Essentials of Anatomy and Physiology

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The Heart 283


fective for pumping blood (see Box 12–4: Arrhyth- stroke volume is 60 to 80 mL per beat. A simple for-
mias). mula then enables us to determine cardiac output:
Cardiac output stroke volume pulse (heart rate)

HEART RATE Let us put into this formula an average resting
stroke volume, 70 mL, and an average resting pulse,
A healthy adult has a resting heart rate (pulse) of 60 to 70 beats per minute (bpm):
80 beats per minute, which is the rate of depolariza- Cardiac output 70 mL 70 bpm
tion of the SA node. (The SA node actually has a
slightly faster rate, closer to 100 beats per minute, but Cardiac output 4900 mL per minute
is slowed by parasympathetic nerve impulses to what (approximately 5 liters)
we consider a normal resting rate.) A rate less than 60 Naturally, cardiac output varies with the size of the
(except for athletes) is called bradycardia; a prolonged person, but the average resting cardiac output is 5 to 6
or consistent rate greater than 100 beats per minute is liters per minute. Notice that this amount is just about
called tachycardia. the same as a person’s average volume of blood. At
A child’s normal heart rate may be as high as 100
beats per minute, that of an infant as high as 120, and rest, the heart pumps all of the blood in the body
within about a minute. Changes are possible, depend-
that of a near-term fetus as high as 140 beats per ing on circumstances and extent of physical activity.
minute. These higher rates are not related to age, but If we now reconsider the athlete, you will be able to
rather to size: the smaller the individual, the higher the see precisely why the athlete has a low resting pulse. In
metabolic rate and the faster the heart rate. Parallels our formula, we will use an average resting cardiac
may be found among animals of different sizes; the output (5 liters) and an athlete’s pulse rate (50):
heart rate of a mouse is about 200 beats per minute
and that of an elephant about 30 beats per minute. Cardiac output stroke volume pulse
Let us return to the adult heart rate and consider 5000 mL stroke volume 50 bpm
the person who is in excellent physical condition. As
you may know, well-conditioned athletes have low 5000/50 stroke volume
resting pulse rates. Those of basketball players are 100 mL stroke volume
often around 50 beats per minute, and the pulse of a
marathon runner often ranges from 35 to 40 beats per Notice that the athlete’s resting stroke volume is
minute. To understand why this is so, remember that significantly higher than the average. The athlete’s
the heart is a muscle. When our skeletal muscles are more efficient heart pumps more blood with each beat
exercised, they become stronger and more efficient. and so can maintain a normal resting cardiac output
The same is true for the heart; consistent exercise with fewer beats.
makes it a more efficient pump, as you will see in the Now let us see how the heart responds to exercise.
next section. Heart rate (pulse) increases during exercise, and so
does stroke volume. The increase in stroke volume is
the result of Starling’s law of the heart, which states
CARDIAC OUTPUT that the more the cardiac muscle fibers are stretched,
the more forcefully they contract. During exercise,
Cardiac output is the amount of blood pumped by a more blood returns to the heart; this is called venous
ventricle in 1 minute. A certain level of cardiac output return. Increased venous return stretches the myocar-
is needed at all times to transport oxygen to tissues and dium of the ventricles, which contract more forcefully
to remove waste products. During exercise, cardiac and pump more blood, thereby increasing stroke vol-
output must increase to meet the body’s need for more ume. Therefore, during exercise, our formula might
oxygen. We will return to exercise after first consider- be the following:
ing resting cardiac output. Cardiac output stroke volume pulse
To calculate cardiac output, we must know the
pulse rate and how much blood is pumped per beat. Cardiac output 100 mL 100 bpm
Stroke volume is the term for the amount of blood
pumped by a ventricle per beat; an average resting Cardiac output 10,000 mL (10 liters)

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284 The Heart



Table 12–2 PHYSIOLOGY OF THE HEART
Aspect and
Normal Range Description
Heart rate (pulse): Generated by the SA node, propagated through the conduction pathway; parasympathetic
60–80 bpm impulses (vagus nerves) decrease the rate; sympathetic impulses increase the rate
Stroke volume: The amount of blood pumped by a ventricle in one beat
60–80 mL/beat
Cardiac output: The volume of blood pumped by a ventricle in 1 minute; stroke volume x pulse
5–6 L/min
Ejection fraction: The percentage of blood within a ventricle that is pumped out per beat
60%–70%
Cardiac reserve: The difference between resting cardiac output and maximum cardiac output during exercise
15 liters or more



This exercise cardiac output is twice the resting to different situations. The nervous system can and
cardiac output we first calculated, which should not be does bring about necessary changes in heart rate as
considered unusual. The cardiac output of a healthy well as in force of contraction.
young person may increase up to four times the rest- The medulla of the brain contains the two cardiac
ing level during strenuous exercise. This difference is centers, the accelerator center and the inhibitory
the cardiac reserve, the extra volume the heart can center. These centers send impulses to the heart
pump when necessary. If resting cardiac output is 5 along autonomic nerves. Recall from Chapter 8 that
liters and exercise cardiac output is 20 liters, the car- the autonomic nervous system has two divisions: sym-
diac reserve is 15 liters. The marathon runner’s cardiac pathetic and parasympathetic. Sympathetic impulses
output may increase six times or more compared to from the accelerator center along sympathetic nerves
the resting level, and cardiac reserve is even greater increase heart rate and force of contraction during
than for the average young person; this is the result of exercise and stressful situations. Parasympathetic
the marathoner’s extremely efficient heart. Because of impulses from the inhibitory center along the vagus
Starling’s law, it is almost impossible to overwork a nerves decrease the heart rate. At rest these impulses
healthy heart. No matter how much the volume of slow down the depolarization of the SA node to what
venous return increases, the ventricles simply pump we consider a normal resting rate, and they also slow
more forcefully and increase the stroke volume and the heart after exercise is over.
cardiac output. Our next question might be: What information is
Also related to cardiac output, and another measure received by the medulla to initiate changes? Because
of the health of the heart, is the ejection fraction. the heart pumps blood, it is essential to maintain nor-
This is the percent of the blood in a ventricle that is mal blood pressure. Blood contains oxygen, which all
pumped during systole. A ventricle does not empty tissues must receive continuously. Therefore, changes
completely when it contracts, but should pump out in blood pressure and oxygen level of the blood are
60% to 70% of the blood within it. A lower percent- stimuli for changes in heart rate.
age would indicate that the ventricle is weakening. You may also recall from Chapter 9 that presso-
These aspects of physiology are summarized in Table receptors and chemoreceptors are located in the
12–2. carotid arteries and aortic arch. Pressoreceptors in
the carotid sinuses and aortic sinus detect changes in
blood pressure. Chemoreceptors in the carotid
REGULATION OF HEART RATE bodies and aortic body detect changes in the oxygen
content of the blood. The sensory nerves for the
Although the heart generates and maintains its own carotid receptors are the glossopharyngeal (9th cra-
beat, the rate of contraction can be changed to adapt nial) nerves; the sensory nerves for the aortic arch

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The Heart 285



Carotid sinus and
carotid body

Glossopharyngeal
nerves
Common
carotid
arteries


Vagus (sensory) Aortic arch
nerve
Aortic sinus
and aortic body


Accelerator center Vagus (motor) nerves
Inhibitory center
SA node
Medulla
Bundle of His
Sympathetic nerves AV node
Thoracic spinal cord



Right ventricle


Figure 12–7. Nervous regulation of the heart. The brain and spinal cord are shown on
the left. The heart and major blood vessels are shown on the right.
QUESTION: Sympathetic impulses to the ventricles will have what effect?

receptors are the vagus (10th cranial) nerves. If we When blood pressure to the brain is restored to nor-
now put all of these facts together in a specific exam- mal, the heart receives more parasympathetic impulses
ple, you will see that the regulation of heart rate is a from the inhibitory center along the vagus nerves to
reflex. Figure 12–7 depicts all of the structures just the SA node and AV node. These parasympathetic
mentioned. impulses slow the heart rate to a normal resting pace.
A person who stands up suddenly from a lying posi- The heart will also be the effector in a reflex stim-
tion may feel light-headed or dizzy for a few moments, ulated by a decrease in the oxygen content of the
because blood pressure to the brain has decreased blood. The aortic receptors are strategically located so
abruptly. The drop in blood pressure is detected by as to detect such an important change as soon as blood
pressoreceptors in the carotid sinuses—notice that leaves the heart. The reflex arc in this situation would
they are “on the way” to the brain, a very strategic be (1) aortic chemoreceptors, (2) vagus nerves (sen-
location. The drop in blood pressure causes fewer sory), (3) accelerator center in the medulla, (4) sympa-
impulses to be generated by the pressoreceptors. thetic nerves, and (5) the heart muscle, which will
These impulses travel along the glossopharyngeal increase its rate and force of contraction to circulate
nerves to the medulla, and the decrease in the fre- more oxygen to correct the hypoxia.
quency of impulses stimulates the accelerator center. Recall also from Chapter 10 that the hormone
The accelerator center generates impulses that are car- epinephrine is secreted by the adrenal medulla in
ried by sympathetic nerves to the SA node, AV node, stressful situations. One of the many functions of epi-
and ventricular myocardium. As heart rate and force nephrine is to increase heart rate and force of contrac-
increase, blood pressure to the brain is raised to nor- tion. This will help supply more blood to tissues in
mal, and the sensation of light-headedness passes. need of more oxygen.

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286 The Heart


sion may be slow, or may be rapid. The heart valves
AGING AND THE HEART
may become thickened by fibrosis, leading to heart
murmurs and less efficient pumping. Arrhythmias are
The heart muscle becomes less efficient with age, and also more common with age, as the cells of the con-
there is a decrease in both maximum cardiac output duction pathway become less efficient.
and heart rate, although resting levels may be more
than adequate. The health of the myocardium
depends on its blood supply, and with age there is
greater likelihood that atherosclerosis will narrow the SUMMARY
coronary arteries. Atherosclerosis is the deposition of
cholesterol on and in the walls of the arteries, which As you can see, the nervous system regulates the func-
decreases blood flow and forms rough surfaces that tioning of the heart based on what the heart is sup-
may cause intravascular clot formation. posed to do. The pumping of the heart maintains
High blood pressure (hypertension) causes the left normal blood pressure and proper oxygenation of tis-
ventricle to work harder; it may enlarge and outgrow sues, and the nervous system ensures that the heart
its blood supply, thus becoming weaker. A weak ven- will be able to meet these demands in different situa-
tricle is not an efficient pump, and such weakness may tions. Blood pressure and the blood vessels are the
progress to congestive heart failure; such a progres- subjects of the next chapter.




STUDY OUTLINE

The heart pumps blood, which creates blood 3. The right and left atria are the upper chambers,
pressure, and circulates oxygen, nutrients, separated by the interatrial septum. The atria
and other substances. The heart is located in receive blood from veins.
the mediastinum, the area between the 4. The right and left ventricles are the lower cham-
lungs in the thoracic cavity. bers, separated by the interventricular septum. The
ventricles pump blood into arteries.
Pericardial Membranes—three layers that
enclose the heart (see Fig. 12–1)
Right Atrium
1. The outer, fibrous pericardium, made of fibrous 1. Receives blood from the upper body by way of the
connective tissue, is a loose-fitting sac that sur- superior vena cava and receives blood from the
rounds the heart and extends over the diaphragm lower body by way of the inferior vena cava.
and the bases of the great vessels. 2. The tricuspid (right AV) valve prevents backflow of
2. The parietal pericardium is a serous membrane blood from the right ventricle to the right atrium
that lines the fibrous pericardium. when the right ventricle contracts.
3. The visceral pericardium, or epicardium, is a serous
membrane on the surface of the myocardium.
Left Atrium
4. Serous fluid between the parietal and visceral peri-
1. Receives blood from the lungs by way of four pul-
cardial membranes prevents friction as the heart
monary veins.
beats.
2. The mitral (left AV or bicuspid) valve prevents
Chambers of the Heart (see Fig. 12–2 and backflow of blood from the left ventricle to the left
Table 12–1) atrium when the left ventricle contracts.
1. Cardiac muscle tissue, the myocardium, forms the 3. The walls of the atria produce atrial natriuretic
walls of the four chambers of the heart. peptide when stretched by increased blood volume
2. Endocardium lines the chambers and covers the or BP. ANP increases the loss of Na ions and
valves of the heart; is simple squamous epithelium water in urine, which decreases blood volume and
that is very smooth and prevents abnormal clotting. BP to normal.

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The Heart 287


Right Ventricle—has relatively thin walls 4. Ventricular contraction pumps all blood into the
1. Pumps blood to the lungs through the pulmonary arteries. The ventricles then relax. Meanwhile,
artery. blood is filling the atria, and the cycle begins again.
2. The pulmonary semilunar valve prevents backflow 5. Systole means contraction; diastole means relax-
of blood from the pulmonary artery to the right ation. In the cardiac cycle, atrial systole is followed
ventricle when the right ventricle relaxes. by ventricular systole. When the ventricles are in
3. Papillary muscles and chordae tendineae prevent systole, the atria are in diastole.
inversion of the right AV valve when the right ven- 6. The mechanical events of the cardiac cycle keep
tricle contracts. blood moving from the veins through the heart and
into the arteries.
Left Ventricle—has thicker walls than does
the right ventricle Heart Sounds—two sounds per heartbeat:
1. Pumps blood to the body through the aorta. lub-dup
2. The aortic semilunar valve prevents backflow of 1. The first sound is created by closure of the AV
blood from the aorta to the left ventricle when the valves during ventricular systole.
left ventricle relaxes. 2. The second sound is created by closure of the aor-
3. Papillary muscles and chordae tendineae prevent tic and pulmonary semilunar valves.
inversion of the left AV valve when the left ventri- 3. Improper closing of a valve results in a heart mur-
cle contracts. mur.
4. The heart is a double pump: The right side of the
heart receives deoxygenated blood from the body Cardiac Conduction Pathway—the pathway
and pumps it to the lungs; the left side of the heart of impulses during the cardiac cycle (see Fig.
receives oxygenated blood from the lungs and 12–6)
pumps it to the body. Both sides of the heart work 1. The SA node in the wall of the right atrium initi-
simultaneously.
ates each heartbeat; the cells of the SA node are

more permeable to Na ions and depolarize more
Coronary Vessels (see Fig. 12–4) rapidly than any other part of the myocardium.
1. Pathway: ascending aorta to right and left coronary 2. The AV node is in the lower interatrial septum.
arteries, to smaller arteries, to capillaries, to coro- Depolarization of the SA node spreads to the AV
nary veins, to the coronary sinus, to the right node and to the atrial myocardium and brings
atrium. about atrial systole.
2. Coronary circulation supplies oxygenated blood to 3. The AV bundle (bundle of His) is in the upper
the myocardium. interventricular septum; the first part of the ventri-
3. Obstruction of a coronary artery causes a myocar- cles to depolarize.
dial infarction: death of an area of myocardium due 4. The right and left bundle branches in the interven-
to lack of oxygen. tricular septum transmit impulses to the Purkinje
fibers in the ventricular myocardium, which com-
Cardiac Cycle—the sequence of events in one plete ventricular systole.
heartbeat (see Fig. 12–5) 5. An electrocardiogram (ECG) depicts the electrical
1. The atria continually receive blood from the veins; activity of the heart (see Fig. 12–6).
as pressure within the atria increases, the AV valves 6. If part of the conduction pathway does not function
are opened. properly, the next part will initiate contraction, but
2. Two-thirds of the atrial blood flows passively at a slower rate.
into the ventricles; atrial contraction pumps the 7. Arrhythmias are irregular heartbeats; their effects
remaining blood into the ventricles; the atria then range from harmless to life-threatening.
relax.
3. The ventricles contract, which closes the AV valves Heart Rate
and opens the aortic and pulmonary semilunar 1. Healthy adult: 60 to 80 beats per minute (heart rate
valves. equals pulse); children and infants have faster

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288 The Heart


pulses because of their smaller size and higher Regulation of Heart Rate (see Fig. 12–7)
metabolic rate. 1. The heart generates its own beat, but the nervous
2. A person in excellent physical condition has a slow system brings about changes to adapt to different
resting pulse because the heart is a more efficient situations.
pump and pumps more blood per beat. 2. The medulla contains the cardiac centers: the
accelerator center and the inhibitory center.
Cardiac Output (see Table 12–2) 3. Sympathetic impulses to the heart increase rate and
1. Cardiac output is the amount of blood pumped by force of contraction; parasympathetic impulses
a ventricle in 1 minute. (vagus nerves) to the heart decrease heart rate.
2. Stroke volume is the amount of blood pumped by a 4. Pressoreceptors in the carotid and aortic sinuses
ventricle in one beat; average is 60 to 80 mL. detect changes in blood pressure.
3. Cardiac output equals stroke volume pulse; aver- 5. Chemoreceptors in the carotid and aortic bodies
age resting cardiac output is 5 to 6 liters. detect changes in the oxygen content of the blood.
4. Starling’s law of the heart—the more cardiac mus- 6. The glossopharyngeal nerves are sensory for the
cle fibers are stretched, the more forcefully they carotid receptors. The vagus nerves are sensory for
contract. the aortic receptors.
5. During exercise, stroke volume increases as venous 7. If blood pressure to the brain decreases, presso-
return increases and stretches the myocardium of receptors in the carotid sinuses detect this decrease
the ventricles (Starling’s law). and send fewer sensory impulses along the glos-
6. During exercise, the increase in stroke volume and sopharyngeal nerves to the medulla. The accelera-
the increase in pulse result in an increase in cardiac tor center dominates and sends motor impulses
output: two to four times the resting level. along sympathetic nerves to increase heart rate and
7. Cardiac reserve is the difference between resting force of contraction to restore blood pressure to
cardiac output and the maximum cardiac output; normal.
may be 15 liters or more. 8. A similar reflex is activated by hypoxia.
8. The ejection fraction is the percent of its total 9. Epinephrine from the adrenal medulla increases
blood that a ventricle pumps per beat; average is heart rate and force of contraction during stressful
60% to 70%. situations.




REVIEW QUESTIONS

1. Describe the location of the heart with respect to 7. Describe the coronary system of vessels and
the lungs and to the diaphragm. (p. 274) explain the purpose of coronary circulation. (p.
278)
2. Name the three pericardial membranes. Where
is serous fluid found and what is its function? 8. Define systole, diastole, and cardiac cycle.
(p. 274) (p. 278)

3. Describe the location and explain the function of 9. Explain how movement of blood from atria to
endocardium. (p. 274) ventricles differs from movement of blood from
ventricles to arteries. (p. 279)
4. Name the veins that enter the right atrium; name 10. Explain why the heart is considered a double
those that enter the left atrium. For each, where pump. Trace the path of blood from the right
does the blood come from? (p. 275)
atrium back to the right atrium, naming the
5. Name the artery that leaves the right ventricle; chambers of the heart and their vessels through
name the artery that leaves the left ventricle. For which the blood passes. (pp. 275, 277)
each, where is the blood going? (p. 275)
11. Name the parts, in order, of the cardiac conduc-
6. Explain the purpose of the right and left AV valves tion pathway. Explain why it is the SA node that
and the purpose of the aortic and pulmonary semi- generates each heartbeat. State a normal range of
lunar valves. (p. 275) heart rate for a healthy adult. (pp. 279–283)

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The Heart 289


12. Calculate cardiac output if stroke volume is 75 have what effect? Name the parasympathetic
mL and pulse is 75 bpm. Using the cardiac output nerves to the heart. (p. 284)
you just calculated as a resting normal, what is the 14. State the locations of arterial pressoreceptors and
stroke volume of a marathoner whose resting chemoreceptors, what they detect, and their sen-
pulse is 40 bpm? (p. 283) sory nerves. (p. 284)
13. Name the two cardiac centers and state their loca- 15. Describe the reflex arc to increase heart rate and
tion. Sympathetic impulses to the heart have what force when blood pressure to the brain decreases.
effect? Parasympathetic impulses to the heart (p. 285)




FOR FURTHER THOUGHT

1. Endocarditis may be caused by bacteria or fungi 3. A neighbor, Mrs. G., age 62, tells you that she
that erode, or wear away, the heart valves, or in “doesn’t feel right” and is suddenly tired for no
some cases make the valves bumpy (these bumps apparent reason. She denies having chest pain,
are called vegetations—think cauliflower). Explain though she admits to a “full” feeling she calls indi-
the possible consequences of this. gestion. You suspect that she may be having a heart
attack. What question can you ask to help you be
2. Bob, a college freshman, is telling his new friends more sure? Explain the physiological basis for your
that he has been running seriously for 6 years, question.
and can run a marathon in a little over 3 hours. His
friends aren’t sure they should believe him, but 4. Several types of artificial hearts are being devel-
don’t want to spend 3 hours waiting while Bob oped and tested. What are the three essential
runs 26 miles. Bob says that he can prove he is characteristics a truly useful artificial heart must
telling the truth in 1 minute. Can he? Explain why have? One is obvious, the others, perhaps not as
or why not. much so.

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C


















CHAPTER 13





Chapter Outline Student Objectives
Arteries • Describe the structure of arteries and veins, and
Veins relate their structure to function.
Anastomoses • Explain the purpose of arterial and venous anasto-
Capillaries moses.
Exchanges in Capillaries • Describe the structure of capillaries, and explain
Pathways of Circulation the exchange processes that take place in capillar-
Pulmonary Circulation ies.
Systemic Circulation • Describe the pathway and purpose of pulmonary
Hepatic Portal Circulation circulation.
Fetal Circulation • Name the branches of the aorta and their distribu-
Velocity of Blood Flow tions.
Blood Pressure • Name the major systemic veins, and the parts of
Maintenance of Systemic Blood Pressure the body they drain of blood.
Distribution of Blood Flow • Describe the pathway and purpose of hepatic por-
Regulation of Blood Pressure tal circulation.
Intrinsic Mechanisms • Describe the modifications of fetal circulation, and
Nervous Mechanisms explain the purpose of each.
Aging and the Vascular System • Explain the importance of slow blood flow in cap-
illaries.
BOX 13–1 DISORDERS OF ARTERIES • Define blood pressure, and state the normal
BOX 13–2 DISORDERS OF VEINS ranges for systemic and pulmonary blood pressure.
BOX 13–3 PULSE SITES • Explain the factors that maintain systemic blood
BOX 13–4 HYPERTENSION pressure.
BOX 13–5 CIRCULATORY SHOCK • Explain how the heart and kidneys are involved in
the regulation of blood pressure.
• Explain how the medulla and the autonomic nerv-
ous system regulate the diameter of blood vessels.








290

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The Vascular System






New Terminology Related Clinical Terminology
Anastomosis (a-NAS-ti-MOH-sis) Anaphylactic (AN-uh-fi-LAK-tik)
Arteriole (ar-TIR-ee-ohl) Aneurysm (AN-yur-izm)
Circle of Willis (SIR-kuhl of WILL-iss) Arteriosclerosis (ar-TIR-ee-oh-skle-ROH-sis)
Ductus arteriosus (DUK-tus ar-TIR-ee-OH-sis) Hypertension (HIGH-per-TEN-shun)
Foramen ovale (for-RAY-men oh-VAHL-ee) Hypovolemic (HIGH-poh-voh-LEEM-ik)
Hepatic portal (hep-PAT-ik POOR-tuhl) Phlebitis (fle-BY-tis)
Peripheral resistance (puh-RIFF-uh-ruhl ree-ZIS- Pulse deficit (PULS DEF-i-sit)
tense) Septic shock (SEP-tik SHAHK)
Placenta (pluh-SEN-tah) Varicose veins (VAR-i-kohs VAINS).
Precapillary sphincter (pre-KAP-i-lar-ee SFINK-ter)
Sinusoid (SIGH-nuh-soyd)
Umbilical arteries (uhm-BILL-i-kull AR-tuh-rees)
Umbilical vein (uhm-BILL-i-kull VAIN)
Venule (VEN-yool)



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























291

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292 The Vascular System




The role of blood vessels in the circulation of blood cle tissue brings about dilation of the vessel. Smooth
has been known since 1628, when William Harvey, an muscle also has a nerve supply; sympathetic nerve
English anatomist, demonstrated that blood in veins impulses bring about vasoconstriction. Fibrous con-
always flowed toward the heart. Before that time, it nective tissue forms the outer layer, the tunica
was believed that blood was static or stationary, some externa. This tissue is very strong, which is important
of it within the vessels but the rest sort of in puddles to prevent the rupture or bursting of the larger arter-
throughout the body. Harvey showed that blood ies that carry blood under high pressure (see Box 13–1:
indeed does move, and only in the blood vessels Disorders of Arteries).
(though he did not know of the existence of capillar- The outer and middle layers of large arteries are
ies). In the centuries that followed, the active (rather quite thick. In the smallest arterioles, only individual
than merely passive) roles of the vascular system were smooth muscle cells encircle the tunica intima. As
discovered, and all contribute to homeostasis. mentioned, the smooth muscle layer enables arteries
The vascular system consists of the arteries, capil- to constrict or dilate. Such changes in diameter are
laries, and veins through which the heart pumps blood regulated by the medulla and autonomic nervous sys-
throughout the body. As you will see, the major “busi- tem, and will be discussed in a later section on blood
ness” of the vascular system, which is the exchange of pressure.
materials between the blood and tissues, takes place in
the capillaries. The arteries and veins, however, are
just as important, transporting blood between the cap- VEINS
illaries and the heart.
Another important topic of this chapter will be
Veins carry blood from capillaries back to the heart;
blood pressure (BP), which is the force the blood
the smaller veins are called venules. The same three
exerts against the walls of the vessels. Normal blood
tissue layers are present in veins as in the walls of
pressure is essential for circulation and for some of the
arteries, but there are some differences when com-
material exchanges that take place in capillaries.
pared to the arterial layers. The inner layer of veins is
smooth endothelium, but at intervals this lining is
folded to form valves (see Fig. 13–1). Valves prevent
ARTERIES backflow of blood and are most numerous in veins of
the legs, where blood must often return to the heart
Arteries carry blood from the heart to capillaries; against the force of gravity.
smaller arteries are called arterioles. If we look at an The middle layer of veins is a thin layer of smooth
artery in cross-section, we find three layers (or tunics) muscle. It is thin because veins do not regulate blood
of tissues, each with different functions (Fig. 13–1). pressure and blood flow into capillaries as arteries do.
The innermost layer, the tunica intima, is the only Veins can constrict extensively, however, and this
part of a vessel that is in contact with blood. It is made function becomes very important in certain situations
of simple squamous epithelium called endothelium. such as severe hemorrhage. The outer layer of veins is
This lining is the same type of tissue that forms the also thin; not as much fibrous connective tissue is nec-
endocardium, the lining of the chambers of the heart. essary because blood pressure in veins is very low.
As you might guess, its function is also the same: Its
extreme smoothness prevents abnormal blood clot- ANASTOMOSES
ting. The endothelium of vessels, however, also pro-
duces nitric oxide (NO), which is a vasodilator. The An anastomosis is a connection, or joining, of vessels,
tunica media, or middle layer, is made of smooth that is, artery to artery or vein to vein. The general
muscle and elastic connective tissue. Both of these tis- purpose of these connections is to provide alternate
sues are involved in the maintenance of normal blood pathways for the flow of blood if one vessel becomes
pressure, especially diastolic blood pressure when the obstructed.
heart is relaxed. The smooth muscle is the tissue An arterial anastomosis helps ensure that blood
affected by the vasodilator NO; relaxation of this mus- will get to the capillaries of an organ to deliver oxygen

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The Vascular System 293


Endothelium (lining)
Internal elastic
Tunica lamina
External elastic media
lamina Artery
Tunica externa
Arteriole
Endothelial
cells
Smooth muscle
Precapillary
sphincter




Figure 13–1. Structure of Capillary
an artery, arteriole, capillary
network, venule, and vein.
See text for description. Tunica Blood flow
QUESTION: What tissue is the intima
tunica media made of, and Venule
how is this layer different in an Vein
artery and in a vein? Tunica
externa
Tunica
media













Valve

and nutrients and to remove waste products. There are actually the extension of the endothelium, the sim-
are arterial anastomoses, for example, between some ple squamous lining, of arteries and veins (see Fig.
of the coronary arteries that supply blood to the 13–1). Some tissues do not have capillaries; these are
myocardium. the epidermis, cartilage, and the lens and cornea of the
A venous anastomosis helps ensure that blood will eye.
be able to return to the heart in order to be pumped Most tissues, however, have extensive capillary net-
again. Venous anastomoses are most numerous among works. The quantity or volume of capillary networks
the veins of the legs, where the possibility of obstruc- in an organ reflects the metabolic activity of the organ.
tion increases as a person gets older (see Box 13–2: The functioning of the kidneys, for example, depends
Disorders of Veins). upon a good blood supply. Fig. 18–2 shows a vascular
cast of a kidney; you can see how dense the vessels are,
most of which are capillaries. In contrast, a tendon
CAPILLARIES such as the Achilles tendon at the heel or the patellar
tendon at the knee would have far fewer vessels,
Capillaries carry blood from arterioles to venules. because fibrous connective tissue is far less metaboli-
Their walls are only one cell in thickness; capillaries cally active.

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294 The Vascular System


BOX 13–1 DISORDERS OF ARTERIES

Arteriosclerosis—although commonly called The most common sites for aneurysm formation
“hardening of the arteries,” arteriosclerosis really are the cerebral arteries and the aorta, especi-
means that the arteries lose their elasticity, and their ally the abdominal aorta. Rupture of a cerebral
walls become weakened. Arteries carry blood under aneurysm is a possible cause of a cerebrovascular
high pressure, so deterioration of their walls is part accident (CVA). Rupture of an aortic aneurysm is
of the aging process. life-threatening and requires immediate corrective
Aneurysm—a weak portion of an arterial wall surgery. The damaged portion of the artery is
may bulge out, forming a sac or bubble called an removed and replaced with a graft. Such surgery
aneurysm. Arteriosclerosis is a possible cause, but may also be performed when an aneurysm is found
some aneurysms are congenital. An aneurysm may before it ruptures.
be present for many years without any symptoms Atherosclerosis—this condition has been men-
and may only be discovered during diagnostic pro- tioned previously; see Chapters 2 and 12.
cedures for some other purpose.



Blood flow into capillary networks is regulated by increase blood flow. These automatic responses ensure
smooth muscle cells called precapillary sphincters, that blood, the volume of which is constant, will circu-
found at the beginning of each network (see Fig. 13–1). late where it is needed most.
Precapillary sphincters are not regulated by the nerv- Some organs have another type of capillary called
ous system but rather constrict or dilate depending on sinusoids, which are larger and more permeable than
the needs of the tissues. Because there is not enough are other capillaries. The permeability of sinusoids
blood in the body to fill all of the capillaries at once, permits large substances such as proteins and blood
precapillary sphincters are usually slightly constricted. cells to enter or leave the blood. Sinusoids are found
In an active tissue that requires more oxygen, such as in the red bone marrow and spleen, where blood cells
exercising muscle, the precapillary sphincters dilate to enter or leave the blood, and in organs such as the



BOX 13–2 DISORDERS OF VEINS

Phlebitis—inflammation of a vein. This condition to pool in the leg veins, stretching their walls. If the
is most common in the veins of the legs, because veins become overly stretched, the valves within
they are subjected to great pressure as the blood is them no longer close properly. These incompetent
returned to the heart against the force of gravity. valves no longer prevent backflow of blood, leading
Often no specific cause can be determined, but to further pooling and even further stretching of
advancing age, obesity, and blood disorders may the walls of the veins. Varicose veins may cause dis-
be predisposing factors. comfort and cramping in the legs, or become even
If a superficial vein is affected, the area may be more painful. Severe varicosities may be removed
tender or painful, but blood flow is usually main- surgically.
tained because there are so many anastomoses This condition may also develop during preg-
among these veins. Deep vein phlebitis is potentially nancy, when the enlarged uterus presses against
more serious, with the possibility of clot formation the iliac veins and slows blood flow into the inferior
(thrombophlebitis) and subsequent dislodging of vena cava. Varicose veins of the anal canal are called
the clot to form an embolism. hemorrhoids, which may also be a result of preg-
Varicose veins—swollen and distended veins that nancy or of chronic constipation and straining to
occur most often in the superficial veins of the legs. defecate. Hemorrhoids that cause discomfort or
This condition may develop in people who must sit pain may also be removed surgically. Developments
or stand in one place for long periods of time. in laser surgery have made this a simpler procedure
Without contraction of the leg muscles, blood tends than it was in the past.

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The Vascular System 295


liver and pituitary gland, which produce and secrete here is about 30 to 35 mmHg, and the pressure of the
proteins into the blood. surrounding tissue fluid is much lower, about 2
mmHg. Because the capillary blood pressure is higher,
the process of filtration occurs, which forces plasma
EXCHANGES IN CAPILLARIES
and dissolved nutrients out of the capillaries and into
Capillaries are the sites of exchanges of materials tissue fluid. This is how nutrients such as glucose,
between the blood and the tissue fluid surrounding amino acids, and vitamins are brought to cells.
cells. Some of these substances move from the blood Blood pressure decreases as blood reaches the
to tissue fluid, and others move from tissue fluid to the venous end of capillaries, but notice that proteins
blood. The processes by which these substances are such as albumin have remained in the blood. Albumin
exchanged are illustrated in Fig. 13–2. contributes to the colloid osmotic pressure (COP)
Gases move by diffusion, that is, from their area of of blood; this is an “attracting” pressure, a “pulling”
greater concentration to their area of lesser concen- rather than a “pushing” pressure. At the venous end
tration. Oxygen, therefore, diffuses from the blood in of capillaries, the presence of albumin in the blood
systemic capillaries to the tissue fluid, and carbon pulls tissue fluid into the capillaries, which also brings
dioxide diffuses from tissue fluid to the blood to be into the blood the waste products produced by
brought to the lungs and exhaled. cells. The tissue fluid that returns to the blood
Let us now look at the blood pressure as blood also helps maintain normal blood volume and blood
enters capillaries from the arterioles. Blood pressure pressure.


Precapillary sphincter Tissue fluid Cells Capillary






Tissue fluid -
Hydrostatic pressure
Plasma, glucose,
2 mmHg
amino acids, vitamins CO 2
CO 2 COP 4 mmHg
CO 2
O 2
O 2
O 2
B. P. 33 mmHg
Tissue fluid,
waste products
Albumin - COP 25 mmHg
B. P. 15 mmHg
Albumin - COP 25 mmHg


ARTERIAL END VENOUS END
Outward forces 37 Outward forces 19
Inward forces 27 Inward forces 27


Figure 13–2. Exchanges between blood in a systemic capillary and the surrounding tis-
sue fluid. Arrows depict the direction of movement. Filtration takes place at the arterial end
of the capillary. Osmosis takes place at the venous end. Gases are exchanged by diffusion.
QUESTION: Of all the pressures shown here, which one is the highest, and what process
does it bring about?

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296 The Vascular System


The amount of tissue fluid formed is slightly body take blood to the superior vena cava. These two
greater than the amount returned to the capillaries. If caval veins return blood to the right atrium. The major
this were to continue, blood volume would be gradu- arteries and veins are shown in Figs. 13–3 to 13–5, and
ally depleted. The excess tissue fluid, however, enters their functions are listed in Tables 13–1 and 13–2.
lymph capillaries. Now called lymph, it will be The aorta is a continuous vessel, but for the sake of
returned to the blood to be recycled again as plasma, precise description it is divided into sections that are
thus maintaining blood volume. This is discussed fur- named anatomically: ascending aorta, aortic arch, tho-
ther in Chapter 14. racic aorta, and abdominal aorta. The ascending aorta
is the first inch that emerges from the top of the left
ventricle. The arch of the aorta curves posteriorly over
PATHWAYS OF CIRCULATION the heart and turns downward. The thoracic aorta
continues down through the chest cavity and through
The two major pathways of circulation are pulmonary the diaphragm. Below the level of the diaphragm, the
and systemic. Pulmonary circulation begins at the abdominal aorta continues to the level of the 4th lum-
right ventricle, and systemic circulation begins at the bar vertebra, where it divides into the two common
left ventricle. Hepatic portal circulation is a special iliac arteries. Along its course, the aorta has many
segment of systemic circulation that will be covered branches through which blood travels to specific
separately. Fetal circulation involves pathways that are organs and parts of the body.
present only before birth and will also be discussed The ascending aorta has only two branches: the
separately. right and left coronary arteries, which supply blood to
the myocardium. This pathway of circulation was
PULMONARY CIRCULATION described previously in Chapter 12.
The aortic arch has three branches that supply
The right ventricle pumps blood into the pulmonary blood to the head and arms: the brachiocephalic
artery (or trunk), which divides into the right and left artery, left common carotid artery, and left subclavian
pulmonary arteries, one going to each lung. Within artery. The brachiocephalic (literally, “arm-head”)
the lungs each artery branches extensively into smaller artery is very short and divides into the right common
arteries and arterioles, then to capillaries. The pul- carotid artery and right subclavian artery. The right
monary capillaries surround the alveoli of the lungs; it and left common carotid arteries extend into the neck,
is here that exchanges of oxygen and carbon dioxide where each divides into an internal carotid artery and
take place. The capillaries unite to form venules, external carotid artery, which supply the head. The
which merge into veins, and finally into the two pul- right and left subclavian arteries are in the shoulders
monary veins from each lung that return blood to the behind the clavicles and continue into the arms. As the
left atrium. This oxygenated blood will then travel artery enters another body area (it may not “branch,”
through the systemic circulation. (Notice that the pul- simply continue), its name changes: The subclavian
monary veins contain oxygenated blood; these are the artery becomes the axillary artery, which becomes the
only veins that carry blood with a high oxygen con- brachial artery. The branches of the carotid and sub-
tent. The blood in systemic veins has a low oxygen clavian arteries are diagrammed in Figs. 13–3 and
content; it is systemic arteries that carry oxygenated 13–5. As you look at these diagrams, keep in mind that
blood.)
the name of the vessel often tells us where it is. The
facial artery, for example, is found in the face.
SYSTEMIC CIRCULATION
Some of the arteries in the head contribute to an
The left ventricle pumps blood into the aorta, the important arterial anastomosis, the circle of Willis
largest artery of the body. We will return to the aorta (or cerebral arterial circle), which is a “circle” of arter-
and its branches in a moment, but first we will sum- ies around the pituitary gland (Fig. 13–6). The circle
marize the rest of systemic circulation. The branches of Willis is formed by the right and left internal
of the aorta take blood into arterioles and capillary net- carotid arteries and the basilar artery, which is the
works throughout the body. Capillaries merge to form union of the right and left vertebral arteries (branches
venules and veins. The veins from the lower body take of the subclavian arteries). The brain is always active,
blood to the inferior vena cava; veins from the upper
(text continued on page 299)

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The Vascular System 297






Maxillary
Occipital
Facial
Internal carotid External carotid

Vertebral Common carotid
Subclavian
Brachiocephalic
Axillary
Aortic arch
Pulmonary


Celiac Intercostal
Left gastric Brachial
Hepatic Renal
Splenic Gonadal

Inferior mesenteric
Superior mesenteric
Radial
Abdominal aorta
Ulnar
Right common iliac
Deep palmar arch
Internal iliac
External iliac Superficial palmar arch




Deep femoral

Femoral




Popliteal


Anterior tibial


Posterior tibial









Figure 13–3. Systemic arteries. The aorta and its major branches are shown in anterior
view.
QUESTION: Can you find three arteries named after bones? After organs?

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298 The Vascular System


Superior sagittal sinus
Inferior sagittal sinus
Straight sinus
Transverse sinus
Anterior facial
Vertebral
External jugular
Internal jugular
Superior vena cava
Subclavian
Axillary Brachiocephalic
Cephalic Pulmonary
Hemiazygos
Hepatic
Intercostal Hepatic portal
Left gastric
Inferior vena cava
Renal
Brachial
Splenic
Basilic
Inferior mesenteric
Gonadal
Superior mesenteric
Internal iliac
Common iliac External iliac


Dorsal arch

Volar digital

Femoral

Great saphenous


Popliteal


Small saphenous




Anterior tibial




Dorsal arch




Figure 13–4. Systemic veins shown in anterior view.
QUESTION: Can you find three veins with the same name as the accompanying artery?

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The Vascular System 299
























































Figure 13–5. Arteries and veins of the head and neck shown in right lateral view. Veins
are labeled on the left. Arteries are labeled on the right.
QUESTION: Which vein is the counterpart of the common carotid artery?
even during sleep, and must have a constant flow of ply the abdominal wall and organs and to the common
blood to supply oxygen and remove waste products. iliac arteries, which continue into the legs. Notice in
For this reason there are four vessels that bring blood Fig. 13–3 that the common iliac artery becomes the
to the circle of Willis. From this anastomosis, several external iliac artery, which becomes the femoral artery,
paired arteries (the cerebral arteries) extend into the which becomes the popliteal artery; the same vessel
brain itself. has different names based on location. These vessels
The thoracic aorta and its branches supply the are also listed in Table 13–1 (see Box 13–3: Pulse
chest wall and the organs within the thoracic cavity. Sites).
These vessels are listed in Table 13–1. The systemic veins drain blood from organs or
The abdominal aorta gives rise to arteries that sup- parts of the body and often parallel their correspond-

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300 The Vascular System



Table 13–1 MAJOR SYSTEMIC ARTERIES
Branches of the Ascending Aorta and Aortic Arch
Artery Branch of Region supplied
Coronary a. Ascending aorta • Myocardium
Brachiocephalic a. Aortic arch • Right arm and head
Right common carotid a. Brachiocephalic a. • Right side of head
Right subclavian a. Brachiocephalic a. • Right shoulder and arm
Left common carotid a. Aortic arch • Left side of head
Left subclavian a. Aortic arch • Left shoulder and arm
External carotid a. Common carotid a. • Superficial head
Superficial temporal a. External carotid a. • Scalp
Internal carotid a. Common carotid a. • Brain (circle of Willis)
Ophthalmic a. Internal carotid a. • Eye
Vertebral a. Subclavian a. • Cervical vertebrae and circle of Willis
Axillary a. Subclavian a. • Armpit
Brachial a. Axillary a. • Upper arm
Radial a. Brachial a. • Forearm
Ulnar a. Brachial a. • Forearm
Volar arch Radial and ulnar a. • Hand
Branches of the Thoracic Aorta
Artery Region Supplied

Intercostal a. (9 pairs) • Skin, muscles, bones of trunk
Superior phrenic a. • Diaphragm
Pericardial a. • Pericardium
Esophageal a. • Esophagus
Bronchial a. • Bronchioles and connective tissue of the lungs
Branches of the Abdominal Aorta
Artery Region Supplied
Inferior phrenic a. • Diaphragm
Lumbar a. • Lumbar area of back
Middle sacral a. • Sacrum, coccyx, buttocks
Celiac a. • (see branches)
Hepatic a. • Liver
Left gastric a. • Stomach
Splenic a. • Spleen, pancreas
Superior mesenteric a. • Small intestine, part of colon
Suprarenal a. • Adrenal glands
Renal a. • Kidneys
Inferior mesenteric a. • Most of colon and rectum

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The Vascular System 301



Table 13–1 MAJOR SYSTEMIC ARTERIES (Continued)
Branches of the Abdominal Aorta
Artery Region Supplied
Testicular or ovarian a. • Testes or ovaries
Common iliac a. • The two large vessels that receive blood from the
abdominal aorta; each branches as follows:
Internal iliac a. • Bladder, rectum, reproductive organs
External iliac a. • Lower pelvis to leg
Femoral a. • Thigh
Popliteal a. • Back of knee
Anterior tibial a. • Front of lower leg
Dorsalis pedis • Top of ankle and foot
Plantar arches • Foot
Posterior tibial a. • Back of lower leg
Peroneal a. • Medial lower leg
Plantar arches • Foot




ing arteries. The most important veins are dia- then circulate through the kidneys, some of the glu-
grammed in Fig. 13–4 and listed in Table 13–2. cose might be lost in urine. However, blood from the
small intestine passes first through the liver sinusoids,
HEPATIC PORTAL CIRCULATION and the liver cells remove the excess glucose and store
it as glycogen. The blood that returns to the heart will
Hepatic portal circulation is a subdivision of sys- then have a blood glucose level in the normal range.
temic circulation in which blood from the abdominal Another example: Alcohol is absorbed into the cap-
digestive organs and spleen circulates through the illaries of the stomach. If it were to circulate directly
liver before returning to the heart. throughout the body, the alcohol would rapidly impair
Blood from the capillaries of the stomach, small the functioning of the brain. Portal circulation, how-
intestine, colon, pancreas, and spleen flows into two ever, takes blood from the stomach to the liver, the
large veins, the superior mesenteric vein and the organ that can detoxify the alcohol and prevent its
splenic vein, which unite to form the portal vein (Fig. detrimental effects on the brain. Of course, if alcohol
13–7). The portal vein takes blood into the liver, consumption continues, the blood alcohol level rises
where it branches extensively and empties blood into faster than the liver’s capacity to detoxify, and the well-
the sinusoids, the capillaries of the liver (see also Fig. known signs of alcohol intoxication appear.
16–6). From the sinusoids, blood flows into hepatic As you can see, this portal circulation pathway
veins, to the inferior vena cava and back to the right enables the liver to modify the blood from the diges-
atrium. Notice that in this pathway there are two sets tive organs and spleen. Some nutrients may be stored
of capillaries, and keep in mind that it is in capillaries or changed, bilirubin from the spleen is excreted into
that exchanges take place. Let us use some specific bile, and potential poisons are detoxified before the
examples to show the purpose and importance of por- blood returns to the heart and the rest of the body.
tal circulation.
Glucose from carbohydrate digestion is absorbed FETAL CIRCULATION
into the capillaries of the small intestine; after a big
meal this may greatly increase the blood glucose level. The fetus depends upon the mother for oxygen and
If this blood were to go directly back to the heart and nutrients and for the removal of carbon dioxide and

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302 The Vascular System



Table 13–2 MAJOR SYSTEMIC VEINS
Vein Vein Joined Region Drained

Head and Neck
Cranial venous sinuses Internal jugular v. • Brain, including reabsorbed CSF
Internal jugular v. Brachiocephalic v. • Face and neck
External jugular v. Subclavian v. • Superficial face and neck
Subclavian v. Brachiocephalic v. • Shoulder
Brachiocephalic v. Superior vena cava • Upper body
Superior vena cava Right atrium • Upper body
Arm and Shoulder
Radial v. Brachial v. • Forearm and hand
Ulnar v. Brachial v. • Forearm and hand
Cephalic v. Axillary v. • Superficial arm and forearm
Basilic v. Axillary v. • Superficial upper arm
Brachial v. Axillary v. • Upper arm
Axillary v. Subclavian v. • Armpit
Subclavian v. Brachiocephalic v. • Shoulder
Trunk
Brachiocephalic v. Superior vena cava • Upper body
Azygos v. Superior vena cava • Deep structures of chest and abdomen; links
inferior vena cava to superior vena cava
Hepatic v. Inferior vena cava • Liver
Renal v. Inferior vena cava • Kidney
Testicular or ovarian v. Inferior vena cava • Testes or ovaries
and left renal v.
Internal iliac v. Common iliac v. • Rectum, bladder, reproductive organs
External iliac v. Common iliac v. • Leg and abdominal wall
Common iliac v. Inferior vena cava • Leg and lower abdomen
Leg and Hip
Anterior and posterior tibial v. Popliteal v. • Lower leg and foot
Popliteal v. Femoral v. • Knee
Small saphenous v. Popliteal v. • Superficial leg and foot
Great saphenous v. Femoral v. • Superficial foot, leg, and thigh
Femoral v. External iliac v. • Thigh
External iliac v. Common iliac v. • Leg and abdominal wall
Common iliac v. Inferior vena cava • Leg and lower abdomen
Inferior vena cava Right atrium • Lower body

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The Vascular System 303










Posterior communicating


Posterior
cerebral
Anterior cerebral

Middle
cerebral
Circle of Willis
Basilar


Anterior communicating Internal
carotid
ARTERIES
External carotid
Anterior
cerebral
Anterior
Cerebrum communicating
(frontal lobe)
Anterior
cerebral Vertebral
Middle
cerebral
Internal
carotid
Posterior
communicating
Posterior
cerebral
Right Left
Cerebrum
(temporal lobe) common carotid common carotid
Basilar
Pons
Medulla Vertebral
Cerebellum
Spinal cord

Figure 13–6. Circle of Willis. This anastomosis is formed by the following arteries: inter-
nal carotid, anterior communicating, posterior communicating, and basilar. The cerebral
arteries extend from the circle of Willis into the brain. The box shows these vessels in an
inferior view of the brain.
QUESTION: Why do so many vessels contribute to the circle of Willis?

other waste products. The site of exchange between The fetus is connected to the placenta by the
fetus and mother is the placenta, which contains fetal umbilical cord, which contains two umbilical arteries
and maternal blood vessels that are very close to one and one umbilical vein (see Fig. 13–8). The umbilical
another (see Figs. 13–8 and 21–5). The blood of the arteries are branches of the fetal internal iliac arteries;
fetus does not mix with the blood of the mother; sub- they carry blood from the fetus to the placenta. In the
stances are exchanged by diffusion and active trans- placenta, carbon dioxide and waste products in the
port mechanisms. fetal blood enter maternal circulation, and oxygen

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304 The Vascular System



BOX 13–3 PULSE SITES

A pulse is the heartbeat that is felt at an arterial site. Popliteal—the popliteal artery at the back of the
What is felt is not actually the force exerted by the knee.
blood, but the force of ventricular contraction Dorsalis pedis—the dorsalis pedis artery on the
transmitted through the walls of the arteries. This is top of the foot (commonly called the pedal pulse).
why pulses are not felt in veins; they are too far Pulse rate is, of course, the heart rate. However,
from the heart for the force to be detectable. if the heart is beating weakly, a radial pulse may be
The most commonly used pulse sites are: lower than an apical pulse (listening to the heart
Radial—the radial artery on the thumb side of the itself with a stethoscope). This is called a pulse
wrist. deficit and indicates heart disease of some kind.
Carotid—the carotid artery lateral to the larynx in When taking a pulse, the careful observer also
the neck. notes the rhythm and force of the pulse. Abnormal
Temporal—the temporal artery just in front of the rhythms may reflect cardiac arrhythmias, and the
ear. force of the pulse (strong or weak) is helpful in
Femoral—the femoral artery at the top of the assessing the general condition of the heart and
thigh. arteries.




and nutrients from the mother’s blood enter fetal passes through the ductus venosus to the inferior
circulation. vena cava, to the right atrium. After birth, when the
The umbilical vein carries this oxygenated blood umbilical cord is cut, the remnants of these fetal ves-
from the placenta to the fetus. Within the body of the sels constrict and become nonfunctional.
fetus, the umbilical vein branches: One branch takes The other modifications of fetal circulation con-
some blood to the fetal liver, but most of the blood cern the fetal heart and large arteries (also shown in



Inferior vena cava
Hepatic vein Left gastric V.
Right gastric V.
Splenic V.
Spleen
Figure 13–7. Hepatic por-
Stomach
tal circulation. Portions of
Liver Pancreas some of the digestive organs
Left gastroepiploic V. have been removed to show
Portal vein the veins that unite to form
Inferior mesenteric V.
Inferior mesenteric V.
the portal vein. See text for
Superior Left colic V. description.
mesenteric V
QUESTION: The blood in the
Descending colon portal vein is going to what
organ? Where is the blood
Right colic V.
coming from?

Ascending colon

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The Vascular System 305





Aortic arch
Ductus arteriosus
Pulmonary artery

Left atrium

Foramen
ovale
Aorta
Right
atrium Ductus venosus
Umbilical cord
Inferior vena cava
Umbilical arteries
Umbilical vein
Navel of fetus Placenta














Maternal
blood vessels


Internal
iliac arteries

Figure 13–8. Fetal circulation. Fetal heart and blood vessels are shown on the left.
Arrows depict the direction of blood flow. The placenta and umbilical blood vessels are
shown on the right. See text for description.
QUESTION: Find the foramen ovale in the fetal heart. Which way does blood flow through
it, and why?



Fig. 13–8). Because the fetal lungs are deflated and do artery to the aorta, to the body. Both the foramen
not provide for gas exchange, blood is shunted away ovale and the ductus arteriosus permit blood to bypass
from the lungs and to the body. The foramen ovale is the fetal lungs.
an opening in the interatrial septum that permits some Just after birth, the baby breathes and expands its
blood to flow from the right atrium to the left atrium, lungs, which pulls more blood into the pulmonary cir-
not, as usual, to the right ventricle. The blood that culation. More blood then returns to the left atrium,
does enter the right ventricle is pumped into the pul- and a flap on the left side of the foramen ovale is
monary artery. The ductus arteriosus is a short ves- closed. The ductus arteriosus constricts, probably in
sel that diverts most of the blood in the pulmonary response to the higher oxygen content of the blood,

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306 The Vascular System







Aorta Arteries Arterioles Capillaries Venules Veins Vena Cavae













A
Figure 13–9. Characteristics of the vas-
4,000 cular system. (A) Schematic of the branch-
ing of vessels. (B) Cross-sectional area in
Cross-sectional area (cm 2 ) 3,000 square centimeters. (C) Blood velocity in
centimeters per second. (D) Systemic
blood pressure changes. Notice that sys-
2,000
pressure in the capillaries.
1,000 B tolic and diastolic pressures become one
QUESTION: Look at the cross-sectional
Blood velocity (cm/sec) 20 area and blood velocity. As area increases,
30
what happens to velocity? Where is veloc-
ity slowest?
10
C
120
Systolic pressure
Pulse pressure 100


80
60 Diastolic pressure
Blood pressure (mm Hg) 40


20
0 D


and pulmonary circulation becomes fully functional other goes down) to the cross-sectional area of the
within a few days. particular segment of the vascular system. Refer to
Fig. 13–9 as you read the following. The aorta receives
all the blood from the left ventricle, its cross-sectional
VELOCITY OF BLOOD FLOW area is small, about 3 cm (1 sq. inch), and the blood
2
moves very rapidly, at least 30 cm per second (about 12
The velocity, or speed, with which blood flows differs inches). Each time the aorta or any artery branches,
in the various parts of the vascular system. Velocity is the total cross-sectional area becomes larger, and the
inversely related (meaning as one value goes up, the speed of blood flow decreases. Think of a river that

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The Vascular System 307


begins in a narrow bed and is flowing rapidly. If the tricle. As blood travels farther away from the heart,
river bed widens, the water spreads out to fill it and blood pressure decreases (see Fig. 13–9). The brachial
flows more slowly. If the river were to narrow again, artery is most often used to take a blood pressure read-
the water would flow faster. This is just what happens ing; here a normal systolic range is 90 to 120 mmHg,
in the vascular system. and a normal diastolic range is 60 to 80 mmHg. In the
The capillaries in total have the greatest cross- arterioles, blood pressure decreases further, and sys-
sectional area, and blood velocity there is slowest, less tolic and diastolic pressures merge into one pressure.
than 0.1 cm per second. When capillaries unite to At the arterial end of capillary networks, blood pres-
form venules, and then veins, the cross-sectional area sure is about 30 to 35 mmHg, decreasing to 12 to 15
decreases and blood flow speeds up. mmHg at the venous end of capillaries. This is high
Recall that it is in capillary networks that exchanges enough to permit filtration but low enough to prevent
of nutrients, wastes, and gases take place between the rupture of the capillaries. As blood flows through
blood and tissue fluid. The slow rate of blood flow in veins, the pressure decreases further, and in the caval
capillaries permits sufficient time for these essential veins, blood pressure approaches zero as blood enters
exchanges. Think of a train slowing down (not actually the right atrium.
stopping) at stations to allow people to jump on The upper limit of the normal blood pressure range
and off, then speeding up again to get to the next sta- is now 120/80 mmHg. The levels of 125 to 139/85 to
tion. The capillaries are the “stations” of the vascular 89 mmHg, once considered high-normal, are now
system. called “prehypertension,” that is, with the potential to
The more rapid blood velocity in other vessels become even higher. A systemic blood pressure con-
makes circulation time quite short. This is the time it sistently higher than the normal range is called hyper-
takes for blood to go from the right ventricle to the tension (see also Box 13–4: Hypertension). A lower
lungs, back to the heart to be pumped by the left ven- than normal blood pressure is called hypotension.
tricle to the body, and return to the heart again. The regulation of systemic blood pressure is discussed
Circulation time is about 1 minute or less, and ensures in a later section.
an adequate exchange of gases. Pulmonary blood pressure is created by the right
ventricle, which has relatively thin walls and thus exerts
about one-sixth the force of the left ventricle. The
BLOOD PRESSURE result is that pulmonary arterial pressure is always low:
20 to 25/8 to 10 mmHg, and in pulmonary capillaries
Blood pressure is the force the blood exerts against is lower still. This is important to prevent filtration in
the walls of the blood vessels. Filtration in capillaries pulmonary capillaries, which in turn prevents tissue
depends upon blood pressure; filtration brings nutri- fluid from accumulating in the alveoli of the lungs.
ents to tissues, and as you will see in Chapter 18, is the
first step in the formation of urine. Blood pressure is MAINTENANCE OF SYSTEMIC
one of the “vital signs” often measured, and indeed a BLOOD PRESSURE
normal blood pressure is essential to life. Because blood pressure is so important, many physio-
The pumping of the ventricles creates blood pres- logical factors and processes interact to keep blood
sure, which is measured in mmHg (millimeters of pressure within normal limits:
mercury). When a systemic blood pressure reading is
taken, two numbers are obtained: systolic and dias- 1. Venous return—the amount of blood that returns
tolic, as in 110/70 mmHg. Systolic pressure is always to the heart by way of the veins. Venous return
the higher of the two and represents the blood pres- is important because the heart can pump only
sure when the left ventricle is contracting. The lower the blood it receives. If venous return decreases,
number is the diastolic pressure, when the left ventri- the cardiac muscle fibers will not be stretched, the
cle is relaxed and does not exert force. Diastolic pres- force of ventricular systole will decrease (Starling’s
sure is maintained by the arteries and arterioles and is law), and blood pressure will decrease. This is what
discussed in a later section. might happen following a severe hemorrhage.
Systemic blood pressure is highest in the aorta, When the body is horizontal, venous return can
which receives all of the blood pumped by the left ven- be maintained fairly easily, but when the body is

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308 The Vascular System


BOX 13–4 HYPERTENSION

Hypertension is high blood pressure, that is, a rest- trophy. This abnormal growth of the myocardium,
ing systemic pressure consistently above the normal however, is not accompanied by a corresponding
range (90 to 120/60 to 80 mmHg). Clinicians now growth in coronary capillaries, and the blood supply
consider 125 to 139/85 to 89 mmHg to be prehy- of the left ventricle may not be adequate for all sit-
pertension. A systolic reading of 140 to 159 mmHg uations. Exercise, for example, puts further
or a diastolic reading of 90 to 99 mmHg may be demands on the heart, and the person may experi-
called stage 1 hypertension, and a systolic reading ence angina due to a lack of oxygen or a myocar-
above 160 mmHg or a diastolic reading above 100 dial infarction if there is a severe oxygen deficiency.
mmHg may be called stage 2 hypertension. Although several different kinds of medications
The term “essential hypertension” means that no (diuretics, vasodilators) are used to treat hyperten-
specific cause can be determined; most cases are in sion, people with moderate hypertension may limit
this category. For some people, however, an over- their dependence on medications by following cer-
production of renin by the kidneys is the cause tain guidelines:
of their hypertension. Excess renin increases the 1. Don’t smoke, because nicotine stimulates vaso-
production of angiotensin II, which raises blood constriction, which raises BP. Smoking also dam-
pressure. Although hypertension often produces no ages arteries, contributing to arteriosclerosis.
symptoms, the long-term consequences may be 2. Lose weight if overweight. A weight loss of as lit-
very serious. Chronic hypertension has its greatest tle as 10 pounds can lower BP. A diet high in
effects on the arteries and on the heart. fruits and vegetables may, for some people, con-
Although the walls of arteries are strong, hyper- tribute to lower BP.
tension weakens them and contributes to arte- 3. Cut salt intake in half. Although salt consump-
riosclerosis. Such weakened arteries may rupture or tion may not be the cause of hypertension,
develop aneurysms, which may in turn lead to a reducing salt intake may help lower blood pres-
CVA or kidney damage. sure by decreasing blood volume.
Hypertension affects the heart because the left 4. Exercise on a regular basis. A moderate amount
ventricle must now pump blood against the higher of aerobic exercise (such as a half hour walk
arterial pressure. The left ventricle works harder every day) is beneficial for the entire cardiovas-
and, like any other muscle, enlarges as more work is cular system and may also contribute to weight
demanded; this is called left ventricular hyper- loss.




vertical, gravity must be overcome to return blood 2. Heart rate and force—in general, if heart rate and
from the lower body to the heart. Three mecha- force increase, blood pressure increases; this is
nisms help promote venous return: constriction of what happens during exercise. However, if the
veins, the skeletal muscle pump, and the respira- heart is beating extremely rapidly, the ventricles
tory pump. may not fill completely between beats, and cardiac
Veins contain smooth muscle, which enables output and blood pressure will decrease.
them to constrict and force blood toward the heart; 3. Peripheral resistance—this term refers to the
the valves prevent backflow of blood. The second resistance the vessels offer to the flow of blood.
mechanism is the skeletal muscle pump, which is The arteries and veins are usually slightly con-
especially effective for the deep veins of the legs. stricted, which maintains normal diastolic blood
These veins are surrounded by skeletal muscles pressure. It may be helpful to think of the vessels
that contract and relax during normal activities as the “container” for the blood. If a person’s body
such as walking. Contractions of the leg muscles has 5 liters of blood, the “container” must be
squeeze the veins to force blood toward the heart. smaller in order for the blood to exert a pressure
The third mechanism is the respiratory pump, against its walls. This is what normal vasoconstric-
which affects veins that pass through the chest cav- tion does: It makes the container (the vessels)
ity. The pressure changes of inhalation and exhala- smaller than the volume of blood so that the blood
tion alternately expand and compress the veins, and will exert pressure even when the left ventricle is
blood is returned to the heart. relaxed.

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The Vascular System 309


If more vasoconstriction occurs, blood pressure pressure and blood flow to the brain. Although a
will increase (the container has become even person may survive loss of 50% of the body’s total
smaller). This is what happens in a stress situation, blood, the possibility of brain damage increases as
when greater vasoconstriction is brought about more blood is lost and not rapidly replaced.
by sympathetic impulses. If vasodilation occurs, 7. Hormones—several hormones have effects on
blood pressure will decrease (the container is blood pressure. You may recall them from Chapters
larger). After eating a large meal, for example, there 10 and 12, but we will summarize them here and in
is extensive vasodilation in the digestive tract to Fig. 13–10. The adrenal medulla secretes norepi-
supply more oxygenated blood for digestive activi- nephrine and epinephrine in stress situations.
ties. To keep blood pressure within the normal Norepinephrine stimulates vasoconstriction, which
range, vasoconstriction must, and does, occur else- raises blood pressure. Epinephrine also causes vaso-
where in the body. This is why strenuous exercise constriction, and increases heart rate and force of
should be avoided right after eating; there is not contraction, both of which increase blood pressure.
enough blood to completely supply oxygen to exer- Antidiuretic hormone (ADH) is secreted by the
cising muscles and an active digestive tract at the posterior pituitary gland when the water content of
same time. the body decreases. ADH increases the reabsorp-
4. Elasticity of the large arteries—when the left tion of water by the kidneys to prevent further loss
ventricle contracts, the blood that enters the large of water in urine and a further decrease in blood
arteries stretches their walls. The arterial walls are pressure.
elastic and absorb some of the force. When the left Aldosterone, a hormone from the adrenal cor-
ventricle relaxes, the arterial walls recoil or snap tex, has a similar effect on blood volume. When
back, which helps keep diastolic pressure within blood pressure decreases, secretion of aldosterone
the normal range. Normal elasticity, therefore, stimulates the reabsorption of Na ions by the kid-

lowers systolic pressure, raises diastolic pressure, neys. Water follows sodium back to the blood,
and maintains a normal pulse pressure. (Pulse pres- which maintains blood volume to prevent a further
sure is the difference between systolic and diastolic drop in blood pressure.
pressure. The usual ratio of systolic to diastolic to Atrial natriuretic peptide (ANP), secreted by the
pulse pressure is approximately 3:2:1. For example, atria of the heart, functions in opposition to aldos-
with a blood pressure of 120/80 mmHg, the pulse terone. ANP increases the excretion of Na ions

pressure is 40, and the ratio is 120:80:40, or 3:2:1.) and water by the kidneys, which decreases blood
5. Viscosity of the blood—normal blood viscosity volume and lowers blood pressure.
depends upon the presence of red blood cells and
plasma proteins, especially albumin. Having too DISTRIBUTION OF BLOOD FLOW
many red blood cells is rare but does occur in the
disorder called polycythemia vera and in people An individual’s blood volume remains relatively con-
who are heavy smokers. This will increase blood stant within the normal range appropriate to the size
viscosity and blood pressure. of the person. Active tissues, however, require more
A decreased number of red blood cells, as is seen blood, that is, more oxygen, than do less active tissues.
with severe anemia, or decreased albumin, as may As active tissues and organs receive a greater propor-
occur in liver disease or kidney disease, will tion of the total blood flow, less active organs must
decrease blood viscosity and blood pressure. In receive less, or blood pressure will decrease markedly.
these situations, other mechanisms such as vaso- As mentioned previously, precapillary sphincters
constriction will maintain blood pressure as close to dilate in active tissues and constrict in less active ones.
normal as is possible. The arterioles also constrict to reduce blood flow to
6. Loss of blood—a small loss of blood, as when less active organs. This ensures that metabolically
donating a pint of blood, will cause a temporary active organs will receive enough oxygen to function
drop in blood pressure followed by rapid compen- properly and that blood pressure for the body as a
sation in the form of a more rapid heart rate and whole will be maintained within normal limits.
greater vasoconstriction. After a severe hemor- An example will be helpful here; let us use the
rhage, however, these compensating mechanisms body at rest and the body during exercise. Consult
may not be sufficient to maintain normal blood Fig. 13–11 as you read the following. Resting cardiac

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310 The Vascular System



Adrenal gland
Norepinephrine
Vasoconstriction

Epinephrine
Raises
B.P.
Increases rate
and force of
contraction
Aldosterone

Pituitary
Kidney Increases
reabsorption of
+
Na ,
water follows

ADH
Increases
reabsorption
of H O
Heart 2
Increases
excretion of Lowers
ANP
+
Na , B.P.
water follows


Figure 13–10. Hormones that affect blood pressure. See text for further description.
QUESTION: Which two hormones have opposite functions, and what are these functions?

output is approximately 5000 mL per minute. Exercise tion of blood ensure sufficient oxygen for active tissues
cardiac output is three times that, about 15,000 mL and an appropriate blood pressure for the body as a
per minute. Keep in mind that the volume of blood is whole.
the same in both cases, but that during exercise the
blood is being circulated more rapidly.
Compare the amounts of blood flowing to various REGULATION OF BLOOD PRESSURE
organs and tissues during exercise and at rest. During
exercise, the heart receives about three times as much The mechanisms that regulate systemic blood pressure
blood as it does when the body is at rest. The very may be divided into two types: intrinsic mechanisms
active skeletal muscles receive about ten times as much and nervous mechanisms. The nervous mechanisms
blood. The skin, as an organ of heat loss, receives involve the nervous system, and the intrinsic mecha-
about four times as much blood. Other organs, how- nisms do not require nerve impulses.
ever, can function adequately with less blood. Blood
flow is reduced to the digestive tract, to the kidneys, INTRINSIC MECHANISMS
and to other parts of the body such as bones.
When the exercise ceases, cardiac output will The term intrinsic means “within.” Intrinsic mecha-
gradually return to the resting level, as will blood flow nisms work because of the internal characteristics of
to the various organs. These changes in the distribu- certain organs. The first such organ is the heart. When

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The Vascular System 311






Blood distribution (mL/min)
Resting cardiac output Exercise cardiac output
5,000 mL/min 15,000 mL/min

















Heart Skeletal muscle Brain Skin GI tract Kidneys Rest of body
Resting
215 mL 1,035 mL 645 mL 430 mL 1,205 mL 950 mL 515 mL
% of 4% 21% 13% 9% 24% 19% 10%
total
Exercise
645 mL 10,710 mL 645 mL 1,635 mL 510 mL 510 mL 330 mL
% of 4.5% 71% 4.5% 11% 3.5% 3.5% 2%





total
Figure 13–11. Blood flow through various organs when the body is at rest and during
exercise. For each organ, the percentage of the total blood flow is given.
QUESTION: During exercise, which organs have the greatest increase in blood flow?
Which organs have the greatest decrease?

venous return increases, cardiac muscle fibers are angiotensin mechanism. When blood pressure
stretched, and the ventricles pump more forcefully decreases, the kidneys secrete the enzyme renin,
(Starling’s law). Thus, cardiac output and blood pres- which initiates a series of reactions that result in the
sure increase. This is what happens during exercise, formation of angiotensin II. These reactions are
when a higher blood pressure is needed. When exer- described in Table 13–3 and depicted in Fig. 13–12.
cise ends and venous return decreases, the heart pumps Angiotensin II causes vasoconstriction and stimulates
less forcefully, which helps return blood pressure to a secretion of aldosterone by the adrenal cortex, both of
normal resting level. which will increase blood pressure.
The second intrinsic mechanism involves the kid-
neys. When blood flow through the kidneys decreases, NERVOUS MECHANISMS
the process of filtration decreases and less urine is
formed. This decrease in urinary output preserves The medulla and the autonomic nervous system are
blood volume so that it does not decrease further. directly involved in the regulation of blood pressure.
Following severe hemorrhage or any other type of The first of these nervous mechanisms concerns the
dehydration, this is very important to maintain blood heart; this was described previously, so we will not
pressure. review it here but refer you to Chapter 12 and Fig.
The kidneys are also involved in the renin- 12–7, as well as Fig. 13–13.

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312 The Vascular System


Lung and
vascular endothelium


CONVERTING ENZYME

Liver

ANGIOTENSINOGEN ANGIOTENSIN I ANGIOTENSIN II



Adrenal cortex


Systemic
ALDOSTERONE arteries
RENIN


Vasoconstriction
Decreased
B. P.

+
Increased Na and Increased
H 2 O reabsorption B. P.
Kidney


Figure 13–12. The renin-angiotensin mechanism. Begin at “Decreased B. P.” and see
Table 13–3 for numbered steps.
QUESTION: Where is renin produced? What are the functions of angiotensin II?


The second nervous mechanism involves periph- center, which consists of a vasoconstrictor area and a
eral resistance, that is, the degree of constriction of the vasodilator area. The vasodilator area may depress the
arteries and arterioles and, to a lesser extent, the veins vasoconstrictor area to bring about vasodilation,
(see Fig. 13–13). The medulla contains the vasomotor which will decrease blood pressure. The vasoconstric-
tor area may bring about more vasoconstriction by
way of the sympathetic division of the autonomic
Table 13–3 THE RENIN-ANGIOTENSIN nervous system.
MECHANISM Sympathetic vasoconstrictor fibers innervate the
smooth muscle of all arteries and veins, and several
1. Decreased blood pressure stimulates the kidneys to
secrete renin. impulses per second along these fibers maintain nor-
2. Renin splits the plasma protein angiotensinogen (syn- mal vasoconstriction. More impulses per second bring
thesized by the liver) to angiotensin I. about greater vasoconstriction, and fewer impulses per
3. Angiotensin I is converted to angiotensin II by an second cause vasodilation. The medulla receives the
enzyme (called converting enzyme) secreted by lung information to make such changes from the presso-
tissue and vascular endothelium.
4. Angiotensin II: receptors in the carotid sinuses and the aortic sinus.
• causes vasoconstriction The inability to maintain normal blood pressure is one
• stimulates the adrenal cortex to secrete aldosterone aspect of circulatory shock (see Box 13–5: Circulatory
Shock).

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Inhibit Vasodilation
vasomotor decreases
center peripheral
resistance








Pressoreceptors in
carotid and aortic Stimulate Heart rate
slows,
sinuses stimulated cardioinhibitory decreases
center
cardiac output
B. P.
decreases to
normal range
B. P.
increases
further



B. P.
decreases
further

B. P.
increases to Vasoconstriction
normal
increases
range
peripheral Stimulate Pressoreceptors in
resistance vasomotor carotid and aortic
center sinuses inhibited
Heart rate
and cardiac
output
increase






Stimulate
cardio-
accelerator
center


Figure 13–13. Nervous mechanisms that regulate blood pressure. See text for
description.
QUESTION: What kind of sensory information is used to make changes in BP, and
where are the receptors located?



313

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314 The Vascular System


BOX 13–5 CIRCULATORY SHOCK

Circulatory shock is any condition in which cardiac Stages of Shock
output decreases to the extent that tissues are Compensated shock—the responses by the body
deprived of oxygen and waste products accumulate. maintain cardiac output. Following a small hemor-
rhage, for example, the heart rate increases,
Causes of Shock
the blood vessels constrict, and the kidneys
Cardiogenic shock occurs most often after a decrease urinary output to conserve water. These
severe myocardial infarction but may also be the responses help preserve blood volume and main-
result of ventricular fibrillation. In either case, the tain blood pressure, cardiac output, and blood flow
heart is no longer an efficient pump, and cardiac to tissues.
output decreases. Progressive shock—the state of shock leads to
Hypovolemic shock is the result of decreased more shock. Following a severe hemorrhage, car-
blood volume, often due to severe hemorrhage. diac output decreases and the myocardium itself is
Other possible causes are extreme sweating (heat deprived of blood. The heart weakens, which fur-
stroke) or extreme loss of water through the kid- ther decreases cardiac output. Arteries that are
neys (diuresis) or intestines (diarrhea). In these deprived of their blood supply cannot remain con-
situations, the heart simply does not have enough stricted. As the arteries dilate, venous return
blood to pump, and cardiac output decreases. decreases, which in turn decreases cardiac output.
Anaphylactic shock, also in this category, is a mas- Progressive shock is a series of such vicious cycles,
sive allergic reaction in which great amounts of and medical intervention is required to restore car-
histamine increase capillary permeability and diac output to normal.
vasodilation throughout the body. Much plasma is Irreversible shock—no amount of medical assis-
then lost to tissue spaces, which decreases blood tance can restore cardiac output to normal. The
volume, blood pressure, and cardiac output. usual cause of death is that the heart has been dam-
Septic shock is the result of septicemia, the aged too much to recover. A severe myocardial
presence of bacteria in the blood. The bacteria and infarction, massive hemorrhage, or septicemia may
damaged tissues release inflammatory chemicals all be fatal despite medical treatment.
that cause vasodilation and extensive loss of plasma
into tissue spaces.





returned to the heart against the force of gravity.
AGING AND THE
Varicose veins and phlebitis are more likely to occur
VASCULAR SYSTEM among elderly people.
It is believed that the aging of blood vessels, especially
arteries, begins in childhood, although the effects are
not apparent for decades. The cholesterol deposits SUMMARY
of atherosclerosis are to be expected with advancing
age, with the most serious consequences in the coro- Although the vascular system does form passageways
nary arteries. A certain degree of arteriosclerosis is to for the blood, you can readily see that the blood ves-
be expected, and average resting blood pressure may sels are not simply pipes through which the blood
increase, which further damages arterial walls. flows. The vessels are not passive tubes, but rather
Consequences include stroke and left-sided heart active contributors to homeostasis. The arteries and
failure. veins help maintain blood pressure, and the capillaries
The veins also deteriorate with age; their thin walls provide sites for the exchanges of materials between
weaken and stretch, making their valves incompetent. the blood and the tissues. Some very important sites of
This is most likely to occur in the veins of the legs; exchange are discussed in the following chapters: the
their walls are subject to great pressure as blood is lungs, the digestive tract, and the kidneys.

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The Vascular System 315


STUDY OUTLINE

The vascular system consists of the arteries, 4. BP in capillaries brings nutrients to tissues and
capillaries, and veins through which blood forms tissue fluid in the process of filtration.
travels. 5. Albumin in the blood provides colloid osmotic
pressure, which pulls waste products and tissue fluid
Arteries (and arterioles) (see Fig. 13–1) into capillaries. The return of tissue fluid maintains
1. Carry blood from the heart to capillaries; three lay- blood volume and BP.
ers in their walls. 6. Precapillary sphincters regulate blood flow into
2. Inner layer (tunica intima): simple squamous capillary networks based on tissue needs; in active
epithelial tissue (endothelium), very smooth to pre- tissues they dilate; in less active tissues they con-
vent abnormal blood clotting; secretes nitric oxide strict.
(NO), a vasodilator. 7. Sinusoids are very permeable capillaries found in
3. Middle layer (tunica media): smooth muscle and the liver, spleen, pituitary gland, and red bone mar-
elastic connective tissue; contributes to mainte- row to permit proteins and blood cells to enter or
nance of diastolic blood pressure (BP). leave the blood.
4. Outer layer (tunica externa): fibrous connective tis-
sue to prevent rupture. Pathways of Circulation
5. Constriction or dilation is regulated by the auto- 1. Pulmonary: Right ventricle → pulmonary artery →
nomic nervous system. pulmonary capillaries (exchange of gases) → pul-
monary veins → left atrium.
Veins (and venules) (see Fig. 13–1) 2. Systemic: left ventricle → aorta → capillaries in
1. Carry blood from capillaries to the heart; three lay- body tissues → superior and inferior caval veins →
ers in walls. right atrium (see Table 13–1 and Fig. 13–3 for sys-
2. Inner layer: endothelium folded into valves to pre- temic arteries and Table 13–2 and Fig. 13–4 for sys-
vent the backflow of blood. temic veins).
3. Middle layer: thin smooth muscle, because veins 3. Hepatic portal circulation: blood from the diges-
are not as important in the maintenance of BP. tive organs and spleen flows through the portal
4. Outer layer: thin fibrous connective tissue because vein to the liver before returning to the heart.
veins do not carry blood under high pressure. Purpose: the liver stores some nutrients or regu-
lates their blood levels and detoxifies potential poi-
Anastomoses—connections between vessels sons before blood enters the rest of peripheral
of the same type circulation (see Fig. 13–7).
1. Provide alternate pathways for blood flow if one
vessel is blocked. Fetal Circulation—the fetus depends on the
2. Arterial anastomoses provide for blood flow to the mother for oxygen and nutrients and for the
capillaries of an organ (e.g., circle of Willis to the removal of waste products (see Fig. 13–8)
brain). 1. The placenta is the site of exchange between fetal
3. Venous anastomoses provide for return of blood to blood and maternal blood.
the heart and are most numerous in veins of the 2. Umbilical arteries (two) carry blood from the fetus
legs. to the placenta, where CO and waste products
2
enter maternal circulation.
Capillaries (see Figs. 13–1 and 13–2) 3. The umbilical vein carries blood with O and nutri-
2
1. Carry blood from arterioles to venules. ents from the placenta to the fetus.
2. Walls are one cell thick (simple squamous epithelial 4. The umbilical vein branches; some blood
tissue) to permit exchanges between blood and tis- flows through the fetal liver; most blood flows
sue fluid. through the ductus venosus to the fetal inferior
3. Oxygen and carbon dioxide are exchanged by vena cava.
diffusion. 5. The foramen ovale permits blood to flow from the

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316 The Vascular System


right atrium to the left atrium to bypass the fetal • Respiratory pump—the pressure changes of
lungs. inhalation and exhalation expand and compress
6. The ductus arteriosus permits blood to flow from the veins in the chest cavity
the pulmonary artery to the aorta to bypass the 2. Heart rate and force—if heart rate and force
fetal lungs. increase, BP increases.
7. These fetal structures become nonfunctional after 3. Peripheral resistance—the resistance of the arteries
birth, when the umbilical cord is cut and breathing and arterioles to the flow of blood. These vessels
takes place. are usually slightly constricted to maintain normal
diastolic BP. Greater vasoconstriction will increase
Velocity of Blood Flow (see Fig. 13–9) BP; vasodilation will decrease BP. In the body,
1. Velocity is inversely related to the cross-sectional vasodilation in one area requires vasoconstriction
area of a segment of the vascular system. in another area to maintain normal BP.
2. The total capillaries have the greatest cross- 4. Elasticity of the large arteries—ventricular systole
sectional area and slowest blood flow. stretches the walls of large arteries, which recoil
3. Slow flow in the capillaries is important to permit during ventricular diastole. Normal elasticity low-
sufficient time for exchange of gases, nutrients, and ers systolic BP, raises diastolic BP, and maintains
wastes. normal pulse pressure.
5. Viscosity of blood—depends on RBCs and plasma
Blood Pressure (BP)—the force exerted by proteins, especially albumin. Severe anemia tends
the blood against the walls of the blood ves- to decrease BP. Deficiency of albumin as in liver or
sels (Fig. 13–9) kidney disease tends to decrease BP. In these cases,
1. BP is measured in mmHg: systolic/diastolic. compensation such as greater vasoconstriction will
Systolic pressure occurs during ventricular con- keep BP close to normal.
traction; diastolic pressure occurs during ventricu- 6. Loss of blood—a small loss will be rapidly com-
lar relaxation. pensated for by faster heart rate and greater vaso-
2. Normal range of systemic arterial BP: 90 to 120/60 constriction. After severe hemorrhage, these
to 80 mmHg. mechanisms may not be sufficient to maintain nor-
3. BP in capillaries is 30 to 35 mmHg at the arterial mal BP.
end and 12 to 15 mmHg at the venous end—high 7. Hormones—(see Fig. 13–10) (a) Norepinephrine
enough to permit filtration but low enough to pre- stimulates vasoconstriction, which raises BP; (b)
vent rupture of the capillaries. epinephrine increases cardiac output and raises BP;
4. BP decreases in the veins and approaches zero in (c) ADH increases water reabsorption by the kid-
the caval veins. neys, which increases blood volume and BP; (d)
5. Pulmonary BP is always low (the right ventricle aldosterone increases reabsorption of Na ions by

pumps with less force): 20 to 25/8 to 10 mmHg. the kidneys; water follows Na and increases blood

This low BP prevents filtration and accumulation volume and BP; (e) ANP increases excretion of
of tissue fluid in the alveoli. Na ions and water by the kidneys, which de-
creases blood volume and BP.
Maintenance of Systemic BP
1. Venous return—the amount of blood that returns Distribution of Blood Flow
to the heart. If venous return decreases, the heart 1. Metabolically active tissues require more oxygen,
contracts less forcefully (Starling’s law) and BP and receive a greater proportion of the blood vol-
decreases. The mechanisms that maintain venous ume as it circulates (see Fig. 13–11).
return when the body is vertical are: 2. Blood flow is increased by the dilation of arterioles
• Constriction of veins with the valves preventing and precapillary sphincters.
backflow of blood 3. In less active tissues, arterioles and precapillary
• Skeletal muscle pump—contraction of skeletal sphincters constrict.
muscles, especially in the legs, squeezes the deep 4. Organs receive sufficient oxygen, and BP for the
veins body is maintained within the normal range.

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The Vascular System 317


Regulation of Blood Pressure—intrinsic Nervous Mechanisms (see Fig. 13–13)
mechanisms and nervous mechanisms 1. Heart rate and force—see also Chapter 12.
Intrinsic Mechanisms 2. Peripheral resistance—the medulla contains the
1. The heart—responds to increased venous return by vasomotor center, which consists of a vasoconstric-
pumping more forcefully (Starling’s law), which tor area and a vasodilator area. The vasodilator area
increases cardiac output and BP. brings about vasodilation by suppressing the vaso-
2. The kidneys—decreased blood flow decreases constrictor area. The vasoconstrictor area main-
filtration, which decreases urinary output to pre- tains normal vasoconstriction by generating several
serve blood volume. Decreased BP stimulates the impulses per second along sympathetic vaso-
kidneys to secrete renin, which initiates the renin- constrictor fibers to all arteries and veins. More
angiotensin mechanism (Table 13–3 and Fig. impulses per second increase vasoconstriction and
13–12) that results in the formation of angiotensin raise BP; fewer impulses per second bring about
II, which causes vasoconstriction and stimulates vasodilation and a drop in BP.
secretion of aldosterone.




REVIEW QUESTIONS

1. Describe the structure of the three layers of the 7. Name the fetal structure with each of the follow-
walls of arteries, and state the function of each ing functions: (pp. 303–305)
layer. Describe the structural differences in these a. Permits blood to flow from the right atrium to
layers in veins, and explain the reason for each dif- the left atrium
ference. (p. 292) b. Carries blood from the placenta to the fetus
c. Permits blood to flow from the pulmonary
2. Describe the structure and purpose of anasto-
artery to the aorta
moses, and give a specific example. (pp. 292–293)
d. Carry blood from the fetus to the placenta
3. Describe the structure of capillaries. State the e. Carries blood from the umbilical vein to the
process by which each of the following is inferior vena cava
exchanged between capillaries and tissue fluid:
8. Describe the three mechanisms that promote
nutrients, oxygen, waste products, and carbon
venous return when the body is vertical.
dioxide. (pp. 293–295)
(pp. 307–308)
4. State the part of the body supplied by each of the 9. Explain how the normal elasticity of the large
following arteries: (pp. 297, 300) arteries affects both systolic and diastolic blood
a. Bronchial pressure. (p. 309)
b. Femoral
10. Explain how Starling’s law of the heart is involved
c. Hepatic
in the maintenance of blood pressure. (p. 307)
d. Brachial
e. Inferior mesenteric 11. Name two hormones involved in the maintenance
f. Internal carotid of blood pressure, and state the function of each.
g. Subclavian (p. 309)
h. Intercostal
12. Describe two different ways the kidneys respond
5. Describe the pathway of blood flow in hepatic por- to decreased blood flow and blood pressure.
tal circulation. Use a specific example to explain the (p. 311)
purpose of portal circulation. (pp. 301, 304) 13. State two compensations that will maintain blood
pressure after a small loss of blood. (p. 309)
6. Begin at the right ventricle and describe the path-
way of pulmonary circulation. Explain the purpose 14. State the location of the vasomotor center and
of this pathway. (p. 296) name its two parts. Name the division of the

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318 The Vascular System


autonomic nervous system that carries impulses greater vasoconstriction is brought about. Explain
to blood vessels. Which blood vessels? Which how vasodilation is brought about. How will
tissue in these vessels? Explain why normal each of these changes affect blood pressure?
vasoconstriction is important. Explain how (pp. 312–313)




FOR FURTHER THOUGHT

1. Some old textbooks used the term descending aorta. 3. A friend tells you that her grandmother has a ten-
Explain what is meant by that, and why it is not a dency to develop blood clots in the veins of her
very good term. Explain why an aneurysm of the legs. Your friend fears that her grandmother will
aorta is quite likely to rupture sooner or later. have a stroke as a result. How would you explain
2. Renee, a nurse, is first on the scene of a car acci- that a stroke from a clot there is not likely? Because
dent. The driver has been thrown from the car, and you are a good friend, you want to explain the seri-
even from 15 feet away, Renee knows that a large ous result that may occur. How would you do that?
artery in the man’s leg has been severed. How does 4. Some people with hypertension take prescribed
she know this? What two things does she see? diuretics. Some call these “water pills.” Is this an
Renee stops the bleeding, but the ambulance has accurate name? How can a diuretic help lower
not arrived. She wants to assess the man’s condition blood pressure? What disadvantage does the use of
after he has lost so much blood. She cannot take a diuretics have?
blood pressure, but what other vital sign can be
helpful? Explain. If Renee could take a blood pres- 5. Sinusoids are found in the liver and pituitary gland.
sure reading, what might it be? Might it be within For each of these organs, name four specific large
the normal range? Explain. molecules that enter the blood by way of sinusoids.

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CHAPTER 14




The Lymphatic System

and Immunity

























































319

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CHAPTER 14





Chapter Outline Student Objectives
Lymph • Describe the functions of the lymphatic system.
Lymph Vessels • Describe how lymph is formed.
Lymphatic Tissue • Describe the system of lymph vessels, and explain
Lymph Nodes and Nodules how lymph is returned to the blood.
Spleen • State the locations and functions of the lymph
Thymus nodes and nodules.
Immunity • State the location and functions of the spleen and
Innate Immunity thymus.
Barriers • Explain what is meant by immunity.
Defensive cells • Describe the aspects of innate immunity.
Chemical defenses • Describe adaptive immunity: cell-mediated and
Adaptive Immunity antibody-mediated.
Cell-Mediated Immunity • Describe the responses to a first and second expo-
Antibody-Mediated Immunity sure to a pathogen.
Antibody Responses • Explain the difference between genetic immunity
Types of Immunity and acquired immunity.
Aging and the Lymphatic System • Explain the difference between passive acquired
immunity and active acquired immunity.
BOX 14–1 HODGKIN’S DISEASE • Explain how vaccines work.
BOX 14–2 AIDS
BOX 14–3 DIAGNOSTIC TESTS
BOX 14–4 VACCINES
BOX 14–5 ALLERGIES
BOX 14–6 VACCINES THAT HAVE CHANGED OUR
LIVES











320

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The Lymphatic System

and Immunity






New Terminology Related Clinical Terminology
Acquired immunity (uh-KWHY-erd) AIDS (AYDS)
Active immunity (AK-tiv) Allergy (AL-er-jee)
Antibody-mediated immunity (AN-ti-BAH-dee Antibody titer (AN-ti-BAH-dee TIGH-ter)
ME-dee-ay-ted) Attenuated (uh-TEN-yoo-AY-ted)
Antigen (AN-ti-jen) Complement fixation test (KOM-ple-ment
B cells (B SELLS) fik-SAY-shun)
Cell-mediated immunity (SELL ME-dee-ay-ted) Fluorescent antibody test (floor-ESS-ent)
Complement (KOM-ple-ment) Hodgkin’s disease (HODJ-kinz)
Cytokines (SIGH-toh-kines) Tonsillectomy (TAHN-si-LEK-toh-mee)
Genetic immunity (je-NET-ik) Toxoid (TOK-soyd)
Humoral immunity (HYOO-mohr-uhl) Vaccine (vak-SEEN)
Interferon (in-ter-FEER-on)
Lymph (LIMF)
Lymph nodes (LIMF NOHDS)
Lymph nodules (LIMF NAHD-yools)
Opsonization (OP-sah-ni-ZAY-shun)
Passive immunity (PASS-iv)
Plasma cell (PLAZ-mah SELL)
Spleen (SPLEEN)
T cells (T SELLS)
Thymus (THIGH-mus)
Tonsils (TAHN-sills)



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











321

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322 The Lymphatic System and Immunity




A child falls and scrapes her knee. Is this likely to return. The smooth muscle layer of the larger lymph
be a life-threatening injury? Probably not, even vessels constricts, and the one-way valves (just like
though the breaks in the skin have permitted the entry those of veins) prevent backflow of lymph. Lymph ves-
of thousands or even millions of bacteria. Those bac- sels in the extremities, especially the legs, are com-
teria, however, will be quickly destroyed by the cells pressed by the skeletal muscles that surround them;
and organs of the lymphatic system. this is the skeletal muscle pump. The respiratory
Although the lymphatic system may be considered pump alternately expands and compresses the lymph
part of the circulatory system, we will consider it sep- vessels in the chest cavity and keeps the lymph moving.
arately because its functions are so different from Where is the lymph going? Back to the blood to
those of the heart and blood vessels. Keep in mind, become plasma again. Refer to Fig. 14–3 as you read
however, that all of these functions are interdepend- the following. The lymph vessels from the lower body
ent. The lymphatic system is responsible for returning unite in front of the lumbar vertebrae to form a vessel
tissue fluid to the blood and for protecting the body called the cisterna chyli, which continues upward in
against foreign material. The parts of the lymphatic front of the backbone as the thoracic duct. Lymph
system are the lymph, the system of lymph vessels, and vessels from the upper left quadrant of the body join
lymphatic tissue, which includes lymph nodes and the thoracic duct, which empties lymph into the left
nodules, the spleen, and the thymus gland. subclavian vein. Lymph vessels from the upper right
quadrant of the body unite to form the right lymphatic
duct, which empties lymph into the right subclavian
LYMPH vein. Flaps in both subclavian veins permit the entry of
lymph but prevent blood from flowing into the lymph
Lymph is the name for tissue fluid that enters lymph vessels.
capillaries. As you may recall from Chapter 13, filtra-
tion in capillaries creates tissue fluid from blood
plasma, most of which returns almost immediately to LYMPHATIC TISSUE
the blood in the capillaries by osmosis. Some tissue
fluid, however, remains in interstitial spaces and must Lymphatic tissue consists mainly of lymphocytes in a
be returned to the blood by way of the lymphatic ves- mesh-like framework of connective tissue. Recall that
sels. Without this return, blood volume and blood most lymphocytes are produced from stem cells in the
pressure would very soon decrease. The relationship red bone marrow, then migrate to the lymph nodes
of the lymphatic vessels to the cardiovascular system is and nodules, to the spleen, and to the thymus. In these
depicted in Fig. 14–1. structures, lymphocytes become activated and prolif-
erate in response to infection (this is a function of all
lymphatic tissue). The thymus has stem cells that pro-
LYMPH VESSELS duce a significant portion of the T lymphocytes.

The system of lymph vessels begins as dead-end LYMPH NODES AND NODULES
lymph capillaries found in most tissue spaces (Fig.
14–2). Lymph capillaries are very permeable and col- Lymph nodes and nodules are masses of lymphatic
lect tissue fluid and proteins. Lacteals are specialized tissue. Nodes and nodules differ with respect to size
lymph capillaries in the villi of the small intestine; they and location. Nodes are usually larger, 10 to 20 mm in
absorb the fat-soluble end products of digestion, such length, and are encapsulated; nodules range from a
as fatty acids and vitamins A, D, E, and K. fraction of a millimeter to several millimeters in length
Lymph capillaries unite to form larger lymph ves- and do not have capsules.
sels, whose structure is very much like that of veins. Lymph nodes are found in groups along the path-
There is no pump for lymph (as the heart is the pump ways of lymph vessels, and lymph flows through these
for blood), but the lymph is kept moving within lymph nodes on its way to the subclavian veins. Lymph enters
vessels by the same mechanisms that promote venous a node through several afferent lymph vessels and

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The Lymphatic System and Immunity 323


Subclavian vein






Lymphatic
vessel
Valve

Lymph node






Figure 14–1. Relationship of lym- Heart
phatic vessels to the cardiovascular
system. Lymph capillaries collect tis-
sue fluid, which is returned to the
blood. The arrows indicate the direc-
tion of flow of the blood and lymph.
QUESTION: To which large veins is
lymph returned, and why is this
return important?
Lymph
flow



Blood
flow
Lymph
capillaries
Blood
capillaries









leaves through one or two efferent vessels (Fig. 14–4). gic locations. These are the cervical, axillary, and
As lymph passes through a lymph node, bacteria and inguinal lymph nodes (see Fig. 14–3). Notice that
other foreign materials are phagocytized by fixed (sta- these are at the junctions of the head and extremities
tionary) macrophages. Plasma cells develop from with the trunk of the body. Breaks in the skin, with
lymphocytes exposed to pathogens in the lymph and entry of pathogens, are much more likely to occur in
produce antibodies. These antibodies will eventually the arms or legs or head rather than in the trunk. If
reach the blood and circulate throughout the body. these pathogens get to the lymph, they will be
There are many groups of lymph nodes along all destroyed by the lymph nodes before they get to the
the lymph vessels throughout the body, but three trunk, before the lymph is returned to the blood in the
paired groups deserve mention because of their strate- subclavian veins.

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324 The Lymphatic System and Immunity


contain. A tonsillectomy is the surgical removal of
Tissue fluid
Lymph the palatine tonsils and the adenoid and may be per-
capillary Cells formed if the tonsils are chronically inflamed and
swollen, as may happen in children. As mentioned ear-
Arteriole lier, the body has redundant structures to help ensure
survival if one structure is lost or seriously impaired.
Thus, there are many other lymph nodules in the
pharynx to take over the function of the surgically
removed tonsils.

SPLEEN
Venule
The spleen is located in the upper left quadrant of the
Blood capillary
abdominal cavity, just below the diaphragm, behind
the stomach. The lower rib cage protects the spleen
from physical trauma (see Fig. 14–3).
Figure 14–2. Dead-end lymph capillaries found in tis-
sue spaces. Arrows indicate the movement of plasma, In the fetus, the spleen produces red blood cells, a
lymph, and tissue fluid. function assumed by the red bone marrow after birth.
QUESTION: Just before water enters lymph capillaries, After birth the spleen is very much like a large lymph
what name does it have? node, except that its functions affect the blood that
flows through it rather than lymph.
The functions of the spleen after birth are:
You may be familiar with the expression “swollen 1. Contains plasma cells that produce antibodies to
glands,” as when a child has a strep throat (an inflam- foreign antigens.
mation of the pharynx caused by Streptococcus bacteria). 2. Contains fixed macrophages (RE cells) that phago-
These “glands” are the cervical lymph nodes that have cytize pathogens or other foreign material in the
enlarged as their macrophages attempt to destroy the blood. The macrophages of the spleen also phago-
bacteria in the lymph from the pharynx (see Box 14–1: cytize old red blood cells and form bilirubin. By
Hodgkin’s Disease). way of portal circulation, the bilirubin is sent to the
Lymph nodules are small masses of lymphatic tis- liver for excretion in bile.
sue found just beneath the epithelium of all mucous 3. Stores platelets and destroys them when they are
membranes. The body systems lined with mucous no longer useful.
membranes are those that have openings to the envi-
ronment: the respiratory, digestive, urinary, and repro- The spleen is not considered a vital organ, because
ductive tracts. You can probably see that these are also other organs compensate for its functions if the spleen
strategic locations for lymph nodules, because any nat- must be removed. The liver and red bone marrow
ural body opening is a possible portal of entry for will remove old red blood cells and platelets from cir-
pathogens. For example, if bacteria in inhaled air get culation. The many lymph nodes and nodules will
through the epithelium of the trachea, lymph nodules phagocytize pathogens (as will the liver) and have lym-
with their macrophages are in position to destroy these phocytes to be activated and plasma cells to produce
bacteria before they get to the blood. antibodies. Despite this redundancy, a person without
Some of the lymph nodules have specific names. a spleen is somewhat more susceptible to certain bac-
Those of the small intestine are called Peyer’s terial infections such as pneumonia and meningitis.
patches, and those of the pharynx are called tonsils. THYMUS
The palatine tonsils are on the lateral walls of the
pharynx, the adenoid (pharyngeal tonsil) is on the pos- The thymus is located inferior to the thyroid gland.
terior wall, and the lingual tonsils are on the base of In the fetus and infant, the thymus is large and extends
the tongue. The tonsils, therefore, form a ring of lym- under the sternum (Fig. 14–5). With increasing age,
phatic tissue around the pharynx, which is a common the thymus shrinks, and relatively little thymus tissue
pathway for food and air and for the pathogens they is found in adults, though it is still active.

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The Lymphatic System and Immunity 325






Submaxillary nodes

Cervical nodes

Left subclavian vein

Thoracic duct
Mammary plexus
Axillary nodes


Right lymphatic duct
Spleen

Cisterna chyli

Mesenteric nodes
Cubital nodes
Iliac nodes

Inguinal nodes















Popliteal nodes


















Figure 14–3. System of lymph vessels and the major groups of lymph nodes. Lymph is
returned to the blood in the right and left subclavian veins.
QUESTION: Where are the major paired groups of lymph nodes located?

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326 The Lymphatic System and Immunity




Afferent lymphatic vessel
Capsule

Cortex







Nodal vein
Nodal artery

Hilus
Valve
A
Efferent lymphatic
vessel


Bacteria





Lymphocytes


B

Neutrophil

Antibody molecule
(enlarged)
Plasma cell
Antigen
Macrophage
(enlarged)
Figure 14–4. Lymph node. (A) Section through a lymph node, showing the flow of
lymph. (B) Microscopic detail of bacteria being destroyed within the lymph node.
QUESTION: What is the function of the plasma cells in a lymph node?


The stem cells of the thymus produce T lympho- cytes themselves. The newborn’s immune system is
cytes or T cells; their functions are discussed in the not yet fully mature, and infants are more susceptible
next section. Thymic hormones are necessary for what to certain infections than are older children and
may be called “immunological competence.” To be adults. Usually by the age of 2 years, the immune sys-
competent means to be able to do something well. tem matures and becomes fully functional. This is why
The thymic hormones enable the T cells to participate some vaccines, such as the measles vaccine, are not
in the recognition of foreign antigens and to provide recommended for infants younger than 15 to 18
immunity. This capability of T cells is established months of age. Their immune systems are not mature
early in life and then is perpetuated by the lympho- enough to respond strongly to the vaccine, and the

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The Lymphatic System and Immunity 327


Antigens are chemical markers that identify cells.
BOX 14–1 HODGKIN’S DISEASE
Human cells have their own antigens that identify all
Hodgkin’s disease is a malignant disorder of the cells in an individual as “self ” (recall the HLA
the lymph nodes; the cause is not known. The types mentioned in Chapter 11). When antigens are
first symptom is usually a swollen but painless foreign, or “non-self,” they may be recognized as such
lymph node, often in the cervical region. The and destroyed. Bacteria, viruses, fungi, and protozoa
individual is prompted to seek medical attention are all foreign antigens that activate immune re-
because of other symptoms: chronic fever, sponses, as are cell products such as bacterial toxins.
fatigue, and weight loss. The diagnosis involves Malignant cells, which may be formed within the
biopsy of the lymph node and the finding of body as a result of mutations of normal cells, are also
characteristic cells. recognized as foreign and are usually destroyed before
Treatment of Hodgkin’s disease requires they can establish themselves and cause cancer. Unfor-
chemotherapy, radiation, or both. With early
diagnosis and proper treatment, this malignancy tunately, organ transplants are also foreign tissue, and
is very often curable. the immune system may reject (destroy) a transplanted
kidney or heart. Sometimes the immune system mis-
takenly reacts to part of the body itself and causes
an autoimmune disease; several of these were men-
protection provided by the vaccine may be incom- tioned in previous chapters. Most often, however, the
plete. immune mechanisms function to protect the body
from the microorganisms around us and within us.
Immunity has two main components: innate immu-
IMMUNITY nity and adaptive immunity. Before we describe each
component, a brief general comparison may be help-
Immunity may be defined as the ability to destroy ful. Innate immunity may be called nonspecific, does
pathogens or other foreign material and to prevent not create memory, and its responses are always the
further cases of certain infectious diseases. This ability same regardless of the target. Adaptive immunity is
is of vital importance because the body is exposed to very specific as to its target, may involve antibodies,
pathogens from the moment of birth. does create memory, and may become more efficient.
Both kinds of immunity work together to prevent
damage and disease.

INNATE IMMUNITY

Innate immunity has several aspects: anatomic and
physiological barriers, phagocytic and other defensive
cells, and chemical secretions and reactions, including
inflammation. These are not separate and distinct;
Trachea rather there is a great deal of overlap among them, as
you will see. The innate immune responses are always
Clavicle
the same, and their degree of efficiency does not
First
rib increase with repeated exposure.
Thymus
gland Barriers
The stratum corneum of the epidermis of the skin is
non-living, and when unbroken is an excellent barrier
to pathogens of all kinds. The fatty acids in sebum
help limit the growth of bacteria on the skin. The liv-
ing cells of the epidermis produce defensins, which are
antimicrobial chemicals. The mucous membranes of
Figure 14–5. Location of the thymus in a young child. the respiratory, digestive, urinary, and reproductive
QUESTION: Which blood cells mature in the thymus? tracts are living tissue, yet still a good barrier. The

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328 The Lymphatic System and Immunity


ciliated epithelium of the upper respiratory tract is an Chemical Defenses
especially effective barrier. Dust and pathogens are
trapped on the mucus, the cilia sweep the mucus to the Chemicals that help the body resist infection include
pharynx, and it is swallowed. The hydrochloric acid of the interferons, complement, and the chemicals
the gastric juice destroys most pathogens that enter involved in inflammation. The interferons (alpha-,
the stomach, either in mucus or with food and drink. beta-, and gamma-interferons) are proteins produced
Lysozyme, an enzyme found in saliva and tears, by cells infected with viruses and by T cells. Viruses
inhibits the growth of bacteria in the oral cavity and must be inside a living cell to reproduce, and although
on the warm, wet surface of the eye. The subcuta- interferon cannot prevent the entry of viruses into
neous tissue contains many white blood cells (WBCs), cells, it does block their reproduction. When viral
as does the areolar connective tissue below the epithe- reproduction is blocked, the viruses cannot infect new
lium of mucous membranes. cells and cause disease. Interferon is probably a factor
in the self-limiting nature of many viral diseases (and
is used in the treatment of some diseases, such as hep-
Defensive Cells
atitis C).
Recall from Chapter 11 that many of our defensive Complement is a group of more than 20 plasma
cells are white blood cells. Macrophages, both fixed proteins that circulate in the blood until activated.
and wandering, have receptors for the pathogens They are involved in the lysis of cellular antigens and
humans are likely to encounter (this probably reflects the labeling of noncellular antigens. Some stimulate
millions of years of coexistence) and are very efficient the release of histamine in inflammation; others
phagocytes. Other cells capable of phagocytosis of attract WBCs to the site.
pathogens or other foreign antigens are the neutro- Inflammation is a general response to damage of
phils and, to a lesser extent, the eosinophils. Phago- any kind: microbial, chemical, or physical. Basophils
cytic cells use intracellular enzymes and chemicals and mast cells release histamine and leukotrienes,
such as hydrogen peroxide (H O ) to destroy ingested which affect blood vessels as previously described.
2 2
pathogens. Vasodilation increases blood flow to the damaged area,
The Langerhans cells of the skin, and other den- and capillaries become more permeable; tissue fluid
dritic cells throughout the body, also phagocytize for- and WBCs collect at the site. The purpose of inflam-
eign material, not merely to destroy it, but to take it to mation is to try to contain the damage, keep it from
a lymph node where the lymphocytes of adaptive spreading, eliminate the cause, and permit repair of the
immune mechanisms are then activated. The macro- tissue to begin. From even this brief description you
phages are also involved in activating these lympho- can see why the four signs of inflammation are redness,
cytes. This is a very important link between the two heat, swelling, and pain: redness from greater blood
components of immunity. flow, heat from the blood and greater metabolic activ-
Natural killer cells (NK cells) circulate in the ity, swelling from the accumulation of tissue fluid, and
blood but are also found in the red bone marrow, pain from the damage itself and perhaps the swelling.
spleen, and lymph nodes. They are a small portion As mentioned in Chapter 10, inflammation is a pos-
(about 10%) of the total lymphocytes, but are able to itive feedback mechanism that may become a vicious
destroy many kinds of pathogens and tumor cells. NK cycle of damage and more damage. The hormone cor-
cells make direct contact with foreign cells, and kill tisol is one brake that prevents this, and at least one of
them by rupturing their cell membranes (with chemi- the complement proteins has this function as well.
cals called perforins) or by inflicting some other kind There are probably other chemical signals (in general
of chemical damage. called cytokines and chemokines) that help limit
Basophils and mast cells (a type of connective tissue inflammation to an extent that is useful.
cell) are also defensive cells that are found throughout In summary, innate immunity is nonspecific, is
areolar connective tissue. They produce histamine and always the same, does not create memory, and does
leukotrienes. Histamine causes vasodilation and makes not become more efficient upon repeated exposures.
capillaries more permeable; these are aspects of Some cells of innate immune mechanisms also activate
inflammation. Leukotrienes also increase capillary the adaptive immune mechanisms. The aspects of
permeability and attract phagocytic cells to the area. innate immunity are shown in Fig. 14–6.

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The Lymphatic System and Immunity 329


A Barriers

Mucous membranes and lysozyme
Integumentary system
Lacrimal gland
Stratum corneum
Ciliated epithelium
Defensins Langerhans cells
• •
•
Subcutaneous tissue
and WBC's •
•
• •
•
Hydrochloric
•
acid
Salivary glands
B Cells
Natural killer cells
Langerhans cells Phagocytes
•
Bacteria
•
Perforins
•
•
Antigen •
Macrophage
•
Neutrophils Basophils and mast cells
Activate lymphocytes




Histamine and leukotrienes


C Chemicals
Inflammation
Interferon Complement
Histamine and leukotrienes
T cell

•
Lyses cells, Vasodilation •
attracts WBC's
Increased
capillary
permeability
•
Blocks viral Tissue fluid
reproduction and WBCs •


Figure 14–6. Innate immunity. (A) Barriers. (B) Defensive cells. (C) Chemical defenses.
See text for description.
QUESTION: The three aspects of innate immunity are interconnected; describe two of
these connections.

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330 The Lymphatic System and Immunity


ADAPTIVE IMMUNITY called simply cellular immunity), in which T cells and
macrophages participate, and antibody-mediated
To adapt means to become suitable, and adaptive immunity (or humoral immunity), which involves T
immunity can become “suitable” for and respond to cells, B cells, and macrophages.
almost any foreign antigen. Adaptive immunity is spe-
cific and is carried out by lymphocytes and macro-
phages. Cell-Mediated Immunity
The majority of lymphocytes are the T lympho- This mechanism of immunity does not result in the
cytes and B lymphocytes, or, more simply, T cells and production of antibodies, but it is effective against
B cells. In the embryo, T cells are produced in the intracellular pathogens (such as viruses), fungi,
bone marrow and thymus. They must pass through malignant cells, and grafts of foreign tissue. As men-
the thymus, where the thymic hormones bring about tioned earlier, the first step is the recognition of the
their maturation. The T cells then migrate to the foreign antigen by macrophages and helper T cells,
spleen, lymph nodes, and lymph nodules, where they which become activated and are specific. (You may
are found after birth. find it helpful to refer to Fig. 14–7 as you read the fol-
Produced in the embryonic bone marrow, B cells lowing.)
migrate directly to the spleen and lymph nodes and These activated T cells, which are antigen specific,
nodules. When activated during an immune response, divide many times to form memory T cells and cyto-
some B cells will divide many times and become toxic (killer) T cells (also called CD8 T cells). The
plasma cells that produce antibodies to a specific for- memory T cells will remember the specific foreign
eign antigen. antigen and become active if it enters the body again.
The mechanisms of immunity that involve T cells Cytotoxic T cells are able to chemically destroy for-
and B cells are specific, meaning that one foreign anti- eign antigens by disrupting cell membranes. This
gen is the target each time a mechanism is activated. A is how cytotoxic T cells destroy cells infected with
macrophage has receptor sites for foreign chemicals viruses and prevent the viruses from reproducing.
such as those of bacterial cell walls or flagella, and may These T cells also produce cytokines, which are chem-
phagocytize just about any foreign material it comes icals that attract macrophages to the area and activate
across (as will the Langerhans or dendritic cells). T them to phagocytize the foreign antigen and cellular
cells and B cells, however, become very specific, as you debris.
will see. It was once believed that another subset of T cells
The first step in the destruction of a pathogen or served to stop the immune response, but this may not
foreign cell is the recognition of its antigens as for- be so. It seems probable that the CD4 and CD8 T
eign. Both T cells and B cells are capable of this, but cells also produce feedback chemicals to limit the
the immune mechanisms are activated especially well immune response once the foreign antigen has been
when this recognition is accomplished by macro- destroyed. The memory T cells, however, will quickly
phages and a specialized group of T lymphocytes initiate the cell-mediated immune response should
called helper T cells (also called CD4 T cells). The there be a future exposure to the antigen.
foreign antigen is first phagocytized by a macrophage,
and parts of it are “presented” on the macrophage’s Antibody-Mediated Immunity
cell membrane. Also on the macrophage membrane
are “self” antigens that are representative of the This mechanism of immunity does involve the pro-
antigens found on all of the cells of the individual. duction of antibodies and is also diagrammed in Fig.
Therefore, the helper T cell that encounters this 14–7. Again, the first step is the recognition of the for-
macrophage is presented not only with the foreign eign antigen, this time by B cells as well as by
antigen but also with “self” antigens for comparison. macrophages and helper T cells. The sensitized helper
The helper T cell becomes sensitized to and specific T cell presents the foreign antigen to B cells, which
for the foreign antigen, the one that does not belong provides a strong stimulus for the activation of B cells
in the body (see Box 14–2: AIDS). specific for this antigen. The activated B cells begin to
The recognition of an antigen as foreign initiates divide many times, and two types of cells are formed.
one or both of the mechanisms of adaptive immunity. Some of the new B cells produced are memory B
These are cell-mediated immunity (sometimes cells, which will remember the specific antigen and

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Copyright © 2007 by F. A. Davis.

The Lymphatic System and Immunity 331



A Cell-mediated
Self antigens
Macrophage
•
• Receptor sites

•
•

• Cytotoxic T cell
Helper T cell
Foreign antigen
Chemically destroys Produces cytokines to
foreign cells attract macrophages
Memory T cell






B Antibody-mediated
Self antigens
•
Macrophage •
Receptor sites
•
•

Helper T cells Memory B cell


B cell Plasma cell







Antibodies





Opsonization Antigen-antibody complex Complement fixation
lysis of
cellular antigen


Macrophage


Figure 14–7. Adaptive immunity. (A) Cell-mediated immunity. (B) Antibody-mediated
immunity. See text for description.
QUESTION: Adaptive immunity has memory; which cells provide this? What kind of mem-
ory is it?
331

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332 The Lymphatic System and Immunity


BOX 14–2 AIDS

In 1981, young homosexual men in New York and placental transmission of the virus from mother to
California were diagnosed with Kaposi’s sarcoma fetus.
and Pneumocystis carinii pneumonia. At that time, In the United States, most of the cases of AIDS
Kaposi’s sarcoma was known as a rare, slowly grow- during the 1980s were in homosexual men and IV
ing malignancy in elderly men. Pneumocystis pneu- drug users who shared syringes contaminated with
monia was almost unheard of; P. carinii (now P. their blood. By the 1990s, however, it was clear that
jiroveci) is a pathogen that does not cause disease in AIDS was becoming more of a heterosexually trans-
healthy people. That in itself was a clue. These mitted disease, with rapidly increasing case rates
young men were not healthy; their immune sys- among women and teenagers. In much of the rest
tems were not functioning normally. As the number of the world, especially Africa and Asia, the trans-
of patients increased rapidly, the disease was given mission of AIDS has always been primarily by het-
a name (acquired immunodeficiency syndrome— erosexual contact, with equal numbers of women
AIDS) and the pathogen was found. Human and men infected. In many of these countries AIDS
immunodeficiency virus (HIV) is a retrovirus that is an enormous public health problem, and the
infects helper T cells, macrophages, and other annual number of new cases is still rising.
human cells. Once infected, the human cells con- At present we have no medications that will
tain HIV genes for the rest of their lives. Without suf- eradicate HIV, although certain combinations of
ficient helper T cells, the immune system is seriously drugs effectively suppress the virus in some people.
impaired. Foreign antigens are not recognized, B For these people, AIDS may become a chronic but
cells are not activated, and killer T cells are not not fatal disease. Unfortunately, the medications do
stimulated to proliferate. not work for everyone, and they are very expensive,
The person with AIDS is susceptible to oppor- beyond the means of most of the world’s AIDS
tunistic infections, that is, those infections caused patients.
by fungi and protozoa that would not affect aver- Development of an AIDS vaccine has not yet
age healthy adults. Some of these infections may be been successful, although dozens of vaccines are
treated with medications and even temporarily undergoing clinical trials. A vaccine stimulates anti-
cured, but the immune system cannot prevent the body production to a specific pathogen, but every-
next infection, or the next. As of this writing, AIDS one who has died of AIDS had antibodies to HIV.
is considered an incurable disease, although with Those antibodies were not protective because HIV is
proper medical treatment, some people with AIDS a mutating virus; it constantly changes itself, mak-
may live for many years. ing previously produced antibodies ineffective. An
Where did this virus come from? The latest AIDS vaccine may not be entirely effective, may not
research suggests that HIV evolved from a harmless have the 80% to 90% protection rate we have
chimpanzee virus in Africa sometime during the come to expect from vaccines.
1930s. Spread of the virus was very slow at first, If we cannot cure AIDS and we cannot yet pre-
and only when air travel became commonplace did vent it by vaccination, what recourse is left?
the virus spread worldwide. Education. Everyone should know how AIDS is
The incubation period of AIDS is highly variable, spread. The obvious reason is to be able to avoid
ranging from a few months to several years. the high-risk behaviors that make acquiring HIV
An infected person may unknowingly spread HIV to more likely. Yet another reason, however, is that
others before any symptoms appear. It should everyone should know that they need not fear
be emphasized that AIDS, although communicable, casual contact with people with AIDS. Healthcare
is not a contagious disease. It is not spread personnel have a special responsibility, not only to
by casual contact as is measles or the common educate themselves, but to provide education
cold. Transmission of AIDS occurs through sexual about AIDS for their patients and the families of
contact, by contact with infected blood, or by their patients.



initiate a rapid response upon a second exposure. gamma globulins, are proteins shaped somewhat like
Other B cells become plasma cells that produce anti- the letter Y. Antibodies do not themselves destroy for-
bodies specific for this one foreign antigen. eign antigens, but rather become attached to such anti-
Antibodies, also called immune globulins (Ig) or gens to “label” them for destruction. Each antibody