-
to the respiratory muscles. costal muscles pull the ribs downward and inward.
Ventilation is the result of the respiratory muscles pro-
These muscles are the diaphragm and the external ducing changes in the pressure within the alveoli and
and internal intercostal muscles (Fig. 15–6). The bronchial tree.
diaphragm is a dome-shaped muscle below the lungs;

Copyright © 2007 by F. A. Davis.

The Respiratory System 349

BOX 15–1 ASTHMA emphysema. When obstructed bronchioles prevent
ventilation of alveoli, the walls of the alveoli begin
Asthma is usually triggered by an infection or aller- to deteriorate and break down, leaving large cavi-
gic reaction that affects the smooth muscle and ties that do not provide much surface area for gas
glands of the bronchioles. Allergens include foods exchange.
and inhaled substances such as dust and pollen.
Wheezing and dyspnea (difficult breathing) charac- One possible way to prevent such serious lung
terize an asthma attack, which may range from damage is to prevent asthma attacks with a med-
mild to fatal. ication that blocks the release of IgE antibodies. An
allergy is an immune overreaction, and blocking
As part of the allergic response, the smooth mus- such a reaction would prevent the damaging effects
cle of the bronchioles constricts. Because there is no of inflammation. In the United States the incidence
cartilage present in their walls, the bronchioles may of asthma is increasing among children; this may be
close completely. The secretion of mucus increases, a result of higher levels of air pollution, though
perhaps markedly, so the already constricted bron- genetic and immunologic factors may contribute as
chioles may become clogged or completely well.
obstructed with mucus.

Chronic asthma is a predisposing factor for

With respect to breathing, three types of pressure diaphragm contracts, moves downward, and expands
are important: the chest cavity from top to bottom. The external
intercostal muscles pull the ribs up and out, which
1. Atmospheric pressure—the pressure of the air expands the chest cavity from side to side and front to
around us. At sea level, atmospheric pressure is 760 back.
mmHg. At higher altitudes, of course, atmospheric
pressure is lower. As the chest cavity is expanded, the parietal pleura
expands with it. Intrapleural pressure becomes even
2. Intrapleural pressure—the pressure within the more negative as a sort of suction is created between
potential pleural space between the parietal pleura the pleural membranes. The adhesion created by the
and visceral pleura. This is a potential rather than a serous fluid, however, permits the visceral pleura to be
real space. A thin layer of serous fluid causes the expanded too, and this expands the lungs as well.
two pleural membranes to adhere to one another.
Intrapleural pressure is always slightly below As the lungs expand, intrapulmonic pressure falls
atmospheric pressure (about 756 mmHg), and is below atmospheric pressure, and air enters the nose
called a negative pressure. The elastic lungs are and travels through the respiratory passages to the
always tending to collapse and pull the visceral alveoli. Entry of air continues until intrapulmonic
pleura away from the parietal pleura. The serous pressure is equal to atmospheric pressure; this is a nor-
fluid, however, prevents actual separation of the mal inhalation. Of course, inhalation can be continued
pleural membranes (see Box 15–3: Pneumothorax). beyond normal, that is, a deep breath. This requires a
more forceful contraction of the respiratory muscles
3. Intrapulmonic pressure—the pressure within the to further expand the lungs, permitting the entry of
bronchial tree and alveoli. This pressure fluctuates more air.
below and above atmospheric pressure during each
cycle of breathing. EXHALATION

INHALATION Exhalation may also be called expiration and begins
when motor impulses from the medulla decrease and
Inhalation, also called inspiration, is a precise the diaphragm and external intercostal muscles relax.
sequence of events that may be described as follows: As the chest cavity becomes smaller, the lungs are
Motor impulses from the medulla travel along the compressed, and their elastic connective tissue, which
phrenic nerves to the diaphragm and along the inter- was stretched during inhalation, recoils and also com-
costal nerves to the external intercostal muscles. The

Copyright © 2007 by F. A. Davis.

350 The Respiratory System

A Exhalation Respiration
Inhalation
Red blood cells
Alveolus
Capillary Interstitial
space

Type II Oxygen (O2)
surfactant Carbon dioxide
(CO2)
cell
Macrophage

Type I cell

B

Surfactant Primary
and tissue fluid bronchi

Alveolar epithelium Elastin fibers
Respiratory
Capillary endothelium membrane

Basement membrane
of capillary endothelium

Figure 15–5. (A) Alveolar structure showing type I and type II cells, and alveolar
macrophages. The respiratory membrane: the structures and substances through which
gases must pass as they diffuse from air to blood (oxygen) or from blood to air (CO2).
(B) Sections of human lungs embedded in plastic. On the left is a normal adult lung; on
the right is a smoker’s lung. (Photograph by Dan Kaufman.)
QUESTION: Which cells shown here are part of the respiratory membrane? Which cells are
not, and what are their functions?

presses the alveoli. As intrapulmonic pressure rises a passive process, depending to a great extent on the
above atmospheric pressure, air is forced out of the normal elasticity of healthy lungs. In other words,
lungs until the two pressures are again equal. under normal circumstances we must expend energy
to inhale but not to exhale (see Box 15–4: Emphy-
Notice that inhalation is an active process that sema).
requires muscle contraction, but normal exhalation is

Copyright © 2007 by F. A. Davis.

The Respiratory System 351

BOX 15–2 HYALINE MEMBRANE DISEASE lapse after each breath rather than remain inflated.
Each breath, therefore, is difficult, and the newborn
Hyaline membrane disease is also called respira- must expend a great deal of energy just to breathe.
tory distress syndrome (RDS) of the newborn, and
most often affects premature infants whose lungs Premature infants may require respiratory assis-
have not yet produced sufficient quantities of pul- tance until their lungs are mature enough to pro-
monary surfactant. duce surfactant. Use of a synthetic surfactant has
significantly helped some infants, and because they
The first few breaths of a newborn inflate most of can breathe more normally, their dependence on
the previously collapsed lungs, and the presence of respirators is minimized. Still undergoing evaluation
surfactant permits the alveoli to remain open. The are the effects of the long-term use of this surfac-
following breaths become much easier, and normal tant in the most premature babies, who may
breathing is established. require it for much longer periods of time.

Without surfactant, the surface tension of the tis-
sue fluid lining the alveoli causes the air sacs to col-

We can, however, go beyond a normal exhalation PULMONARY VOLUMES
and expel more air, such as when talking, singing, or
blowing up a balloon. Such a forced exhalation is an The capacity of the lungs varies with the size and age
active process that requires contraction of other mus- of the person. Taller people have larger lungs than do
cles. Contraction of the internal intercostal muscles shorter people. Also, as we get older our lung capacity
pulls the ribs down and in and squeezes even more air diminishes as lungs lose their elasticity and the respi-
out of the lungs. Contraction of abdominal muscles, ratory muscles become less efficient. For the following
such as the rectus abdominis, compresses the abdomi- pulmonary volumes, the values given are those for
nal organs and pushes the diaphragm upward, which healthy young adults. These are also shown in Fig.
also forces more air out of the lungs (see Box 15–5: 15–7.
The Heimlich Maneuver).

Inhalation Exhalation
Trachea
Ribs
Lung
Ribs

Figure 15–6. Actions of the res- External
piratory muscles. (A) Inhalation: intercostal
diaphragm contracts downward; muscles
external intercostal muscles pull rib
cage upward and outward; lungs Sternum
are expanded. (B) Normal exhala-
tion: diaphragm relaxes upward;
rib cage falls down and in as exter-
nal intercostal muscles relax; lungs
are compressed.
QUESTION: Why is a normal exha-
lation a passive process?

Diaphragm
AB

Copyright © 2007 by F. A. Davis.

BOX 15–3 PNEUMOTHORAX

Pneumothorax is the presence of air in the pleural trauma, may result from rupture of weakened alve-
space, which causes collapse of the lung on that oli on the lung surface. Pulmonary diseases such as
side. Recall that the pleural space is only a potential emphysema may weaken alveoli.
space because the serous fluid keeps the pleural
membranes adhering to one another, and the Puncture wounds of the chest wall also allow air
intrapleural pressure is always slightly below atmos- into the pleural space, with resulting collapse of a
pheric pressure. Should air at atmospheric pressure lung. In severe cases, large amounts of air push the
enter the pleural cavity, the suddenly higher pres- heart, great vessels, trachea, and esophagus toward
sure outside the lung will contribute to its collapse the opposite side (mediastinal shift), putting pres-
(the other factor is the normal elasticity of the sure on the other lung and making breathing diffi-
lungs). cult. This is called tension pneumothorax, and
requires rapid medical intervention to remove the
A spontaneous pneumothorax, without apparent trapped air.

BOX 15–4 EMPHYSEMA In progressive emphysema, damaged lung tissue
is replaced by fibrous connective tissue (scar tissue),
Emphysema, a form of chronic obstructive pul- which further limits the diffusion of gases. Blood
monary disease (COPD), is a degenerative disease oxygen level decreases, and blood carbon dioxide
in which the alveoli lose their elasticity and cannot level increases. Accumulating carbon dioxide
recoil. Perhaps the most common (and avoidable) decreases the pH of body fluids; this is a respiratory
cause is cigarette smoking; other causes are long- acidosis.
term exposure to severe air pollution or industrial
dusts, or chronic asthma. Inhaled irritants damage One of the most characteristic signs of emphy-
the alveolar walls and cause deterioration of the sema is that the affected person must make an
elastic connective tissue surrounding the alveoli. effort to exhale. The loss of lung elasticity makes
Macrophages migrate to the damaged areas and normal exhalation an active process, rather than the
seem to produce an enzyme that contributes to the passive process it usually is. The person must
destruction of the protein elastin. This is an instance expend energy to exhale in order to make room in
of a useful body response (for cleaning up damaged the lungs for inhaled air. This extra “work” required
tissue) becoming damaging when it is excessive. As for exhalation may be exhausting for the person
the alveoli break down, larger air cavities are cre- and contribute to the debilitating nature of emphy-
ated that are not efficient in gas exchange (see Box sema.
Fig. 15–A).

A Normal Lung B Emphysema

Box Figure 15–A (A) Lung tissue with normal alveoli. (B) Lung tissue in emphysema.

352

Copyright © 2007 by F. A. Davis.

The Respiratory System 353

BOX 15–5 THE HEIMLICH MANEUVER Foreign object

The Heimlich maneuver has received much well- Lung
deserved publicity, and indeed it is a life-saving Diaphragm
technique.

If a person is choking on a foreign object (such
as food) lodged in the pharynx or larynx, the air in
the lungs may be utilized to remove the object. The
physiology of this technique is illustrated in the
accompanying figure.

The person performing the maneuver stands
behind the choking victim and puts both arms
around the victim’s waist. One hand forms a fist
that is placed between the victim’s navel and rib
cage (below the diaphragm), and the other hand
covers the fist. It is important to place hands cor-
rectly, in order to avoid breaking the victim’s ribs.
With both hands, a quick, forceful upward thrust is
made and repeated if necessary. This forces the
diaphragm upward to compress the lungs and force
air out. The forcefully expelled air is often sufficient
to dislodge the foreign object.

Box Figure 15–B The Heimlich maneuver.

1. Tidal volume—the amount of air involved in one tory reserve, and expiratory reserve. Stated another
normal inhalation and exhalation. The average tidal way, vital capacity is the amount of air involved in
volume is 500 mL, but many people often have the deepest inhalation followed by the most force-
lower tidal volumes because of shallow breathing. ful exhalation. Average range of vital capacity is
3500 to 5000 mL.
2. Minute respiratory volume (MRV)—the amount 6. Residual air—the amount of air that remains in
of air inhaled and exhaled in 1 minute. MRV is cal- the lungs after the most forceful exhalation; the
culated by multiplying tidal volume by the number average range is 1000 to 1500 mL. Residual air is
of respirations per minute (average range: 12 to 20 important to ensure that there is some air in the
per minute). If tidal volume is 500 mL and the res- lungs at all times, so that exchange of gases is a con-
piratory rate is 12 breaths per minute, the MRV is tinuous process, even between breaths.
6000 mL, or 6 liters of air per minute, which is
average. Shallow breathing usually indicates a Some of the volumes just described can be deter-
smaller than average tidal volume, and would thus mined with instruments called spirometers, which
require more respirations per minute to obtain the measure movement of air. Trained singers and musi-
necessary MRV. cians who play wind instruments often have vital
capacities much larger than would be expected for
3. Inspiratory reserve—the amount of air, beyond their height and age, because their respiratory muscles
tidal volume, that can be taken in with the deepest have become more efficient with “practice.” The same
possible inhalation. Normal inspiratory reserve is true for athletes who exercise regularly. A person
ranges from 2000 to 3000 mL. with emphysema, however, must “work” to exhale,
and vital capacity and expiratory reserve volume are
4. Expiratory reserve—the amount of air, beyond often much lower than average.
tidal volume, that can be expelled with the most
forceful exhalation. Normal expiratory reserve Another kind of pulmonary volume is alveolar
ranges from 1000 to 1500 mL.

5. Vital capacity—the sum of tidal volume, inspira-

Copyright © 2007 by F. A. Davis.

354 The Respiratory System

Liters
6

5 Figure 15–7. Pulmonary
volumes. See text for descrip-
Inspiratory tion.
reserve QUESTION: Which volumes
make up vital capacity? Which
4 volume cannot be measured
with a spirometer?
Vital
capacity

Total Tidal volume 3
lung (normal breath) 2.5
capacity

2

Expiratory
reserve

1
Residual volume

0

Time

ventilation, which is the amount of air that actually exchange of gases between the air in the alveoli and
reaches the alveoli and participates in gas exchange. the blood in the pulmonary capillaries is called exter-
An average tidal volume is 500 mL, of which 350 to nal respiration. This term may be a bit confusing at
400 mL is in the alveoli at the end of an inhalation. first, because we often think of “external” as being
The remaining 100 to 150 mL of air is anatomic dead outside the body. In this case, however, “external”
space, the air still within the respiratory passages. means the exchange that involves air from the external
Despite the rather grim name, anatomic dead space is environment, though the exchange takes place within
normal; everyone has it. the lungs. Internal respiration is the exchange of
gases between the blood in the systemic capillaries and
Physiological dead space is not normal, and is the the tissue fluid (cells) of the body.
volume of non-functioning alveoli that decrease gas
exchange. Causes of increased physiological dead space The air we inhale (the earth’s atmosphere) is
include bronchitis, pneumonia, tuberculosis, emphy- approximately 21% oxygen and 0.04% carbon diox-
sema, asthma, pulmonary edema, and a collapsed lung. ide. Although most (78%) of the atmosphere is nitro-
gen, this gas is not physiologically available to us, and
The compliance of the thoracic wall and the lungs, we simply exhale it. This exhaled air also contains
that is, their normal expansibility, is necessary for suf- about 16% oxygen and 4.5% carbon dioxide, so it is
ficient alveolar ventilation. Thoracic compliance may apparent that some oxygen is retained within the body
be decreased by fractured ribs, scoliosis, pleurisy, or and the carbon dioxide produced by cells is exhaled.
ascites. Lung compliance will be decreased by any
condition that increases physiologic dead space. Nor- DIFFUSION OF GASES—
mal compliance thus promotes sufficient gas exchange PARTIAL PRESSURES
in the alveoli.
Within the body, a gas will diffuse from an area of
EXCHANGE OF GASES greater concentration to an area of lesser concentra-
tion. The concentration of each gas in a particular site
There are two sites of exchange of oxygen and carbon (alveolar air, pulmonary blood, and so on) is expressed
dioxide: the lungs and the tissues of the body. The in a value called partial pressure. The partial pressure

Copyright © 2007 by F. A. Davis.

The Respiratory System 355

Table 15–1 PARTIAL PRESSURES AND OXYGEN SATURATION

Site PO2 (mmHg) PCO2 (mmHg) Hemoglobin Saturation (SaO2)
160 0.15 —
Atmosphere 104 —
Alveolar air 40 40
Systemic venous blood 45 70–75%
100
(to pulmonary arteries) 40 95–100%
Systemic arterial blood 40
50 —
(from pulmonary veins)
Tissue fluid

Partial pressure is calculated as follows:

% of the gas in the mixture ϫ total pressure ϭ PGAS

Example: O2 in the atmosphere
21% ϫ 760 mmHg ϭ 160 mmHg (PO2)

Example: CO2 in the atmosphere
0.04% ϫ 760 mmHg ϭ 0.15 mmHg (PCO2)

Notice that alveolar partial pressures are not exactly those of the atmosphere. Alveolar air contains significant amounts of

water vapor and the CO2 diffusing in from the blood. Oxygen also diffuses readily from the alveoli into the pulmonary capillaries.
Therefore, alveolar PO2 is lower than atmospheric PO2, and alveolar PCO2 is significantly higher than atmospheric PCO2.

of a gas, measured in mmHg, is the pressure it exerts sue fluid have a low PO2 and a high PCO2 because cells
within a mixture of gases, whether the mixture is actu- continuously use oxygen in cell respiration (energy
ally in a gaseous state or is in a liquid such as blood. production) and produce carbon dioxide in this
The partial pressures of oxygen and carbon dioxide in process. Therefore, in internal respiration, oxygen dif-
the atmosphere and in the sites of exchange in the fuses from the blood to tissue fluid (cells), and carbon
body are listed in Table 15–1. The abbreviation for dioxide diffuses from tissue fluid to the blood. The
partial pressure is “P,” which is used, for example, on blood that enters systemic veins to return to the heart
hospital lab slips for blood gases and will be used here. now has a low PO2 and a high PCO2 and is pumped by
the right ventricle to the lungs to participate in exter-
The partial pressures of oxygen and carbon dioxide nal respiration.
at the sites of external respiration (lungs) and internal
respiration (body) are shown in Fig. 15–8. Because Disorders of gas exchange often involve the lungs,
partial pressure reflects concentration, a gas will dif- that is, external respiration (see Box 15–6: Pulmonary
fuse from an area of higher partial pressure to an area Edema and Box 15–7: Pneumonia).
of lower partial pressure.
TRANSPORT OF GASES
The air in the alveoli has a high PO2 and a low PCO2. IN THE BLOOD
The blood in the pulmonary capillaries, which has just
come from the body, has a low PO2 and a high PCO2. Although some oxygen is dissolved in blood plasma
Therefore, in external respiration, oxygen diffuses and does create the PO2 values, it is only about 1.5%
from the air in the alveoli to the blood, and carbon of the total oxygen transported, not enough to sustain
dioxide diffuses from the blood to the air in the alveoli. life. As you already know, most oxygen is carried in the
The blood that returns to the heart now has a high PO2 blood bonded to the hemoglobin in red blood cells
and a low PCO2 and is pumped by the left ventricle into (RBCs). The mineral iron is part of hemoglobin and
systemic circulation. gives this protein its oxygen-carrying ability.

The arterial blood that reaches systemic capillaries
has a high PO2 and a low PCO2. The body cells and tis-

Copyright © 2007 by F. A. Davis.

356 The Respiratory System

Alveoli

Po2 105
Pco2 40

Pulmonary CtoO2 Pulmonary O2 to Pulmonary
artery alveoli capillaries blood veins
Po2 40 External
respiration Po2 100
Pco2 45 Pco2 40

Right Left Figure 15–8. External respi-
heart heart ration in the lungs and internal
respiration in the body. The
Venae Aorta partial pressures of oxygen and
cavae carbon dioxide are shown at
each site.
QUESTION: In external respi-
ration, describe the movement
of oxygen. In internal respira-
tion, describe the movement
of carbon dioxide.

Veins Internal Arteries
respiration
Po2 40 Systemic Po2 100
Pco2 45 capillaries Pco2 40

CO2 Po2 40 O2 to tissue
to blood Pco2 50

The oxygen–hemoglobin bond is formed in the Another measure of blood oxygen is the percent of
lungs where PO2 is high. This bond, however, is rela- oxygen saturation of hemoglobin (SaO2). The higher
tively unstable, and when blood passes through tissues the PO2, the higher the SaO2, and as PO2 decreases, so
with a low PO2, the bond breaks, and oxygen is does SaO2, though not as rapidly. A PO2 of 100 is an
released to the tissues. The lower the oxygen concen- SaO2 of about 97% , as is found in systemic arteries. A
tration in a tissue, the more oxygen the hemoglobin PO2 of 40, as is found in systemic veins, is an SaO2 of
will release. This ensures that active tissues, such as about 75%. Notice that venous blood still has quite a
exercising muscles, receive as much oxygen as possible bit of oxygen. Had this blood flowed through a very
to continue cell respiration. Other factors that increase active tissue, more of its oxygen would have been
the release of oxygen from hemoglobin are a high released from hemoglobin. This venous reserve of
PCO2 (actually a lower pH) and a high temperature, oxygen provides active tissues with the oxygen they
both of which are also characteristic of active tissues. need (see also Box 15–8: Carbon Monoxide).

Copyright © 2007 by F. A. Davis.

The Respiratory System 357

BOX 15–6 PULMONARY EDEMA capillaries. As blood pressure increases in the pul-
monary capillaries, filtration creates tissue fluid that
Pulmonary edema is the accumulation of fluid in collects in the alveoli.
the alveoli. This is often a consequence of conges-
tive heart failure in which the left side of the heart Fluid-filled alveoli are no longer sites of efficient
(or the entire heart) is not pumping efficiently. If the gas exchange, and the resulting hypoxia leads to
left ventricle does not pump strongly, the chamber the symptoms of dyspnea and increased respira-
does not empty as it should and cannot receive all tory rate. The most effective treatment is that which
the blood flowing in from the left atrium. Blood restores the pumping ability of the heart to normal.
flow, therefore, is “congested,” and blood backs up
in the pulmonary veins and then in the pulmonary

BOX 15–7 PNEUMONIA that accumulates in the air sacs. Many neutrophils
migrate to the site of infection and attempt to
Pneumonia is a bacterial infection of the lungs. phagocytize the bacteria. The alveoli become filled
Although many bacteria can cause pneumonia, with fluid, bacteria, and neutrophils (this is called
the most common one is probably Streptococcus consolidation); this decreases the exchange of
pneumoniae. This species is estimated to cause at gases.
least 500,000 cases of pneumonia every year in the
United States, with 50,000 deaths. Pneumovax is a vaccine for this type of pneumo-
nia. It contains only the capsules of S. pneumoniae
S. pneumoniae is a transient inhabitant of the and cannot cause the disease. The vaccine is rec-
upper respiratory tract, but in otherwise healthy ommended for people over the age of 60 years,
people, the ciliated epithelium and the immune sys- and for those with chronic pulmonary disorders or
tem prevent infection. Most cases of pneumonia any debilitating disease. It has also been approved
occur in elderly people following a primary infec- for administration to infants.
tion such as influenza.

When the bacteria are able to establish them-
selves in the alveoli, the alveolar cells secrete fluid

BOX 15–8 CARBON MONOXIDE light skin as cyanosis, a bluish cast to the skin, lips,
and nail beds. This is because hemoglobin is dark
Carbon monoxide (CO) is a colorless, odorless gas red unless something (usually oxygen) is bonded to
that is produced during the combustion of fuels it. When hemoglobin bonds to CO, however, it
such as gasoline, coal, oil, and wood. As you know, becomes a bright, cherry red. This color may be
CO is a poison that may cause death if inhaled in seen in light skin and may be very misleading; the
more than very small quantities or for more than a person with CO poisoning is in a severely hypoxic
short period of time. state.

The reason CO is so toxic is that it forms a very Although CO is found in cigarette smoke, it is
strong and stable bond with the hemoglobin in present in such minute quantities that it is not
RBCs (carboxyhemoglobin). Hemoglobin with CO lethal. Heavy smokers, however, may be in a mild
bonded to it cannot bond to and transport oxygen. but chronic hypoxic state because much of their
The effect of CO, therefore, is to drastically decrease hemoglobin is firmly bonded to CO. As a compen-
the amount of oxygen carried in the blood. As little sation, RBC production may increase, and a heavy
as 0.1% CO in inhaled air can saturate half the total smoker may have a hematocrit over 50%.
hemoglobin with CO.

Lack of oxygen is often apparent in people with

Copyright © 2007 by F. A. Davis.

358 The Respiratory System

Carbon dioxide transport is a little more compli- their contraction. The result is inhalation. As the
cated. Some carbon dioxide is dissolved in the plasma, lungs inflate, baroreceptors in lung tissue detect this
and some is carried by hemoglobin (carbaminohemo- stretching and generate sensory impulses to the
globin), but these account for only about 20% of total medulla; these impulses begin to depress the inspira-
CO2 transport. Most carbon dioxide is carried in the tion center. This is called the Hering-Breuer inflation
plasma in the form of bicarbonate ions (HCO3–). Let reflex, which also helps prevent overinflation of the
us look at the reactions that transform CO2 into a lungs.
bicarbonate ion.
As the inspiration center is depressed, the result is
When carbon dioxide enters the blood, most dif- a decrease in impulses to the respiratory muscles,
fuses into red blood cells, which contain the enzyme which relax to bring about exhalation. Then the inspi-
carbonic anhydrase. This enzyme (which contains ration center becomes active again to begin another
zinc) catalyzes the reaction of carbon dioxide and cycle of breathing. When there is a need for more
water to form carbonic acid: forceful exhalations, such as during exercise, the inspi-
ration center activates the expiration center, which
CO2 + H2O → H2CO3 generates impulses to the internal intercostal and
abdominal muscles.
The carbonic acid then dissociates:
The two respiratory centers in the pons work with
H2CO3 → H+ + HCO3– the inspiration center to produce a normal rhythm of
breathing. The apneustic center prolongs inhalation,
The bicarbonate ions diffuse out of the red blood and is then interrupted by impulses from the pneu-
cells into the plasma, leaving the hydrogen ions (H+) motaxic center, which contributes to exhalation. In
in the red blood cells. The many H+ ions would tend normal breathing, inhalation lasts 1 to 2 seconds, fol-
to make the red blood cells too acidic, but hemoglobin lowed by a slightly longer (2 to 3 seconds) exhalation,
acts as a buffer to prevent acidosis. To maintain an producing the normal respiratory rate range of 12 to
ionic equilibrium, chloride ions (Cl–) from the plasma 20 breaths per minute.
enter the red blood cells; this is called the chloride
shift. Where is the CO2? It is in the plasma as part of What has just been described is normal breathing,
HCO3– ions. When the blood reaches the lungs, an but variations are possible and quite common. Emo-
area of lower PCO2, these reactions are reversed, and tions often affect respiration; a sudden fright may bring
CO2 is re-formed and diffuses into the alveoli to be about a gasp or a scream, and anger usually increases
exhaled. the respiratory rate. In these situations, impulses from
the hypothalamus modify the output from the
REGULATION OF RESPIRATION medulla. The cerebral cortex enables us to voluntar-
ily change our breathing rate or rhythm to talk, sing,
Two types of mechanisms regulate breathing: nervous breathe faster or slower, or even to stop breathing for
mechanisms and chemical mechanisms. Because any 1 or 2 minutes. Such changes cannot be continued
changes in the rate or depth of breathing are ulti- indefinitely, however, and the medulla will eventually
mately brought about by nerve impulses, we will con- resume control.
sider nervous mechanisms first.
Coughing and sneezing are reflexes that remove
NERVOUS REGULATION irritants from the respiratory passages; the medulla
contains the centers for both of these reflexes. Sneez-
The respiratory centers are located in the medulla ing is stimulated by an irritation of the nasal mucosa,
and pons, which are parts of the brain stem (see Fig. and coughing is stimulated by irritation of the mucosa
15–9). Within the medulla are the inspiration center of the pharynx, larynx, or trachea. The reflex action is
and expiration center. essentially the same for both: An inhalation is followed
by exhalation beginning with the glottis closed to
The inspiration center automatically generates build up pressure. Then the glottis opens suddenly,
impulses in rhythmic spurts. These impulses travel and the exhalation is explosive. A cough directs the
along nerves to the respiratory muscles to stimulate exhalation out the mouth, while a sneeze directs the
exhalation out the nose.

Copyright © 2007 by F. A. Davis.

The Respiratory System 359

A Stimulatory
Inhibitory

Cerebral cortex
External intercostal muscles

Hypothalamus Pons
Medulla
Pneumotaxic
center

Apneustic
center

B
Expiration center

Inspiration center C Diaphragm Baroreceptors in
lungs

Figure 15–9. Nervous regulation of respiration. (A) Midsagittal section of brain.
(B) Respiratory centers in medulla and pons. (C) Respiratory muscles. See text for descrip-
tion.
QUESTION: Which center directly stimulates inhalation? How can you tell from this
picture?

Hiccups, also a reflex, are spasms of the diaphragm. or accumulation of carbon dioxide, but we really do
The result is a quick inhalation that is stopped when not know. Nor do we know why yawning is conta-
the glottis snaps shut, causing the “hic” sound. The gious, but seeing someone yawn is almost sure to elicit
stimulus may be irritation of the phrenic nerves a yawn of one’s own. You may even have yawned while
or nerves of the stomach. Excessive alcohol is an irri- reading this paragraph about yawning.
tant that can cause hiccups. Some causes are simply
unknown. CHEMICAL REGULATION

Yet another respiratory reflex is yawning. Most of Chemical regulation refers to the effect on breathing
us yawn when we are tired, but the stimulus for and of blood pH and blood levels of oxygen and carbon
purpose of yawning are not known with certainty. dioxide. This is shown in Fig. 15–10. Chemorecep-
There are several possibilities, such as lack of oxygen

Copyright © 2007 by F. A. Davis.

360 The Respiratory System

O2 Chemoreceptors Medulla; Increase rate More O2 available
in carotid and inspiration and depth of to enter blood
CO2 or aortic bodies center respiration
pH More CO2 exhaled
Chemoreceptors pH
in medulla

Figure 15–10. Chemical regulation of respiration. See text for description.
QUESTION: The body’s response to two very different changes (less O2 or more CO2)
is the same. Explain why.

tors that detect changes in blood gases and pH are was available to do so if needed. Also, the residual air
located in the carotid and aortic bodies and in the in the lungs supplies oxygen to the blood even if
medulla itself. breathing rate slows.

A decrease in the blood level of oxygen (hypoxia) is Therefore, carbon dioxide must be the major regu-
detected by the chemoreceptors in the carotid and lator of respiration, and the reason is that carbon diox-
aortic bodies. The sensory impulses generated by ide affects the pH of the blood. As was just mentioned,
these receptors travel along the glossopharyngeal and an excess of CO2 causes the blood pH to decrease, a
vagus nerves to the medulla, which responds by process that must not be allowed to continue.
increasing respiratory rate or depth (or both). This Therefore, any increase in the blood CO2 level is
response will bring more air into the lungs so that quickly compensated for by increased breathing to
more oxygen can diffuse into the blood to correct the exhale more CO2. If, for example, you hold your
hypoxic state. breath, what is it that makes you breathe again? Have
you run out of oxygen? Probably not, for the reasons
Carbon dioxide becomes a problem when it is pres- mentioned. What has happened is that accumulating
ent in excess in the blood, because excess CO2 (hyper- CO2 has lowered blood pH enough to stimulate the
capnia) lowers the pH when it reacts with water to medulla to start the breathing cycle again.
form carbonic acid (a source of H+ ions). That is,
excess CO2 makes the blood or other body fluids less In some situations, oxygen does become the major
alkaline (or more acidic). The medulla contains regulator of respiration. People with severe, chronic
chemoreceptors that are very sensitive to changes pulmonary diseases such as emphysema have de-
in pH, especially decreases. If accumulating CO2 low- creased exchange of both oxygen and carbon dioxide
ers blood pH, the medulla responds by increasing res- in the lungs. The decrease in pH caused by accumu-
piration. This is not for the purpose of inhaling, but lating CO2 is corrected by the kidneys, but the blood
rather to exhale more CO2 to raise the pH back to oxygen level keeps decreasing. Eventually, the oxygen
normal. level may fall so low that it does provide a very strong
stimulus to increase the rate and depth of respiration.
Of the two respiratory gases, which is the more
important as a regulator of respiration? Our guess RESPIRATION AND
might be oxygen, because it is essential for energy pro- ACID–BASE BALANCE
duction in cell respiration. However, the respiratory
system can maintain a normal blood level of oxygen As you have just seen, respiration affects the pH of
even if breathing decreases to half the normal rate or body fluids because it regulates the amount of carbon
stops for a few moments. Recall that exhaled air is
16% oxygen. This oxygen did not enter the blood but

Copyright © 2007 by F. A. Davis.

The Respiratory System 361

dioxide in these fluids. Remember that CO2 reacts tration of body fluids is increased. Respiratory com-
with water to form carbonic acid (H2CO3), which ion- pensation involves an increase in the rate and depth of
izes into H+ ions and HCO3– ions. The more hydro- respiration to exhale more CO2 to decrease H+ ion
gen ions present in a body fluid, the lower the pH, and formation, which will raise the pH toward the normal
the fewer hydrogen ions present, the higher the pH. range.

The respiratory system may be the cause of a pH Metabolic alkalosis is not a common occurrence
imbalance, or it may help correct a pH imbalance cre- but may be caused by ingestion of excessive amounts
ated by some other cause. of alkaline medications such as those used to relieve
gastric disturbances. Another possible cause is vomit-
RESPIRATORY ACIDOSIS ing of stomach contents only. In such situations, the
AND ALKALOSIS H+ ion concentration of body fluids is decreased.
Respiratory compensation involves a decrease in res-
Respiratory acidosis occurs when the rate or effi- piration to retain CO2 in the body to increase H+ ion
ciency of respiration decreases, permitting carbon formation, which will lower the pH toward the normal
dioxide to accumulate in body fluids. The excess CO2 range.
results in the formation of more H+ ions, which
decrease the pH. Holding one’s breath can bring about Respiratory compensation for an ongoing meta-
a mild respiratory acidosis, which will soon stimulate bolic pH imbalance cannot be complete, because there
the medulla to initiate breathing again. More serious are limits to the amounts of CO2 that may be exhaled
causes of respiratory acidosis are pulmonary diseases or retained. At most, respiratory compensation is only
such as pneumonia and emphysema, or severe asthma. about 75% effective. A complete discussion of acid–
Each of these impairs gas exchange and allows excess base balance is found in Chapter 19.
CO2 to remain in body fluids.
AGING AND THE
Respiratory alkalosis occurs when the rate of res- RESPIRATORY SYSTEM
piration increases, and CO2 is very rapidly exhaled.
Less CO2 decreases H+ ion formation, which increases Perhaps the most important way to help your respira-
the pH. Breathing faster for a few minutes can bring tory system age gracefully is not to smoke. In the
about a mild state of respiratory alkalosis. Babies who absence of chemical assault, respiratory function does
cry for extended periods (crying is a noisy exhalation) diminish but usually remains adequate. The respira-
put themselves in this condition. In general, however, tory muscles, like all skeletal muscles, weaken with
respiratory alkalosis is not a common occurrence. age. Lung tissue loses its elasticity and alveoli are
Severe physical trauma and shock, or certain states lost as their walls deteriorate. All of this results in de-
of mental or emotional anxiety, may be accompanied creased ventilation and lung capacity, but the remain-
by hyperventilation and also result in respiratory ing capacity is usually sufficient for ordinary activities.
alkalosis. In addition, traveling to a higher altitude The cilia of the respiratory mucosa deteriorate with
(less oxygen in the atmosphere) may cause a tempo- age, and the alveolar macrophages are not as efficient,
rary increase in breathing rate before compensation which make elderly people more prone to pneumonia,
occurs (increased rate of RBC production—see a serious pulmonary infection.
Chapter 11).
Chronic alveolar hypoxia from diseases such as
RESPIRATORY COMPENSATION emphysema or chronic bronchitis may lead to pul-
monary hypertension, which in turn overworks the
If a pH imbalance is caused by something other than a right ventricle of the heart. Systemic hypertension
change in respiration, it is called metabolic acidosis or often weakens the left ventricle of the heart, leading to
alkalosis. In either case, the change in pH stimulates a congestive heart failure and pulmonary edema, in
change in respiration that may help restore the pH of which excess tissue fluid collects in the alveoli and
body fluids to normal. decreases gas exchange. Though true at any age, the
interdependence of the respiratory and circulatory
Metabolic acidosis may be caused by untreated systems is particularly apparent in elderly people.
diabetes mellitus (ketoacidosis), kidney disease, or
severe diarrhea. In such situations, the H+ ion concen-

Copyright © 2007 by F. A. Davis.

362 The Respiratory System

SUMMARY cell respiration. Breathing also regulates the level of
CO2 within the body, and this contributes to the
As you have learned, respiration is much more than maintenance of the acid–base balance of body fluids.
the simple mechanical actions of breathing. Inhalation Although the respiratory gases do not form structural
provides the body with the oxygen that is necessary for components of the body, their contributions to the
the production of ATP in the process of cell respira- chemical level of organization are essential to the
tion. Exhalation removes the CO2 that is a product of functioning of the body at every level.

STUDY OUTLINE 2. Oropharynx—behind the mouth; a passageway for
both air and food. Palatine tonsils are on the lateral
The respiratory system moves air into and walls.
out of the lungs, which are the site of
exchange for O2 and CO2 between the air 3. Laryngopharynx—a passageway for both air and
and the blood. The functioning of the respi- food; opens anteriorly into the larynx and posteri-
ratory system depends directly on the orly into the esophagus.
proper functioning of the circulatory system.
1. The upper respiratory tract consists of those parts Larynx—the voice box and the airway
between the pharynx and trachea (see Fig.
outside the chest cavity. 15–2)
2. The lower respiratory tract consists of those parts 1. Made of nine cartilages; the thyroid cartilage is the

within the chest cavity. largest and most anterior.
2. The epiglottis is the uppermost cartilage; covers
Nose—made of bone and cartilage covered
with skin the larynx during swallowing.
1. Hairs inside the nostrils block the entry of dust. 3. The vocal cords are lateral to the glottis, the open-

Nasal Cavities—within the skull; separated ing for air (see Fig. 15–3).
by the nasal septum (see Fig. 15–1) 4. During speaking, the vocal cords are pulled across
1. Nasal mucosa is ciliated epithelium with goblet
the glottis and vibrated by exhaled air, producing
cells; surface area is increased by the conchae. sounds that may be turned into speech.
2. Nasal mucosa warms and moistens the incoming 5. The cranial nerves for speaking are the vagus and
accessory.
air; dust and microorganisms are trapped on mucus
and swept by the cilia to the pharynx. Trachea—extends from the larynx to the pri-
3. Olfactory receptors respond to vapors in inhaled mary bronchi (see Fig. 15–4)
air. 1. Sixteen to 20 C-shaped cartilages in the tracheal
4. Paranasal sinuses in the maxillae, frontal, sphenoid,
and ethmoid bones open into the nasal cavities: wall keep the trachea open.
functions are to lighten the skull and provide reso- 2. Mucosa is ciliated epithelium with goblet cells; cilia
nance for the voice.
sweep mucus, trapped dust, and microorganisms
Pharynx—posterior to nasal and oral cavities upward to the pharynx.
(see Fig. 15–1)
1. Nasopharynx—above the level of the soft palate, Bronchial Tree—extends from the trachea to
the alveoli (see Fig. 15–4)
which blocks it during swallowing; a passageway 1. The right and left primary bronchi are branches of
for air only. The eustachian tubes from the middle
ears open into it. The adenoid is a lymph nodule on the trachea; one to each lung; same structure as the
the posterior wall. trachea.

Copyright © 2007 by F. A. Davis.

The Respiratory System 363

2. Secondary bronchi: to the lobes of each lung (three • Intrapulmonic pressure is within the bronchial
right, two left) tree and alveoli; fluctuates during breathing.

3. Bronchioles—no cartilage in their walls. Inhalation (inspiration)
1. Motor impulses from medulla travel along phrenic
Pleural Membranes—serous membranes of
the thoracic cavity nerves to diaphragm, which contracts and moves
1. Parietal pleura lines the chest wall. down. Impulses are sent along intercostal nerves to
2. Visceral pleura covers the lungs. external intercostal muscles, which pull ribs up and
3. Serous fluid between the two layers prevents fric- out.
2. The chest cavity is expanded and expands the pari-
tion and keeps the membranes together during etal pleura.
breathing. 3. The visceral pleura adheres to the parietal pleura
and is also expanded and in turn expands the lungs.
Lungs—on either side of the heart in the 4. Intrapulmonic pressure decreases, and air rushes
chest cavity; extend from the diaphragm into the lungs.
below up to the level of the clavicles
1. The rib cage protects the lungs from mechanical Exhalation (expiration)
1. Motor impulses from the medulla decrease, and the
injury.
2. Hilus—indentation on the medial side: primary diaphragm and external intercostal muscles relax.
2. The chest cavity becomes smaller and compresses
bronchus and pulmonary artery and veins enter
(also bronchial vessels). the lungs.
3. The elastic lungs recoil and further compress the
Alveoli—the sites of gas exchange in the
lungs alveoli.
1. Made of alveolar type I cells, simple squamous 4. Intrapulmonic pressure increases, and air is forced

epithelium; thin to permit diffusion of gases. out of the lungs. Normal exhalation is passive.
2. Surrounded by pulmonary capillaries, which are 5. Forced exhalation: contraction of the internal

also made of simple squamous epithelium (see Fig. intercostal muscles pulls the ribs down and in; con-
15–4). traction of the abdominal muscles forces the
3. Elastic connective tissue between alveoli is impor- diaphragm upward.
tant for normal exhalation.
4. A thin layer of tissue fluid lines each alveolus; essen- Pulmonary Volumes (see Fig. 15–7)
tial to permit diffusion of gases (see Fig. 15–5). 1. Tidal volume—the amount of air in one normal
5. Alveolar type II cells produce pulmonary surfactant
that mixes with the tissue fluid lining to decrease inhalation and exhalation.
surface tension to permit inflation of the alveoli. 2. Minute respiratory volume—the amount of air
6. Alveolar macrophages phagocytize foreign mate-
rial. inhaled and exhaled in 1 minute.
3. Inspiratory reserve—the amount of air beyond
Mechanism of Breathing
1. Ventilation is the movement of air into and out of tidal in a maximal inhalation.
4. Expiratory reserve—the amount of air beyond tidal
the lungs: inhalation and exhalation.
2. Respiratory centers are in the medulla and pons. in the most forceful exhalation.
3. Respiratory muscles are the diaphragm and exter- 5. Vital capacity—the sum of tidal volume, inspira-

nal and internal intercostal muscles (see Fig. 15–6). tory and expiratory reserves.
• Atmospheric pressure is air pressure: 760 mmHg 6. Residual volume—the amount of air that remains

at sea level. in the lungs after the most forceful exhalation; pro-
• Intrapleural pressure is within the potential pleu- vides for continuous exchange of gases.
7. Alveolar ventilation—air that reaches the alveoli
ral space; always slightly below atmospheric for gas exchange; depends on normal thoracic and
pressure (“negative”). lung compliance.
• Anatomic dead space—air still in the respiratory

passages at the end of inhalation (is normal).

Copyright © 2007 by F. A. Davis.

364 The Respiratory System

• Physiological dead space—the volume of non- Nervous Regulation of Respiration
functional alveoli; decreases compliance. (see Fig. 15–9)
1. The medulla contains the inspiration center and
Exchange of Gases
1. External respiration is the exchange of gases expiration center.
2. Impulses from the inspiration center to the respira-
between the air in the alveoli and the blood in the
pulmonary capillaries. tory muscles cause their contraction; the chest cav-
2. Internal respiration is the exchange of gases ity is expanded.
between blood in the systemic capillaries and tissue 3. Baroreceptors in lung tissue detect stretching and
fluid (cells). send impulses to the medulla to depress the inspi-
3. Inhaled air (atmosphere) is 21% O2 and 0.04% ration center. This is the Hering-Breuer inflation
CO2. Exhaled air is 16% O2 and 4.5% CO2. reflex, which also prevents overinflation of the
4. Diffusion of O2 and CO2 in the body occurs lungs.
because of pressure gradients (see Table 15–1). A 4. The expiration center is stimulated by the inspi-
gas will diffuse from an area of higher partial pres- ration center when forceful exhalations are needed.
sure to an area of lower partial pressure. 5. In the pons: the apneustic center prolongs inhala-
5. External respiration: PO2 in the alveoli is high, and tion, and the pneumotaxic center helps bring about
PO2in the pulmonary capillaries is low, so O2 dif- exhalation. These centers work with the inspiration
fuses from the air to the blood. PCO2 in the alveoli center in the medulla to produce a normal breath-
is low, and PCO2 in the pulmonary capillaries is ing rhythm.
high, so CO2 diffuses from the blood to the air and 6. The hypothalamus influences changes in breathing
is exhaled (see Fig. 15–8). in emotional situations. The cerebral cortex per-
6. Internal respiration: PO2 in the systemic capillaries mits voluntary changes in breathing.
is high, and PO2 in the tissue fluid is low, so O2 dif- 7. Coughing and sneezing remove irritants from the
fuses from the blood to the tissue fluid and cells. upper respiratory tract; the centers for these
PCO2 in the systemic capillaries is low, and PCO2 in reflexes are in the medulla.
the tissue fluid is high, so CO2 diffuses from the tis-
sue fluid to the blood (see Fig. 15–8). Chemical Regulation of Respiration
(see Fig. 15–10)
Transport of Gases in the Blood 1. Decreased blood O2 is detected by chemoreceptors
1. Oxygen is carried by the iron of hemoglobin (Hb)
in the carotid body and aortic body. Response:
in the RBCs. The O2–Hb bond is formed in the increased respiration to take more air into the
lungs where the PO2 is high. lungs.
2. In tissues, Hb releases much of its O2; the impor- 2. Increased blood CO2 level is detected by chemo-
tant factors are low PO2 in tissues, high PCO2 in tis- receptors in the medulla. Response: increased res-
sues, and a high temperature in tissues. piration to exhale more CO2.
3. Oxygen saturation of hemoglobin (SaO2) is 95% to 3. CO2 is the major regulator of respiration because
97% in systemic arteries and averages 70% to 75% excess CO2 decreases the pH of body fluids (CO2 +
in systemic veins. H2O → H2CO3 → H+ + HCO3–). Excess H+ ions
4. Most CO2 is carried as HCO3– ions in blood lower pH.
plasma. CO2 enters the RBCs and reacts with H2O 4. Oxygen becomes a major regulator of respiration
to form carbonic acid (H2CO3). Carbonic anhy- when blood level is very low, as may occur with
drase is the enzyme that catalyzes this reaction. severe, chronic pulmonary disease.
H2CO3 dissociates to H+ ions and HCO3– ions.
The HCO3– ions leave the RBCs and enter the Respiration and Acid–Base Balance
plasma; Hb buffers the H+ ions that remain in the 1. Respiratory acidosis: a decrease in the rate or effi-
RBCs. Cl– ions from the plasma enter the RBCs to
maintain ionic equilibrium (the chloride shift). ciency of respiration permits excess CO2 to accu-
5. When blood reaches the lungs, CO2 is re-formed, mulate in body fluids, resulting in the formation of
diffuses into the alveoli, and is exhaled. excess H+ ions, which lower pH. Occurs in severe
pulmonary disease.
2. Respiratory alkalosis: an increase in the rate of res-

Copyright © 2007 by F. A. Davis.

The Respiratory System 365

piration increases the CO2 exhaled, which de- increased respiration to exhale CO2 to decrease H+
creases the formation of H+ ions and raises pH. ion formation to raise pH to normal.
Occurs during hyperventilation or when first at a 4. Respiratory compensation for metabolic alkalosis:
high altitude. decreased respiration to retain CO2 to increase H+
3. Respiratory compensation for metabolic acidosis: ion formation to lower pH to normal.

REVIEW QUESTIONS 9. Name the cell, protein, and mineral that transport
oxygen in the blood. State the three factors
1. State the three functions of the nasal mucosa. that increase the release of oxygen in tissues.
(p. 344) (pp. 355–356)

2. Name the three parts of the pharynx; state whether 10. Most carbon dioxide is transported in what part of
each is an air passage only or an air and food pas- the blood, and in what form? Explain the function
sage. (pp. 344–346) of hemoglobin with respect to carbon dioxide
transport. (p. 358)
3. Name the tissue that lines the larynx and trachea,
and describe its function. State the function of the 11. Name the respiratory centers in the medulla and
cartilage of the larynx and trachea. (p. 346) pons, and explain how each is involved in a
breathing cycle. (p. 358)
4. Name the pleural membranes, state the location
of each, and describe the functions of serous fluid. 12. State the location of chemoreceptors affected by a
(pp. 347) low blood oxygen level; describe the body’s
response to hypoxia and its purpose. State the
5. Name the tissue of which the alveoli and pul- location of chemoreceptors affected by a high
monary capillaries are made, and explain the blood CO2 level; describe the body’s response and
importance of this tissue in these locations. Explain its purpose. (p. 360)
the function of pulmonary surfactant. (p. 347)
13. For respiratory acidosis and alkalosis: state a cause
6. Name the respiratory muscles, and describe how and explain what happens to the pH of body flu-
they are involved in normal inhalation and exhala- ids. (p. 361)
tion. Define these pressures and relate them to a
cycle of breathing: atmospheric pressure, intrapul- 14. Explain how the respiratory system may compen-
monic pressure. (pp. 348–349) sate for metabolic acidosis or alkalosis. For an
ongoing pH imbalance, what is the limit of respi-
7. Describe external respiration in terms of partial ratory compensation? (pp. 361)
pressures of oxygen and carbon dioxide. (p. 355)

8. Describe internal respiration in terms of partial
pressures of oxygen and carbon dioxide. (p. 355)

FOR FURTHER THOUGHT 3. As recently as 45 years ago (the early 1960s)
it was believed that mouth-to-mouth resusci-
1. The success of an organ transplant depends on tation was not really helpful to another per-
many factors. What factor would diminish the son. What mistaken belief about the air we exhale
chance of success of a lung transplant, but is not a contributed to that thinking, and what are the
factor at all in a heart transplant? facts?

2. Name four types of tissues that contribute to the 4. You are making a list of vital organs, organs we can-
functioning of the lungs, and describe the physical not live without. Should you include the larynx on
characteristics of each that are important. Name your list? Explain why or why not.
two types of cells that are also important to the
functioning of the lungs.

Copyright © 2007 by F. A. Davis.

366 The Respiratory System

5. At a construction site, a hole caved in and buried a are 32 per minute. What acid–base situation is this
workman up to his shoulders in wet sand. The fore- patient in? State your reasoning step-by-step.
man told the trapped man that a crane would be
there in 20 minutes to pull him out, but another 7. Mrs. D is in the emergency room because of severe
worker said they couldn’t wait, and had to dig now. abdominal pain that may be appendicitis. Her
He was right. Why is this a life-threatening emer- blood pH is 7.47 and her respirations are 34 per
gency? minute. What acid–base situation is Mrs. D in?
State your reasoning step-by-step.
6. A patient’s blood pH is 7.34, and his respirations

Copyright © 2007 by F. A. Davis.

CHAPTER 16

The Digestive System

367

Copyright © 2007 by F. A. Davis.

CHAPTER 16

Chapter Outline BOX 16–1 DISORDERS OF THE STOMACH
Divisions of the Digestive System BOX 16–2 GALLSTONES
Types of Digestion BOX 16–3 DISORDERS OF THE INTESTINES
End Products of Digestion BOX 16–4 INFANT BOTULISM
Oral Cavity BOX 16–5 FIBER
Teeth BOX 16–6 HEPATITIS
Tongue
Salivary Glands Student Objectives
Pharynx
Esophagus • Describe the general functions of the digestive
Structural Layers of the Alimentary Tube
Mucosa system, and name its major divisions.
Submucosa
External Muscle Layer • Explain the difference between mechanical and
Serosa
Stomach chemical digestion, and name the end products of
Small Intestine digestion.
Liver
Gallbladder • Describe the structure and functions of the teeth
Pancreas
Completion of Digestion and Absorption and tongue.
Small Intestine
Absorption • Explain the functions of saliva.
Large Intestine • Describe the location and function of the pharynx
Elimination of Feces
Other Functions of the Liver and esophagus.
Aging and the Digestive System
• Describe the structure and function of each of the
368
four layers of the alimentary tube.

• Describe the location, structure, and function of

the stomach, small intestine, liver, gallbladder, and
pancreas.

• Describe absorption in the small intestine.
• Describe the location and functions of the large

intestine.

• Explain the functions of the normal flora of the

colon.

• Describe the functions of the liver.

Copyright © 2007 by F. A. Davis.

The Digestive System

New Terminology Related Clinical Terminology
Alimentary tube (AL-i-MEN-tah-ree TOOB)
Chemical digestion (KEM-i-kuhl dye-JES-chun) Appendicitis (uh-PEN-di-SIGH-tis)
Common bile duct (KOM-mon BYL DUKT) Diverticulitis (DYE-ver-TIK-yoo-LYE-tis)
Defecation reflex (DEF-e-KAY-shun) Gastric ulcer (GAS-trik UL-ser)
Duodenum (dew-AH-den-um) Hepatitis (HEP-uh-TIGH-tis)
Emulsify (e-MULL-si-fye) Lactose intolerance (LAK-tohs in-TAHL-er-ense)
Enamel (e-NAM-uhl) Lithotripsy (LITH-oh-TRIP-see)
Essential amino acids (e-SEN-shul ah-ME-noh Paralytic ileus (PAR-uh-LIT-ik ILL-ee-us)
Peritonitis (per-i-toh-NIGH-tis)
ASS-ids) Pyloric stenosis (pye-LOR-ik ste-NOH-sis)
External anal sphincter (eks-TER-nuhl AY-nuhl

SFINK-ter)
Ileocecal valve (ILL-ee-oh-SEE-kuhl VALV)
Internal anal sphincter (in-TER-nuhl AY-nuhl

SFINK-ter)
Lower esophageal sphincter (e-SOF-uh-JEE-uhl

SFINK-ter)
Mechanical digestion (muh-KAN-i-kuhl dye-JES-

chun)
Non-essential amino acids (NON-e-SEN-shul ah-

ME-noh ASS-ids)
Normal flora (NOR-muhl FLOOR-ah)
Periodontal membrane (PER-ee-oh-DON-tal)
Pyloric sphincter (pye-LOR-ik SFINK-ter)
Rugae (ROO-gay)
Villi (VILL-eye)

Terms that appear in bold type in the chapter text are defined in the glossary, which begins on page 547.

369

Copyright © 2007 by F. A. Davis.

370 The Digestive System

A hurried breakfast when you are late for work or digestion. Mechanical digestion is the physical break-
ing up of food into smaller pieces. Chewing is an
school . . . Thanksgiving dinner . . . going on a diet to example of this. As food is broken up, more of its sur-
lose 5 pounds . . . what do these experiences all have in face area is exposed for the action of digestive enzymes.
common? Food. We may take food for granted, cele- Enzymes are discussed in Chapter 2. The work of the
brate with it, or wish we wouldn’t eat quite so much of digestive enzymes is the chemical digestion of bro-
it. Although food is not as immediate a need for ken-up food particles, in which complex chemical mol-
human beings as is oxygen, it is a very important part ecules are changed into much simpler chemicals that
of our lives. Food provides the raw materials or nutri- the body can utilize. Such enzymes are specific with
ents that cells use to reproduce and to build new tis- respect to the fat, protein, or carbohydrate food mole-
sue. The energy needed for cell reproduction and cules each can digest. For example, protein-digesting
tissue building is released from food in the process of enzymes work only on proteins, not on carbohydrates
cell respiration. In fact, a supply of nutrients from reg- or fats. Each enzyme is produced by a particular diges-
ular food intake is so important that the body can even tive organ and functions at a specific site. However, the
store any excess for use later. Those “extra 5 pounds” enzyme’s site of action may or may not be its site of
are often stored fat in adipose tissue. production. These digestive enzymes and their func-
tions are discussed in later sections.
The food we eat, however, is not in a form that our
body cells can use. A turkey sandwich, for example, END PRODUCTS OF DIGESTION
consists of complex proteins, fats, and carbohydrates.
The function of the digestive system is to change Before we describe the organs of digestion, let us see
these complex organic nutrient molecules into simple where the process of digestion will take us, or rather,
organic and inorganic molecules that can then be will take our food. The three types of complex organic
absorbed into the blood or lymph to be transported to molecules found in food are carbohydrates, proteins,
cells. In this chapter we will discuss the organs of and fats. Each of these complex molecules is digested
digestion and the contribution each makes to diges- to a much more simple substance that the body can
tion and absorption. then use. Carbohydrates, such as starches and disac-
charides, are digested to monosaccharides such as glu-
DIVISIONS OF THE cose, fructose, and galactose. Proteins are digested to
DIGESTIVE SYSTEM amino acids, and fats are digested to fatty acids and
glycerol. Also part of food, and released during diges-
The two divisions of the digestive system are the ali- tion, are vitamins, minerals, and water.
mentary tube and the accessory organs (Fig. 16–1).
The alimentary tube extends from the mouth to the We will now return to the beginning of the alimen-
anus. It consists of the oral cavity, pharynx, esophagus, tary tube and consider the digestive organs and the
stomach, small intestine, and large intestine. process of digestion.
Digestion takes place within the oral cavity, stomach,
and small intestine; most absorption of nutrients takes ORAL CAVITY
place in the small intestine. Undigestible material, pri-
marily cellulose, is eliminated by the large intestine Food enters the oral cavity (or buccal cavity) by way
(also called the colon). of the mouth. The boundaries of the oral cavity are
the hard and soft palates superiorly; the cheeks later-
The accessory organs of digestion are the teeth, ally; and the floor of the mouth inferiorly. Within the
tongue, salivary glands, liver, gallbladder, and pancreas. oral cavity are the teeth and tongue and the openings
Digestion does not take place within these organs, but of the ducts of the salivary glands.
each contributes something to the digestive process.
TEETH
TYPES OF DIGESTION
The function of the teeth is, of course, chewing. This
The food we eat is broken down in two complemen- is the process that mechanically breaks food into
tary processes: mechanical digestion and chemical smaller pieces and mixes it with saliva. An individual

Copyright © 2007 by F. A. Davis.

Teeth The Digestive System 371

Sublingual gland Tongue
Submandibular gland
Parotid gland
Pharynx

Esophagus

Figure 16–1. The digestive organs Stomach (cut)
shown in anterior view of the trunk
and left lateral view of the head. (The Liver Left lobe
spleen is not a digestive organ but is
included to show its location relative Spleen
to the stomach, pancreas, and colon.) Duodenum
QUESTION: In which parts of the Pancreas
digestive system does digestion actu- Descending colon
ally take place? Small intestine

Right lobe Rectum
Gallbladder Anal canal
Bile duct

Transverse colon
(cut)

Ascending colon
Cecum

Vermiform appendix

develops two sets of teeth: deciduous and permanent. of permanent teeth consists of 32 teeth; the types of
The deciduous teeth begin to erupt through the teeth are incisors, canines, premolars, and molars. The
gums at about 6 months of age, and the set of 20 teeth wisdom teeth are the third molars on either side of
is usually complete by the age of 2 years. These teeth each jawbone. In some people, the wisdom teeth may
are gradually lost throughout childhood and replaced not emerge from the jawbone because there is no
by the permanent teeth, the first of which are molars room for them along the gum line. These wisdom
that emerge around the age of 6 years. A complete set teeth are said to be impacted and may put pressure on

Copyright © 2007 by F. A. Davis.

372 The Digestive System

the roots of the second molars. In such cases, extrac- called papillae, many of which contain taste buds (see
tion of a wisdom tooth may be necessary to prevent also Chapter 9). The sensory nerves for taste are also
damage to other teeth. cranial nerves: the facial (7th) and glossopharyngeal
(9th). As you know, the sense of taste is important
The structure of a tooth is shown in Fig. 16–2. The because it makes eating enjoyable, but the tongue has
crown is visible above the gum (gingiva). The root is other functions as well.
enclosed in a socket in the mandible or maxillae. The
periodontal membrane lines the socket and produces Chewing is efficient because of the action of the
a bone-like cement that anchors the tooth. The outer- tongue in keeping the food between the teeth and
most layer of the crown is enamel, which is made by mixing it with saliva. Elevation of the tongue is the
cells called ameloblasts. Enamel provides a hard chew- first step in swallowing. This is a voluntary action, in
ing surface and is more resistant to decay than are which the tongue contracts and meets the resistance of
other parts of the tooth. Within the enamel is dentin, the hard palate. The mass of food, called a bolus, is
which is very similar to bone and is produced by cells thus pushed backward toward the pharynx. The
called odontoblasts. Dentin also forms the roots of a remainder of swallowing is a reflex, which is described
tooth. The innermost portion of a tooth is the pulp in the section on the pharynx.
cavity, which contains blood vessels and nerve end-
ings of the trigeminal nerve (5th cranial). Erosion of SALIVARY GLANDS
the enamel and dentin layers by bacterial acids (dental
caries or cavities) may result in bacterial invasion of The digestive secretion in the oral cavity is saliva,
the pulp cavity and a very painful toothache. produced by three pairs of salivary glands, which are
shown in Fig. 16–3. The parotid glands are just
TONGUE below and in front of the ears. The submandibular
(also called submaxillary) glands are at the posterior
The tongue is made of skeletal muscle that is inner- corners of the mandible, and the sublingual glands
vated by the hypoglossal nerves (12th cranial). On the are below the floor of the mouth. Each gland has at
upper surface of the tongue are small projections least one duct that takes saliva to the oral cavity.

Crown Enamel Secretion of saliva is continuous, but the amount
Neck varies in different situations. The presence of food
Dentin (or anything else) in the mouth increases saliva secre-
Pulp tion. This is a parasympathetic response mediated
cavity by the facial and glossopharyngeal nerves. The sight
Gingiva or smell of food also increases secretion of saliva.
(gum) Sympathetic stimulation in stress situations decreases
secretion, making the mouth dry and swallowing
Root Cementum difficult.

Periodontal Saliva is mostly water, which is important to dis-
membrane solve food for tasting and to moisten food for swal-
lowing. The digestive enzyme in saliva is salivary
Nerve Blood vessels amylase, which breaks down starch molecules to
shorter chains of glucose molecules, or to maltose, a
Figure 16–2. Tooth structure. Longitudinal section of a disaccharide. Most of us, however, do not chew our
tooth showing internal structure. food long enough for the action of salivary amylase to
QUESTION: Which parts of a tooth are living? How do be truly effective. As you will see, another amylase
you know? from the pancreas is also available to digest starch.
Table 16–1 summarizes the functions of digestive
secretions.

Saliva is made from blood plasma and thus contains
many of the chemicals that are found in plasma.
Considerable research is focused on detecting in saliva
chemical markers for diseases such as cancer, with the

Copyright © 2007 by F. A. Davis.

The Digestive System 373

sure of the epiglottis, and peristalsis of the esophagus.
As you can see, swallowing is rather complicated, but
because it is a reflex we don’t have to think about mak-
ing it happen correctly. Talking or laughing while eat-
ing, however, may interfere with the reflex and cause
food to go into the “wrong pipe,” the larynx. When
that happens, the cough reflex is usually effective in
clearing the airway.

Parotid duct ESOPHAGUS

Parotid The esophagus is a muscular tube that takes food
gland from the pharynx to the stomach; no digestion takes
place here. Peristalsis of the esophagus propels food in
Sublingual Submandibular one direction and ensures that food gets to the stom-
ducts gland ach even if the body is horizontal or upside down. At
the junction with the stomach, the lumen (cavity) of
Sublingual gland Submandibular duct the esophagus is surrounded by the lower esophageal
sphincter (LES or cardiac sphincter), a circular
Figure 16–3. The salivary glands shown in left lateral smooth muscle. The LES relaxes to permit food to
view. enter the stomach, then contracts to prevent the
QUESTION: Why are these exocrine glands? What is backup of stomach contents. If the LES does not close
saliva made from? completely, gastric juice may splash up into the esoph-
agus; this is a painful condition we call heartburn, or
goal of using saliva rather than blood for diagnostic gastroesophageal reflux disease (GERD). Most people
tests. experience heartburn once in a while, and it is merely
uncomfortable, but chronic GERD is more serious.
PHARYNX The lining of the esophagus cannot withstand the cor-
rosive action of gastric acid and will be damaged, per-
As described in the preceding chapter, the oropharynx haps resulting in bleeding or even perforation.
and laryngopharynx are food passageways connecting Medications are available to treat this condition.
the oral cavity to the esophagus. No digestion takes
place in the pharynx. Its only related function is swal- STRUCTURAL LAYERS
lowing, the mechanical movement of food. When the OF THE ALIMENTARY TUBE
bolus of food is pushed backward by the tongue, the
constrictor muscles of the pharynx contract as part of Before we continue with our discussion of the organs
the swallowing reflex. The reflex center for swallow- of digestion, we will first examine the typical structure
ing is in the medulla, which coordinates the many of the alimentary tube. When viewed in cross-section,
actions that take place: constriction of the pharynx, the alimentary tube has four layers (Fig. 16–4): the
cessation of breathing, elevation of the soft palate to mucosa, submucosa, external muscle layer, and serosa.
block the nasopharynx, elevation of the larynx and clo- Each layer has a specific structure, and its functions
contribute to the functioning of the organs of which it
is a part.

MUCOSA

The mucosa, or lining, of the alimentary tube is made
of epithelial tissue, areolar connective tissue, and two

Copyright © 2007 by F. A. Davis.

374 The Digestive System

Table 16–1 THE PROCESS OF DIGESTION

Organ Enzyme or Function Site of Action
Salivary glands Other Secretion
Stomach • Converts starch to maltose Oral cavity
Amylase Stomach
Liver • Converts proteins to polypeptides Stomach
Pancreas Pepsin • Changes pepsinogen to pepsin; maintains pH
HCI Small intestine
Small intestine 1–2; destroys pathogens Small intestine
Bile salts Small intestine
• Emulsify fats Small intestine
Amylase Small intestine
Trypsin • Converts starch to maltose Small intestine
Lipase • Converts polypeptides to peptides Small intestine
• Converts emulsified fats to fatty acids and glycerol Small intestine
Peptidases
Sucrase • Convert peptides to amino acids
Maltase • Converts sucrose to glucose and fructose
Lactase • Converts maltose to glucose (2)
• Converts lactose to glucose and galactose

Salivary Amylase Carbohydrates
glands

Amylase
Sucrase, Maltase, Lactase

Liver Bile Fats
Stomach Pepsin
Lipase
Pancreas Trypsin
Small
intestine

Peptidases Proteins

Table Figure 16–A Functions of digestive secretions.
QUESTION: Proteins are digested by secretions from which organs? How did you decide?

Copyright © 2007 by F. A. Davis.

Small intestine

Mucosa Epithelium
Lacteal

Capillary
network

Lymph
nodule

Lymphatic
vessel

Submucosa Smooth
muscle
External
muscle Venule
layer
Arteriole
A Serosa
Meissner's
Figure 16–4. (A) The four layers of the wall of the plexus
alimentary tube. A small part of the wall of the small
intestine has been magnified to show the four layers Circular
typical of the alimentary tube. (B) Sagittal section smooth muscle
through the abdomen showing the relationship of the
peritoneum and mesentery to the abdominal organs. Longitudinal
QUESTION: What is the function of the external mus- smooth muscle
cle layer?
Auerbach's
plexus

Parietal Diaphragm
peritoneum Liver

Transverse Pancreas
colon Stomach
Greater Duodenum
omentum Mesentery
Small intestine
Uterus
Sigmoid colon
Bladder Rectum

B
375

Copyright © 2007 by F. A. Davis.

376 The Digestive System

thin layers of smooth muscle. In the esophagus the tions and peristalsis, promoting normal digestion. The
epithelium is stratified squamous epithelium; in the parasympathetic nerves are the vagus (10th cranial)
stomach and intestines it is simple columnar epithe- nerves; they truly live up to the meaning of vagus,
lium. The epithelium secretes mucus, which lubricates which is “wanderer.”
the passage of food, and also secretes the digestive
enzymes of the stomach and small intestine. Just SEROSA
below the epithelium, within the areolar connective
tissue, are lymph nodules that contain lymphocytes to Above the diaphragm, for the esophagus, the serosa,
produce antibodies, and macrophages to phagocytize the outermost layer, is fibrous connective tissue. Below
bacteria or other foreign material that get through the the diaphragm, the serosa is the mesentery or visceral
epithelium. The thin layers of smooth muscle create peritoneum, a serous membrane. Lining the abdomi-
folds in the mucosa, and ripples, so that all of the nal cavity is the parietal peritoneum, usually simply
epithelial cells are in touch with the contents of the called the peritoneum. The peritoneum-mesentery is
organ. In the stomach and small intestine this is actually one continuous membrane (see Fig. 16–4).
important for absorption. The serous fluid between the peritoneum and mesen-
tery prevents friction when the alimentary tube con-
SUBMUCOSA tracts and the organs slide against one another.

The submucosa is made of areolar connective tissue The preceding descriptions are typical of the layers
with many blood vessels and lymphatic vessels. Many of the alimentary tube. As noted, variations are possi-
millions of nerve fibers are also present, part of what ble, and any important differences are mentioned in
is called the enteric nervous system, or the “brain of the sections that follow on specific organs.
the gut,” which extends the entire length of the ali-
mentary tube. The nerve networks in the submucosa STOMACH
are called Meissner’s plexus (or submucosal plexus),
and they innervate the mucosa to regulate secretions. The stomach is located in the upper left quadrant of
Parasympathetic impulses increase secretions, whereas the abdominal cavity, to the left of the liver and in
sympathetic impulses decrease secretions. Sensory front of the spleen. Although part of the alimentary
neurons are also present to the smooth muscle (a tube, the stomach is not a tube, but rather a sac that
stretched or cramping gut is painful), as are motor extends from the esophagus to the small intestine.
neurons to blood vessels, to regulate vessel diameter Because it is a sac, the stomach is a reservoir for food,
and blood flow. so that digestion proceeds gradually and we do not
have to eat constantly. Both mechanical and chemical
EXTERNAL MUSCLE LAYER digestion take place in the stomach.

The external muscle layer typically contains two layers The parts of the stomach are shown in Fig. 16–5.
of smooth muscle: an inner, circular layer and an outer, The cardiac orifice is the opening of the esophagus,
longitudinal layer. Variations from the typical do and the fundus is the portion above the level of this
occur, however. In the esophagus, this layer is striated opening. The body of the stomach is the large central
muscle in the upper third, which gradually changes to portion, bounded laterally by the greater curvature and
smooth muscle in the lower portions. The stomach has medially by the lesser curvature. The pylorus is adja-
three layers of smooth muscle, rather than two. cent to the duodenum of the small intestine, and the
pyloric sphincter surrounds the junction of the two
Contractions of this muscle layer help break up organs. The fundus and body are mainly storage areas,
food and mix it with digestive juices. The one-way whereas most digestion takes place in the pylorus.
contractions of peristalsis move the food toward the
anus. Auerbach’s plexus (or myenteric plexus) is the When the stomach is empty, the mucosa appears
portion of the enteric nervous system in this layer, and wrinkled or folded. These folds are called rugae; they
some of its millions of neurons are autonomic. Sympa- flatten out as the stomach is filled and permit expan-
thetic impulses decrease contractions and peristalsis, sion of the lining without tearing it. The gastric pits
whereas parasympathetic impulses increase contrac- are the glands of the stomach and consist of several

Copyright © 2007 by F. A. Davis.

The Digestive System 377

Mucous cell Esophagus Fundus of stomach
Parietal cell

Chief cell Longitudinal muscle
G cell layer

B Cardiac orifice Circular muscle layer

Lesser curvature Oblique muscle layer
Pyloric sphincter
Body
Greater curvature

A

Duodenum Rugae

Pylorus

Figure 16–5. (A) The stomach in anterior view. The stomach wall has been sectioned to
show the muscle layers and the rugae of the mucosa. (B) Gastric pits (glands) showing the
types of cells present. See text for functions.
QUESTION: What is the function of the pyloric sphincter?

types of cells; their collective secretions are called gas- of proteins to polypeptides, and also gives gastric juice
tric juice. Mucous cells secrete mucus, which coats its pH of 1 to 2. This very acidic pH is necessary for
the stomach lining and helps prevent erosion by the pepsin to function and also kills most microorganisms
gastric juice. Chief cells secrete pepsinogen, an inac- that enter the stomach. The parietal cells also secrete
tive form of the enzyme pepsin. Parietal cells pro- intrinsic factor, which is necessary for the absorption
duce hydrochloric acid (HCl); these cells have of vitamin B12. Enteroendocrine cells called G cells
enzymes called proton pumps, which secrete H+ ions secrete the hormone gastrin.
into the stomach cavity. The H+ ions unite with Cl–
ions that have diffused from the parietal cells to form Gastric juice is secreted in small amounts at the
HCl in the lumen of the stomach. HCl converts sight or smell of food. This is a parasympathetic
pepsinogen to pepsin, which then begins the digestion response that ensures that some gastric juice will be
present in the stomach when food arrives. The pres-

Copyright © 2007 by F. A. Davis.

378 The Digestive System

ence of food in the stomach causes the G cells to Within the abdominal cavity, the large intestine encir-
secrete gastrin, a hormone that stimulates the secre- cles the coils of the small intestine (see Fig. 16–1).
tion of greater amounts of gastric juice.
The duodenum is the first 10 inches (25 cm) of the
The external muscle layer of the stomach consists small intestine. The common bile duct enters the duo-
of three layers of smooth muscle: circular, longitudi- denum at the ampulla of Vater (or hepatopancreatic
nal, and oblique layers. These three layers are inner- ampulla). The jejunum is about 8 feet long, and the
vated by the myenteric plexuses of the enteric nervous ileum is about 11 feet in length. In a living person,
system. Stimulatory impulses are carried from the however, the small intestine is always contracted and is
CNS by the vagus nerves (10th cranial) and provide therefore somewhat shorter.
for very efficient mechanical digestion to change food
into a thick liquid called chyme. The pyloric sphincter Digestion is completed in the small intestine, and
is usually contracted when the stomach is churning the end products of digestion are absorbed into the
food; it relaxes at intervals to permit small amounts of blood and lymph. The mucosa (see Fig. 16–4) has
chyme to pass into the duodenum. This sphincter then simple columnar epithelium that includes cells with
contracts again to prevent the backup of intestinal microvilli and goblet cells that secrete mucus.
contents into the stomach (see Box 16–1: Disorders of Enteroendocrine cells secrete the hormones of the
the Stomach). small intestine. Lymph nodules called Peyer’s patches
are especially abundant in the ileum to destroy
SMALL INTESTINE absorbed pathogens. The external muscle layer has the
typical circular and longitudinal smooth muscle layers
The small intestine is about 1 inch (2.5 cm) in diam- that mix the chyme with digestive secretions and pro-
eter and approximately 20 feet (6 m) long and extends pel the chyme toward the colon. Stimulatory impulses
from the stomach to the cecum of the large intestine. to the enteric nerves of these muscle layers are carried
by the vagus nerves. The waves of peristalsis, however,
can take place without stimulation by the central nerv-

BOX 16–1 DISORDERS OF THE STOMACH pyloric stenosis. Correcting this condition requires
surgery to widen the opening in the sphincter.
Vomiting is the expulsion of stomach and intes-
tinal contents through the esophagus and mouth. A gastric ulcer is an erosion of the mucosa of
Stimuli include irritation of the stomach, motion the stomach. Because the normal stomach lining is
sickness, food poisoning, or diseases such as menin- adapted to resist the corrosive action of gastric
gitis. The vomiting center is in the medulla, which juice, ulcer formation is the result of oversecretion
coordinates the simultaneous contraction of the of HCl or undersecretion of mucus.
diaphragm and the abdominal muscles. This
squeezes the stomach and upper intestine, expell- As erosion reaches the submucosa, small blood
ing their contents. As part of the reflex, the lower vessels are ruptured and bleed. If vomiting occurs,
esophageal sphincter relaxes, and the glottis closes. the vomitus has a “coffee-ground” appearance due
If the glottis fails to close, as may happen in alcohol to the presence of blood acted on by gastric juice.
or drug intoxication, aspiration of vomitus may A more serious complication is perforation of the
occur and result in fatal obstruction of the respira- stomach wall, with leakage of gastric contents into
tory passages. the abdominal cavity, and peritonitis.

Pyloric stenosis means that the opening of the The bacterium called Helicobacter pylori is the
pyloric sphincter is narrowed, and emptying of the cause of most gastric ulcers. For many patients, a
stomach is impaired. This is most often a congeni- few weeks of antibiotic therapy to eradicate this
tal disorder caused by hypertrophy of the pyloric bacterium has produced rapid healing of their
sphincter. For reasons unknown, this condition is ulcers. This bacterium also seems to be responsible
more common in male infants than in female for virtually all cases of stomach cancer.
infants. When the stomach does not empty effi-
ciently, its internal pressure increases. Vomiting The medications that decrease the secretion of
relieves the pressure; this is a classic symptom of HCl are useful for ulcer patients not helped by
antibiotics.

Copyright © 2007 by F. A. Davis.

The Digestive System 379

ous system; the enteric nervous system can function rizes the regulation of secretion of all digestive secre-
independently and promote normal peristalsis. tions.

There are three sources of digestive secretions that GALLBLADDER
function within the small intestine: the liver, the pan-
creas, and the small intestine itself. We will return to The gallbladder is a sac about 3 to 4 inches (7.5 to 10
the small intestine after considering these other cm) long located on the undersurface of the right lobe
organs. of the liver. Bile in the hepatic duct of the liver flows
through the cystic duct into the gallbladder (see Fig.
LIVER 16–6), which stores bile until it is needed in the small
intestine. The gallbladder also concentrates bile by
The liver (Fig. 16–6) consists of two large lobes, right absorbing water (see Box 16–2: Gallstones).
and left, and fills the upper right and center of the
abdominal cavity, just below the diaphragm. The When fatty foods enter the duodenum, the
structural unit of the liver is the liver lobule, a enteroendocrine cells of the duodenal mucosa secrete
roughly hexagonal column of liver cells (hepatocytes). the hormone cholecystokinin. This hormone stimu-
Between adjacent lobules are branches of the hepatic lates contraction of the smooth muscle in the wall of
artery and portal vein. The capillaries of a lobule are the gallbladder, which forces bile into the cystic duct,
sinusoids, large and very permeable vessels between then into the common bile duct, and on into the duo-
the rows of liver cells. The sinusoids receive blood denum.
from both the hepatic artery and portal vein, and it is
with this mixture of blood that the liver cells carry out PANCREAS
their functions. The hepatic artery brings oxygenated
blood, and the portal vein brings blood from the The pancreas is located in the upper left abdominal
digestive organs and spleen (see Fig. 13–7). Each lob- quadrant between the curve of the duodenum and the
ule has a central vein. The central veins of all the lob- spleen and is about 6 inches (15 cm) in length. The
ules unite to form the hepatic veins, which take blood endocrine functions of the pancreas were discussed in
out of the liver to the inferior vena cava. Chapter 10, so only the exocrine functions will be
considered here. The exocrine glands of the pancreas
The cells of the liver have many functions (which are called acini (singular: acinus) (Fig. 16–7). They
are discussed in a later section), but their only diges- produce enzymes that are involved in the digestion of
tive function is the production of bile. Bile enters the all three types of complex food molecules.
small bile ducts, called bile canaliculi, on the liver
cells, which unite to form larger ducts and finally The pancreatic enzyme amylase digests starch to
merge to form the hepatic duct, which takes bile out maltose. You may recall that this is the “backup”
of the liver (see Fig. 16–6). The hepatic duct unites enzyme for salivary amylase, though pancreatic amy-
with the cystic duct of the gallbladder to form the lase is responsible for most digestion of starch. Lipase
common bile duct, which takes bile to the duode- converts emulsified fats to fatty acids and glycerol.
num. The emulsifying or fat-separating action of bile salts
increases the surface area of fats so that lipase works
Bile is mostly water and has an excretory function effectively. Trypsinogen is an inactive enzyme that is
in that it carries bilirubin and excess cholesterol to the changed to active trypsin in the duodenum. Trypsin
intestines for elimination in feces. The digestive func- digests polypeptides to shorter chains of amino acids.
tion of bile is accomplished by bile salts, which emul-
sify fats in the small intestine. Emulsification means The pancreatic enzyme juice is carried by small
that large fat globules are broken into smaller glob- ducts that unite to form larger ducts, then finally the
ules. This is mechanical, not chemical, digestion; the main pancreatic duct. An accessory duct may also be
fat is still fat but now has more surface area to facili- present. The main pancreatic duct emerges from the
tate chemical digestion. medial side of the pancreas and joins the common bile
duct to the duodenum (see Fig. 16–7).
Production of bile is stimulated by the hormone
secretin, which is produced by the duodenum when The pancreas also produces a bicarbonate juice
food enters the small intestine. Table 16–2 summa- (containing sodium bicarbonate), which is alkaline.

Copyright © 2007 by F. A. Davis. Inferior vena cava

Right hepatic vein Left hepatic vein
Liver

Cystic
duct

Hepatic duct

Common bile duct

Hepatic artery
Portal vein

A Central vein
(branch of
Gallbladder hepatic vein)

Sinusoids Liver lobule

Bile duct

Branch of portal vein
Branch of hepatic artery

Bile duct
Hepatocytes

B

Figure 16–6. (A) The liver and gallbladder with blood vessels and bile ducts.
(B) Magnified view of one liver lobule. See text for description.
QUESTION: In part B, trace the pathway of blood flow through a liver lobule.

380

Copyright © 2007 by F. A. Davis.

The Digestive System 381

Table 16–2 REGULATION OF DIGESTIVE SECRETIONS

Secretion Nervous Regulation Chemical Regulation
Saliva Presence of food in mouth or sight of food;
None
Gastric juice parasympathetic impulses along 7th and
9th cranial nerves Gastrin—produced by the G cells of
Bile Sight or smell of food; parasympathetic the gastric mucosa when food is
Secretion by impulses along 10th cranial nerves present in the stomach
the liver
None Secretin—produced by the enteroen-
Contraction of docrine cells of the duodenum
the gallbladder None when chyme enters

Enzyme pancreatic juice None Cholecystokinin—produced by the
Bicarbonate pancreatic juice None enteroendocrine cells of the duo-
Intestinal juice Presence of chyme in the duodenum; parasym- denum when chyme enters

pathetic impulses along 10th cranial nerves Cholecystokinin—from the duodenum
Secretin—from the duodenum
None

Because the gastric juice that enters the duodenum cystokinin stimulates the secretion of the pancreatic
is very acidic, it must be neutralized to prevent dam- enzymes.
age to the duodenal mucosa. This neutralizing is
accomplished by the sodium bicarbonate in pancreatic COMPLETION OF DIGESTION
juice, and the pH of the duodenal chyme is raised to AND ABSORPTION
about 7.5.
SMALL INTESTINE
Secretion of pancreatic juice is stimulated by the The secretion of the epithelium of the intestinal
hormones secretin and cholecystokinin, which are glands (or crypts of Lieberkühn) is stimulated by the
produced by the duodenal mucosa when chyme enters
the small intestine. Secretin stimulates the produc-
tion of bicarbonate juice by the pancreas, and chole-

BOX 16–2 GALLSTONES Several treatments are available for gallstones.
Medications that dissolve gallstones work slowly,
One of the functions of the gallbladder is to con- over the course of several months, and are useful if
centrate bile by absorbing water. If the bile contains biliary obstruction is not severe. An instrument that
a high concentration of cholesterol, absorption of generates shock waves (called a lithotripter) may be
water may lead to precipitation and the formation of used to pulverize the stones into smaller pieces that
cholesterol crystals. These crystals are gallstones. may easily pass into the duodenum; this procedure
is called lithotripsy. Surgery to remove the gall-
If the gallstones are small, they will pass through bladder (cholecystectomy) is required in some
the cystic duct and common bile duct to the duo- cases. The hepatic duct is then connected directly
denum without causing symptoms. If large, how- to the common bile duct, and dilute bile flows into
ever, the gallstones cannot pass out of the the duodenum. Following such surgery, the patient
gallbladder, and may cause mild to severe pain that should avoid meals high in fats.
often radiates to the right shoulder. Obstructive
jaundice may occur if bile backs up into the liver
and bilirubin is reabsorbed into the blood.

Copyright © 2007 by F. A. Davis.

382 The Digestive System

Hepatic duct Pyloric sphincter

Cystic duct Main
pancreatic
Common duct
bile duct

Duodenum Pancreas

Ampulla of Vater Superior mesenteric
A artery and vein

Islets of
Langerhans

Ducts
Acini
Capillaries

Delta cell
Beta cell
Alpha cell

B

Figure 16–7. (A) The pancreas, sectioned to show the pancreatic ducts. The main pan-
creatic duct joins the common bile duct. (B) Microscopic section showing acini with their
ducts and several islets of Langerhans.
QUESTION: In part B, what do the acini secrete?

presence of food in the duodenum. The intestinal digest the disaccharides sucrose, maltose, and lactose
enzymes are the peptidases and sucrase, maltase, and to monosaccharides.
lactase. Peptidases complete the digestion of protein
by breaking down short polypeptide chains to amino The enteroendocrine cells of the intestinal glands
acids. Sucrase, maltase, and lactase, respectively, secrete the hormones of the small intestine. Secretion
is stimulated by food entering the duodenum.