The Circulatory System - coursewareobjects.com€¦ · 302 Chapter 12 The Circulatory System HEART Location, Size, and Position No one needs to be told where the heart is or what

300 Chapter 12 The Circulatory System12The Circulatory System

AFTER YOU HAVE COMPLETED THIS CHAPTER, YOU SHOULD BE ABLE TO:

1. Discuss the location, size, and position of the heart in the thoracic cavity and identify the heart chambers, sounds, and valves.

2. Trace blood through the heart and compare the functions of the heart chambers on the right and left sides.

3. List the anatomical components of the heart conduction system and discuss the features of a normal electrocardiogram.

4. Explain the relationship between blood vessel structure and function.

5. Trace the path of blood through the systemic, pulmonary, hepatic portal, and fetal circula-tions.

6. Identify and discuss the primary factors in-volved in the generation and regulation of blood pressure and explain the relationships between these factors.

Objectives

HEART, XX

Location, Size, and Position, XX

Anatomy, XX

Heart Sounds, XX

Blood Flow Through the Heart, XX

Blood Supply to the Heart Muscle, XX

Cardiac Cycle, XX

Conduction System of the Heart, XX

Electrocardiogram, XX

BLOOD VESSELS, XX

Types, XX

Structure, XX

Functions, XX

CIRCULATION, XX

Systemic and Pulmonary Circulation, XX

Hepatic Portal Circulation, XX

Fetal Circulation, XX

BLOOD PRESSURE, XX

Defining Blood Pressure, XX

Factors That Influence Blood Pressure, XX

Fluctuations in Blood Pressure, XX

Outline

300

PULSE, XX

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Differing amounts of nutrients and waste products enter and leave the fl uid surround-ing each body cell continually. In addition,

requirements for hormones, body salts, water, and other critical substances constantly change. How-ever, homeostasis or constancy of the body fl uid con-tents surrounding the billions of cells that make up our bodies is required for survival. The system that supplies our cells’ transportation needs is the circu-latory system. The levels of dozens of substances in the blood can remain constant even though the absolute amounts that are needed or produced may change because we have this extremely effective sys-tem that transports these substances to or from each cell as circumstances change.

We begin the study of the circulatory system with the heart—the pump that keeps blood mov-ing through a closed circuit of blood vessels. De-tails related to heart structure will be followed by a discussion of how the heart functions. This chap-ter concludes with a study of the vessels through which blood fl ows as a result of the pumping ac-tion of the heart. As a group, these vessels are multipurpose structures. Some allow for rapid movement of blood from one body area to another. Others, such as the microscopic capillaries, permit the movement or exchange of many substances between the blood and fl uid surrounding body cells. Chapter 13 covers the lymphatic system and immunity topics that relate in many ways to the structure and functions of the circulatory system.

STUDY TIPS

To make the study of the nervous system more effi cient, we suggest these tips:1. Before studying Chapter 12, review

the synopsis of the circulatory system in Chapter 4. Chapter 12 deals with the heart, the pump that moves the blood; and the vessels, the tubing that carries the blood.

2. The term cardio- refers to the heart; in Chapter 7 you learned that myo- means muscle. Myocardium is the heart muscle.

3. The arteries and veins are composed of three layers of tissue. There is a differ-ence in thickness in these vessels be-cause the arteries carry blood under higher pressure. Arteries and veins carry blood in opposite directions—arteries away from the heart, veins toward the heart. Capillaries need to be thin-walled because this is where the exchange of material between the blood and the tis-sues takes place.

4. A liquid moves from high to low pres-sure, so it is logical that the blood pressure in the cardiovascular system is highest just after leaving the heart and is lowest just before returning to the heart.

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302 Chapter 12 The Circulatory System

HEARTLocation, Size, and PositionNo one needs to be told where the heart is or what it does. Everyone knows that the heart is in the chest, that it beats night and day to keep the blood fl owing, and that if it stops, life stops.

Most of us probably think of the heart as being located on the left side of the body. As you can see in Figure 12-1, the heart is located between the lungs in the lower portion of the mediastinum. Draw an imaginary line through the middle of the trachea in Figure 12-1 and continue the line down through the thoracic cavity to divide it into right and left halves. Note that about two-thirds of the mass of the heart is to the left of this line and one-third is to the right.

The heart is often described as a triangular or-gan, shaped and sized roughly like a closed fi st. In Figure 12-1 you can see that the apex, or blunt point, of the lower edge of the heart lies on the diaphragm, pointing toward the left. Doctors and nurses often listen to the heart sounds by placing a stethoscope on the chest wall directly over the apex of the heart. Sounds of the so-called apical beat are easily heard in this area (that is, in the space between the fi fth and sixth ribs on a line even with the midpoint of the left clavicle).

The heart is positioned in the thoracic cavity between the sternum in front and the bodies of the thoracic vertebrae behind. Because of this place-ment, it can be compressed or squeezed by appli-cation of pressure to the lower portion of the body of the sternum using the heel of the hand. Rhyth-mic compression of the heart in this way can main-tain blood fl ow in cases of cardiac arrest and, if combined with effective artifi cial respiration, the resulting procedure, called cardiopulmonary re-suscitation (CPR), can be lifesaving.

To learn more about the location of the heart, go to AnimationDirect on your CD-ROM.

AnatomyHeart ChambersIf you cut open a heart, you can see many of its main structural features (Figure 12-2). This organ is hol-low, not solid. A partition divides it into right and left sides. The heart contains four cavities, or hollow chambers. The two upper chambers are called atria (AY-tree-ah) (singular, atrium), and the two lower chambers are called ventricles (VEN-tri-kulz). The atria are smaller than the ventricles, and their walls are thinner and less muscular. Atria are often called receiving chambers because blood enters the heart through veins that open into these upper cavities. Eventually, blood is pumped from the heart into ar-teries that exit from the ventricles; therefore, the ven-tricles are sometimes referred to as the discharging chambers of the heart. Each heart chamber is named according to its location. Thus there are right and left atrial chambers above and right and left ventricular chambers below. The wall of each heart chamber is composed of cardiac muscle tissue usually referred to as the myocardium (my-oh-KAR-dee-um). The septum between the atrial chambers is called the in-teratrial septum; the interventricular septum separates the ventricles.

Each chamber of the heart is lined by a thin layer of very smooth tissue called the endocardium (en-doh-KAR-dee-um) (see Figure 12-2). Infl ammation of this lining is referred to as endocarditis (en-doh-kar-DYE-tis). If infl amed, the endocardial lining can become rough and abrasive to RBCs passing over its surface. Blood fl owing over a rough surface is sub-ject to clotting, and a thrombus (THROM-bus), or clot, may form (see Chapter 11). Unfortunately, rough spots caused by endocarditis or injuries to blood vessel walls often cause the release of platelet factors. The result is often the formation of a fatal blood clot.

To learn more about the chambers of the heart, go to AnimationDirect on your CD-ROM.


Chapter 12 The Circulatory System 303

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Superior vena cava

Left common carotid artery

Left subclavian artery

Arch of aorta

Left pulmonary artery

Left atrium

Left pulmonary veins

Great cardiac vein

Branches of leftcoronary arteryand cardiac vein

Left ventricle

Apex

Brachiocephalic trunk

Ascending aorta

Right pulmonary artery

Right pulmonary veins

Right atrium

Right coronary arteryand cardiac vein

Right ventricle

Trachea

Arch of aorta

Diaphragm

Lung

FIGURE 12-1

The heart. The heart and major blood vessels viewed from the front (anterior). Inset shows the relationship of the heart to other structures in the thoracic cavity.


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OUTSIDEOF

HEART INSIDEOF

HEART

Fatty connective tissue

Coronary vessels

Parietal pericardium

Pericardial space

Visceral pericardium (epicardium)

Endocardium

Myocardium

Aorta

Superior vena cava

Pulmonarysemilunar valve

Right atrium

Tricuspidvalve

Pulmonary arteries

Left pulmonary veins

Left atrium

Aortic semilunar valve

Bicuspid valve

Chordaetendineae

Left ventricle

Right ventricle

Interventricular septum

FIGURE 12-2

An internal view of the heart. The inset shows a cross section of the heart wall, including the pericardium.



Covering Sac, or PericardiumThe heart has a covering and a lining. Its cover-ing, called the pericardium (pair-i-KAR-dee-um), consists of two layers of fi brous tissue with a small space in between. The inner layer of the pericar-dium is called the visceral pericardium or epicar-dium (ep-i-KAR-dee-um). It covers the heart the way an apple skin covers an apple. The outer layer of pericardium is called the parietal pericardium. It fi ts around the heart like a loose-fi tting sack, allowing enough room for the heart to beat. It is easy to remember the difference between the endo-cardium, which lines the heart chambers, and the epicardium, which covers the surface of the heart (see Figure 12-2), if you understand the meaning of the prefi xes endo- and epi-. Endo- comes from the Greek word meaning “inside” or “within,” and epi- comes from the Greek word meaning “upon” or “on.”

The two pericardial layers slide against each other without friction when the heart beats be-cause these are serous membranes with moist, not dry, surfaces. A thin fi lm of pericardial fl uid fur-nishes the lubricating moistness between the heart and its enveloping pericardial sac. If the pericar-dium becomes infl amed, a condition called peri-carditis (pair-i-kar-DYE-tis) results.

Heart ActionThe heart serves as a muscular pumping device for distributing blood to all parts of the body. Con-traction of the heart is called systole (SIS-toh-lee), and relaxation is called diastole (dye-ASS-toh-lee). When the heart beats (that is, when it contracts), the atria contract fi rst (atrial systole), forcing blood into the ventricles. Once fi lled, the two ventricles con-tract (ventricular systole) and force blood out of the heart (Figure 12-3). For the heart to be effi cient in its pumping action, more than just the rhythmic con-traction of its muscular fi bers is required. The direc-tion of blood fl ow must be directed and controlled. This is accomplished by four sets of valves located at the entrance and near the exit of the ventricles.

Heart ValvesThe two valves that separate the atrial chambers above from the ventricles below are called AV, or atrioventricular (ay-tree-oh-ven-TRIK-yoo-lar), valves. The two AV valves are called the bicus-

pid, or mitral (MY-tral), valve, located between the left atrium and ventricle, and the tricuspid valve, located between the right atrium and ven-tricle. The AV valves prevent backfl ow of blood into the atria when the ventricles contract. Locate the AV valves in Figures 12-2 and 12-3. Note that a number of stringlike structures called chordae tendineae (KOR-dee ten-DIN-ee) attach the AV valves to the wall of the heart.

The SL, or semilunar (sem-i-LOO-nar), valves are located between the two ventricular chambers and the large arteries that carry blood away from the heart when contraction occurs (see Figure 12-3). The ventricles, like the atria, contract together; therefore, the two semilunar valves open and close at the same time. The pulmonary semilunar valve is located at the beginning of the pulmonary artery and allows blood going to the lungs to fl ow out of the right ventricle but prevents it from fl owing back into the ventricle. The aortic semilunar valve is lo-cated at the beginning of the aorta and allows blood to fl ow out of the left ventricle up into the aorta but prevents backfl ow into this ventricle.

Heart SoundsIf a stethoscope is placed on the anterior chest wall, two distinct sounds can be heard. They are rhythmical and repetitive sounds that are often de-scribed as lub dup.

The fi rst, or lub, sound is caused by the vibra-tion and abrupt closure of the AV valves as the ventricles contract. Closure of the AV valves pre-vents blood from rushing back up into the atria during contraction of the ventricles. This fi rst sound is of longer duration and lower pitch than the second. The pause between this fi rst sound and the dup, or second, sound is shorter than that after the second sound and the lub dup of the next systole. The second heart sound is caused by the closing of both the semilunar valves when the ventricles undergo diastole (relax).

Blood Flow Through the HeartThe heart acts as two separate pumps. The right atrium and the right ventricle perform a task quite different from the left atrium and the left ventricle. When the heart “beats,” fi rst the atria contract simul-


Atrial Systole Ventricular Systole

Aorta

Superiorvena cava

Semilunarvalves closed

Semilunarvalves open

Inferiorvena cava

Atrioventricularvalves open

Atrioventricularvalves open

Atrioventricularvalves closed

Atrioventricularvalves closed

Rightatrium

Leftatrium

Leftventricle

Rightventricle

Rightatrium

Leftatrium

LeftventricleRight

ventricle

Right AV(tricuspid)

valve

Left AV(mitral) valve

Right AV(tricuspid)

valve

Left AV(mitral) valve

Aortic SL valve

Pulmonary SL valve

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FIGURE 12-3

Heart action. A, During atrial systole (contraction) cardiac muscle in the atrial wall contracts, forcing blood through the atrioventricular (AV) valves and into the ventricles. Bottom illustration shows superior view of all four valves, with semilunar (SL) valves closed and AV valves open. B, During ventricular systole that follows, the AV valves close, and blood is forced out of the ventricles through the semilunar valves and into the arteries. Bottom illustration shows superior view of SL valves open and AV valves closed.

A B



taneously. This is atrial systole. Then the ventricles fi ll with blood, and they, too, contract together dur-ing ventricular systole. Although the atria contract as a unit followed by the ventricles below, the right and left sides of the heart act as separate pumps. As we study the blood fl ow through the heart, the separate functions of the two pumps will become clearer.

Note in Figure 12-3 that blood enters the right atrium through two large veins called the supe-rior vena (VEE-nah) cava (KAY-vah) and inferior vena cava. The right heart pump receives oxy-gen-poor blood from the veins. After entering the right atrium, it is pumped through the right AV, or tricuspid valve, and enters the right ven-tricle. When the ventricles contract, blood in the right ventricle is pumped through the pulmo-nary semilunar valve into the pulmonary artery and eventually to the lungs, where oxygen is added and carbon dioxide is lost.

As you can see in Figure 12-3, blood rich in oxy-gen returns to the left atrium of the heart through four pulmonary veins. It then passes through the left AV, or bicuspid valve, into the left ventricle. When the left ventricle contracts, blood is forced through the aortic semilunar valve into the aorta (ay-OR-tah) and is distributed to the body as a whole.

As you can tell from Figure 12-4, the two sides of the heart actually pump blood through two separate “circulations” and function as two sepa-rate pumps. The pulmonary circulation involves movement of blood from the right ventricle to the lungs, and the systemic circulation involves movement of blood from the left ventricle throughout the body as a whole. The pulmonary and systemic circulations are discussed later in this chapter.

Blood Supply to the Heart MuscleTo sustain life, the heart must pump blood through-out the body on a regular and ongoing basis. As a result, the heart muscle, or myocardium, requires a constant supply of blood containing nutrients and oxygen to function effectively. The delivery of oxygen and nutrient-rich arterial blood to cardiac muscle tissue and the return of oxygen-poor blood from this active tissue to the venous system are called coronary circulation.

Blood fl ows into the heart muscle by way of two small vessels—the right and left coronary arteries. The coronary arteries are the aorta’s fi rst branches (Figure 12-5). The openings into these small vessels lie behind the fl aps of the aortic SL valve. During ventricular diastole, blood in the aorta that backs up behind the aortic SL valve can fl ow into the coro-nary arteries.

In both coronary thrombosis and coronary em-bolism (EM-boh-liz-em), a blood clot occludes or plugs up some part of a coronary artery. Blood cannot pass through the occluded vessel and so cannot reach the heart muscle cells it normally supplies. Deprived of oxygen, these cells soon be-come damaged or die. In medical terms, myocar-dial (my-oh-KAR-dee-all) infarction (in-FARK-shun) (MI), or tissue death, occurs. Myocardial infarction, also referred to as a “heart attack,” is a common cause of death during middle and late adulthood. Recovery from a myocardial infarction is possible if the amount of heart tissue damaged was small enough so that the remaining undam-aged heart muscle can pump blood effectively enough to supply the needs of the rest of the heart and the body. The term angina (an-JYE-nah) pec-toris (PEK-tor-is) is used to describe the severe chest pain that occurs when the myocardium is deprived of adequate oxygen. It is often a warning that the coronary arteries are no longer able to supply enough blood and oxygen to the heart muscle. Coronary bypass surgery is a common treatment for those who suffer from severely re-stricted coronary artery blood fl ow. In this proce-dure, veins or arteries are “harvested” or removed from other areas of the body and used to bypass partial blockages in coronary arteries (Figure 12-6). Another treatment used to improve coronary blood fl ow is angioplasty, a procedure in which a device is inserted into a blood vessel to open a channel for blood fl ow.

After blood has passed through the capillary beds in the myocardium, it fl ows into cardiac veins, which empty into the coronary sinus and fi nally into the right atrium.

Cardiac CycleThe beating of the heart is a regular and rhyth-mic process. Each complete heartbeat is called a cardiac cycle and includes the contraction (sys-



QUICKtole) and relaxation (diastole) of atria and ventri-cles. Each cycle takes about 0.8 seconds to com-plete if the heart is beating at an average rate of about 72 beats per minute. The term stroke vol-ume refers to the volume of blood ejected from the ventricles during each beat. Cardiac output, or the volume of blood pumped by one ventricle per minute, averages about 5 L in a normal, rest-ing adult.

1. What are the functions of the atria and ventricles of the heart?

2. What coverings does the heart have? What is the heart’s lining called?

3. What are systole and diastole of the heart?

4. What are the two major “circulations” of the body?

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Circulation totissues of headand upper body

O2CO2Systemic

capillaries

Lung

CO2

CO2

O2

Pulmonarycapillaries

Pulmonary circulation

CO2 O2

Systemic circulation

Circulation totissues oflower body

Lung

O2

FIGURE 12-4

Blood fl ow through the circulatory system. In the pulmonary circulatory route, blood is pumped from the right side of the heart to the gas-exchange tissues of the lungs. In the systemic circulation, blood is pumped from the left side of the heart to all other tissues of the body.



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Superiorvena cava

Aorta

Aorticsemilunar

valve

Rightcoronary

artery

Rightventricle

Leftventricle

Branchesof left coronaryartery

Leftatrium

Leftcoronaryartery

Pulmonarytrunk

Superiorvena cava

Rightatrium

Aorta

Pulmonarytrunk

Leftatrium

Coronarysinus

Greatcardiac vein

Leftventricle

Smallcardiac

vein

Rightventricle

Rightatrium

FIGURE 12-5

Coronary circulation. A, Arteries. B, Veins. Both are anterior views of the heart. Vessels near the anterior surface are more darkly colored than those of the posterior surface seen through the heart.

A B

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Triple bypass

Aorta

Vein graftsfrom the leg

Rightcoronary

artery

Obstruction

Leftcoronaryartery

FIGURE 12-6

Coronary bypass. In coronary bypass surgery, blood vessels are “harvested” from other parts of the body and used to construct detours around blocked coronary arter-ies. Artifi cial vessels can also be used.



Conduction System of the HeartCardiac muscle fi bers can contract rhythmically on their own. However, they must be coordinat-ed by electrical signals (impulses) if the heart is to pump effectively. Although the rate of the car-diac muscle’s rhythm is controlled by autonomic nerve signals, the heart has its own built-in con-duction system for coordinating contractions during the cardiac cycle. The most important thing to realize about this conduction system is that all of the cardiac muscle fi bers in each re-gion of the heart are electrically linked together. The intercalated disks that were fi rst introduced in Chapter 3 (see Figure 3-20, p. ***) are actually electrical connectors that join muscle fi bers into a single unit that can conduct an impulse through the entire wall of a heart chamber without stop-ping. Thus both atrial walls will contract at about the same time because all their fi bers are electri-cally linked. Likewise, both ventricular walls will contract at about the same time.

Four structures embedded in the wall of the heart are specialized to generate strong impulses and conduct them rapidly to certain regions of the heart wall. Thus they make sure that the atria contract and then the ventricles contract in an effi cient manner. The names of the structures that make up this conduction system of the heart follow:

1. Sinoatrial (sye-no-AY-tree-al) node, which is sometimes called the SA node or the pacemaker

2. Atrioventricular (ay-tree-oh-ven-TRIK-yoo-lar) node, or AV node

3. AV bundle, or bundle of His4. Purkinje (pur-KIN-jee) fi bersImpulse conduction normally starts in the heart’s

pacemaker, namely, the SA node. From there, it spreads, as you can see in Figure 12-7, in all direc-tions through the atria. This causes the atrial fi bers to contract. When impulses reach the AV node, it relays them by way of the bundle of His and Pur-kinje fi bers to the ventricles, causing them to con-tract. Normally, therefore, a ventricular beat follows each atrial beat. Various conditions such as endocar-ditis or myocardial infarction, however, can damage the heart’s conduction system and thereby disturb

its rhythmic beating. One such disturbance is the condition commonly called heart block. Impulses are blocked from getting through to the ventricles, re-sulting in the heart beating at a much slower rate than normal. A physician may treat heart block by implanting in the heart an artifi cial pacemaker, an electrical device that causes ventricular contractions at a rate fast enough to maintain an adequate circu-lation of blood.

ElectrocardiogramThe specialized structures of the heart’s con-duction system generate tiny electrical currents that spread through surrounding tissues to the surface of the body. This fact is of great clinical signifi cance because these electrical signals can be picked up from the body surface and trans-formed into visible tracings by an instrument called an electrocardiograph (ee-lek-troh-KAR-dee-oh-graf).

The electrocardiogram (ee-lek-troh-KAR-dee-oh-gram), or ECG, is the graphic record of the heart’s electrical activity. Skilled interpretation of these ECG records may sometimes make the dif-ference between life and death. A normal ECG tracing is shown in Figure 12-8.

A normal ECG tracing has three very character-istic defl ections or waves called the P wave, the QRS complex, and the T wave. These defl ections represent the electrical activity that regulates the contraction or relaxation of the atria or ventricles. The term depolarization describes the electrical activity that triggers contraction of the heart mus-cle. Repolarization begins just before the relax-ation phase of cardiac muscle activity. In the nor-mal ECG shown in Figure 12-8, the small P wave occurs with depolarization of the atria. The QRS complex occurs as a result of depolarization of the ventricles, and the T wave results from electrical activity generated by repolarization of the ventri-cles. You may wonder why no visible record of atrial repolarization is noted in a normal ECG. The reason is simply that the defl ection is very small and is hidden by the large QRS complex that oc-curs at the same time.

Damage to cardiac muscle tissue that is caused by a myocardial infarction or disease affecting the



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heart’s conduction system results in distinctive changes in the ECG. Therefore ECG tracings are extremely valuable in the diagnosis and treatment of heart disease.

1. What structure is the natural “pace-maker” of the heart?

2. What information is in an electrocar-diogram?

BLOOD VESSELSTypesArterial blood is pumped from the heart through a series of large distribution vessels—the arteries. The largest artery in the body is the aorta. Arter-ies subdivide into vessels that become progres-sively smaller and fi nally become tiny arterioles (ar-TEER-ee-ohlz) that control the fl ow into micro-scopic exchange vessels called capillaries (KAP-i-

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Sinoatrial (SA) node(pacemaker)

Atrioventricular (AV) node Purkinje fibers

Right and left branchesof AV bundle (bundle of His)

Superior vena cava

Tricuspid valve

Right ventricle

Inferior vena cava

Left ventricle

Mitral (bicuspid)valve

Pulmonary veins

Pulmonary artery

Aorta

FIGURE 12-7

Conduction system of the heart. Specialized cardiac muscle cells in the wall of the heart rapidly conduct an electrical impulse throughout the myocardium. The signal is initiated by the SA node (pacemaker) and spreads to the rest of the atrial myocardium and to the atrioventricular (AV) node. The AV node then initiates a signal that is conducted through the ventricular myocardium by way of the AV bundle (of His) and Purkinje fi bers.


Depolarized Repolarized

P

P

P

Q

R

S

QRS complex

P wave T wave

QRS complex

P wave T wave

P

Q

R

S

FIGURE 12-8

Events represented by the electrocardiogram (ECG). It is nearly impossible to illustrate the invisible, dynamic events of heart conduction in a few cartoon panels or “snapshots,” but the sketches here give you an idea of what is happening in the heart as the ECG is recorded. A, The heart wall is completely relaxed, with no change in electrical activity, so the ECG remains constant. B, P wave occurs when the AV node and atrial walls depolarize. C, Atrial walls are completely depolarized and thus no change is recorded on the ECG. D, The QRS complex occurs as the atria repolarize and the ventricular walls depolarize. E, The atrial walls are now completely repolarized, and the ventricular walls are now completely depolarized and thus no change is recorded on the ECG. F, The T wave appears on the ECG when the ventricular walls repolarize. G, After the ventricles are completely repolarized, we are back at the baseline of the ECG—essentially back where we began in part A of this fi gure.

A E

F

G

B

C

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lair-eez). In the so-called capillary beds, the exchange of nutrients and respiratory gases occurs between the blood and tissue fl uid around the cells. Blood exits or is drained from the capillary beds and then enters the small venules (VEN-yoolz), which join with other venules and increase in size, becoming veins. The largest veins are the superior vena cava and the inferior vena cava.

As noted previously (see Figure 12-4), arteries carry blood away from the heart and toward capil-laries. Veins carry blood toward the heart and away from capillaries, and capillaries carry blood from the tiny arterioles into tiny venules. The aorta carries blood out of the left ventricle of the heart, and the venae cavae return blood to the

right atrium after the blood has circulated through the body.

StructureArteries, veins, and capillaries differ in struc-ture. Three coats, or layers, are found in both arteries and veins (Figure 12-9). The outermost layer is called the tunica externa (or tunica ad-ventitia). This outer layer is made of connective tissue fi bers, which reinforce the wall of the ves-sel so that it will not burst under pressure. Note that smooth muscle tissue is found in the middle layer, or tunica media, of arteries and veins. However, this muscle layer is much thicker in ar-

Changes in Blood Flow During Exercise

Not only does the overall rate of blood fl ow increase during exercise, the relative blood fl ow through the

different organs of the body also changes. During exercise, blood is routed away from the kidneys and digestive organs and toward the skeletal muscles, cardiac muscle, and skin. Rerouting of blood is ac-complished by contracting precapillary sphincters in some tissues (thus reducing blood fl ow) while relax-ing precapillary sphincters in other tissues (thus in-

Health and Well-Being

creasing blood fl ow). How can homeostasis be better maintained by these changes? One reason is that glucose and oxygen levels are dropping rapidly in muscles as they use up these substances to produce energy. Increased blood fl ow restores normal levels of glucose and oxygen rapidly. Blood warmed up in active muscles fl ows to the skin for cooling. This helps keep the body temperature from getting too high. Can you think of other ways this situation helps maintain homeostasis? Typical changes in organ blood fl ow with exercise are shown in the illustration. The green bar in each pair shows the resting blood fl ow; the blue bar shows the fl ow during exercise.

At rest

During exercise

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Brain Cardiacmuscle

Skeletalmuscle

Skin Abdominalorgans

Kidneys Other

Blo

od fl

ow (

L/m

in)



teries than it is in veins. Why is this important? Because the thicker muscle layer in the artery wall is able to resist great pressures generated by ventricular systole. In arteries, the tunica media plays a critical role in maintaining blood pres-sure and controlling blood distribution. This is a smooth muscle, so it is controlled by the au-tonomic nervous system. The tunica media also includes a thin layer of elastic fi brous tissue.

An inner layer of endothelial cells called the tunica intima lines arteries and veins. The tunica intima is actually a single layer of squamous epi-thelial cells called endothelium (en-doh-THEE-lee-um) that lines the inner surface of the entire circulatory system.

As you can see in Figure 12-9, veins have a unique structural feature not present in arteries. They are equipped with one-way valves that pre-

ARTERY VEIN•Thinner than tunica media in arteries

•Thickest layer in veins

Tunica externa(fibrous connectivetissue)

•Thicker in arteries

•Thinner in veins

Semilunar valve

Tunica media(smooth muscle layerand elastic tissue)

Tunica intima(endothelium)

FIGURE 12-9

Artery and vein. Schematic drawings of an artery, A, and a vein, B, show comparative thicknesses of the three layers: the outer layer or tunica externa, the muscle layer or tunica media, and the tunica intima made of endothelium. Note that the muscle and outer layer are much thinner in veins than in arteries and that veins have valves.

A B



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vent the backfl ow of blood. When a surgeon cuts into the body, only arteries, arterioles, veins, and venules can be seen. Capillaries cannot be seen because they are microscopic. The most important structural feature of capillaries is their extreme thinness—only one layer of fl at, endothelial cells composes the capillary membrane. Instead of three layers or coats, the capillary wall is composed of only one—the tunica intima. Substances such as glucose, oxygen, and wastes can quickly pass through it on their way to or from cells. Smooth muscle cells called precapillary sphincters guard the entrance to the capillary and determine how much blood will fl ow into each capillary.

FunctionsArteries, veins, and capillaries all have different functions. Arteries and arterioles distribute blood from the heart to capillaries in all parts of the body. In addition, by constricting or dilating, arterioles help maintain arterial blood pressure at a normal level. Venules and veins collect blood from capil-laries and return it to the heart. They also serve as blood reservoirs because they carry blood under lower pressure (than arteries) and can expand to hold a larger volume of blood or constrict to hold a much smaller amount. Capillaries function as ex-change vessels. For example, glucose and oxygen move out of the blood in capillaries into interstitial fl uid and on into cells. Carbon dioxide and other substances move in the opposite direction (that is, into the capillary blood from the cells). Fluid is also exchanged between capillary blood and interstitial fl uid (see Chapter 18).

Study Figure 12-10 and Table 12-1 to learn the names of the main arteries of the body and Figures 12-11 and 12-12 and Table 12-2 for the names of the main veins.

1. What are the two main types of blood vessel in the body? How are they different?

2. Can you describe the three major lay-ers of a large blood vessel?

3. What are capillaries?

CIRCULATIONSystemic and Pulmonary CirculationThe term blood circulation is self-explanatory, meaning that blood fl ows through vessels that are arranged to form a circuit or circular pattern. Blood fl ow from the left ventricle of the heart through blood vessels to all parts of the body and back to the right atrium of the heart has already been described as the systemic circulation. The left ventricle pumps blood into the aorta. From there, it fl ows into arteries that carry it into the tissues and organs of the body. As indicated in Figure 12-13, within each structure, blood moves from arteries to arterioles to capillaries. There, the vital two-way exchange of substances oc-curs between blood and cells. Next, blood fl ows out of each organ’s capillary beds by way of its venules and then its veins to drain eventually into the inferior or superior venae cavae. These two great veins return venous blood to the right atrium of the heart to complete the systemic cir-culation. But the blood has not quite come full circle back to its starting point in the left ven-tricle. To do this and start on its way again, it must fi rst fl ow through another circuit, referred to earlier as the pulmonary circulation. Observe in Figure 12-13 that venous blood moves from the right atrium to the right ventricle to the pul-monary artery to lung arterioles and capillaries. There, the exchange of gases between the blood and air takes place, converting the deep crimson typical of venous blood to the scarlet of arterial blood. This oxygenated blood then fl ows through lung venules into four pulmonary veins and re-turns to the left atrium of the heart. From the left atrium, it enters the left ventricle to be pumped again through the systemic circulation.

To learn more about pulmonary circulation and systemic circulation, go to AnimationDirect on your CD-ROM.



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L

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Occipital

Internal carotid

External carotid

Left common carotid

Left subclavianArch of aortaPulmonary

Left coronary

Aorta

Celiac

Splenic

Renal

Inferior mesenteric

Radial

Ulnar

Facial

Right common carotid

Brachiocephalic

Right coronary

Brachial

Axillary

Superior mesenteric

Abdominal aorta

Common iliac

Internal iliac

External iliac

Deep femoral

Femoral

Popliteal

Anterior tibial

FIGURE 12-10

Principal arteries of the body.



Hepatic Portal CirculationThe term hepatic portal circulation refers to the route of blood fl ow through the liver. Veins from the spleen, stomach, pancreas, gallbladder, and intestines do not pour their blood directly into the inferior vena cava as do the veins from other ab-dominal organs. Instead, they send their blood to the liver by means of the hepatic portal vein (Fig-ure 12-14). The blood then must pass through the liver before it reenters the regular venous return pathway to the heart. Blood leaves the liver by way of the hepatic veins, which drain into the in-ferior vena cava. As noted in Figure 12-13, blood normally fl ows from arteries to arterioles to capil-laries to venules to veins and back to the heart. Blood fl ow in the hepatic portal circulation, how-ever, does not follow this typical route. Venous blood, which would ordinarily return directly to the heart, is sent instead through a second cap-

illary bed in the liver. The hepatic portal vein shown in Figure 12-14 is located between two capillary beds—one set in the digestive organs and the other in the liver. Once blood exits from the liver capillary beds, it returns to the normal pathway of blood returning to the heart.

The detour of venous blood through a second capillary bed in the liver before its return to the heart serves some valuable purposes. For exam-ple, when a meal is being absorbed, the blood in the portal vein contains a higher-than-normal concentration of glucose. Liver cells remove the excess glucose and store it as glycogen; therefore, blood leaving the liver usually has a normal blood glucose concentration. Liver cells also re-move and detoxify various poisonous substances that may be present in the blood. The hepatic portal system is an excellent example of how “structure follows function” in helping the body maintain homeostasis.

TABLE 12-1

The Major Arteries

ARTERY TISSUES SUPPLIED

HEAD AND NECK Occipital Posterior head and neckFacial Mouth, pharynx, and faceInternal carotid Anterior brain and meningesExternal carotid Superfi cial neck, face, eyes,

and larynxRight common carotid Right side of the head and

neckLeft common carotid Left side of the head and

neck

THORAXLeft subclavian Left upper extremityBrachiocephalic Head and armArch of aorta Branches to head, neck,

and upper extremitiesCoronary Heart muscle

ABDOMENCeliac Stomach, spleen, and liverSplenic Spleen

ARTERY TISSUES SUPPLIED

Renal KidneysSuperior mesenteric Small intestine; upper half of

the large intestineInferior mesenteric Lower half of the large

intestine

UPPER EXTREMITYAxillary Axilla (armpit)Brachial ArmRadial Lateral side of the handUlnar Medial side of the hand

LOWER EXTREMITYInternal iliac Pelvic viscera and rectumExternal iliac Genitalia and lower trunk

musclesDeep femoral Deep thigh musclesFemoral ThighPopliteal Leg and footAnterior tibial Leg



FIGURE 12-11

Principal veins of the body.

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Occipital

External jugular

Internal jugular

Left brachiocephalicLeft subclavian

Cephalic

Axillary

Great cardiac

Basilic

Long thoracic

Splenic

Inferior mesenteric

Internal iliac

Popliteal

Posterior tibial

Anterior tibial

Great saphenous

Femoral

Common iliac

Superior mesenteric

Median cubital

Hepatic portal

Inferior vena cava

Right pulmonary

Superior vena cava

Right subclavian

Right brachiocephalic

External iliac

Femoral

Common iliac

Facial

Fibular

Hepatic

Small cardiac



Fetal CirculationCirculation in the body before birth differs from circulation after birth because the fetus must se-cure oxygen and food from maternal blood in-stead of from its own lungs and digestive organs. For the exchange of nutrients and oxygen to oc-cur between fetal and maternal blood, special-ized blood vessels must carry the fetal blood to the placenta (plah-SEN-tah), where the exchange occurs, and then return it to the fetal body. Three vessels (shown in Figure 12-15 as part of the umbilical cord) accomplish this purpose. They are the two small umbilical arteries and a sin-gle, much larger umbilical vein. The movement of blood in the umbilical vessels may seem un-usual at fi rst in that the umbilical vein carries oxygenated blood, and the umbilical artery car-ries oxygen-poor blood. Remember that arteries

are vessels that carry blood away from the heart, whereas veins carry blood toward the heart, regardless of the oxygen content these vessels may have.

Another structure unique to fetal circulation is called the ductus venosus (DUK-tus veh-NOH-sus). As you can see in Figure 12-15, it is actually a continuation of the umbilical vein. It serves as a shunt, allowing most of the blood returning from the placenta to bypass the immature liver of the developing baby and empty directly into the infe-rior vena cava. Two other structures in the devel-oping fetus allow most of the blood to bypass the developing lungs, which remain collapsed until birth. The foramen ovale (foh-RAY-men oh-VAL-ee) shunts blood from the right atrium directly into the left atrium, and the ductus arteriosus (DUK-tus ar-teer-ee-OH-sus) connects the aorta and the pulmonary artery.

TABLE 12-2

The Major Veins

VEIN TISSUES DRAINED

HEAD AND NECKSuperior sagittal sinus BrainAnterior facial Anterior and superfi cial faceExternal jugular Superfi cial tissues of the

head and neckInternal jugular Sinuses of the brain

THORAXBrachiocephalic Viscera of the thoraxSubclavian Upper extremitiesSuperior vena cava Head, neck, and upper

extremitiesPulmonary LungsCardiac HeartInferior vena cava Lower body

ABDOMENHepatic LiverLong thoracic Abdominal and thoracic

musclesHepatic portal Liver and gallbladderSplenic Spleen

VEIN TISSUES DRAINED

Superior mesenteric Small intestine and most of the colon

Inferior mesenteric Descending colon and rectum

UPPER EXTREMITYCephalic Lateral armAxillary Axilla and armBasilic Medial armMedian cubital Cephalic vein (to basilic vein)

LOWER EXTREMITYExternal iliac Lower limbInternal iliac Pelvic visceraFemoral ThighGreat saphenous LegPopliteal Lower legPeroneal FootAnterior tibial Deep anterior leg and dor-

sal footPosterior tibial Deep posterior leg and

plantar aspect of foot



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At birth, the baby’s specialized fetal blood ves-sels and shunts must be rendered nonfunctional. When the newborn infant takes its fi rst deep breaths, the circulatory system is subjected to in-creased pressure. The result is closure of the fora-men ovale and rapid collapse of the umbilical blood vessels, the ductus venosus, and ductus arteriosus.

1. How do systemic and pulmonary circulations differ?

2. What is the hepatic portal circulation?

3. How is fetal circulation different than adult circulation?

BLOOD PRESSUREDefi ning Blood PressureA good way to explain blood pressure might be to try to answer a few questions about it. What is blood pressure? Just what the words say—blood pressure is the pressure or “push” of blood as it fl ows through the circulatory system.

Where does blood pressure exist? It exists in all blood vessels, but it is highest in the arteries and lowest in the veins. In fact, if we list blood vessels in order according to the amount of blood pressure in them and draw a graph, as in Figure 12-16, the graph looks like a hill, with aortic blood pressure at the top and vena caval pressure at the bottom. This blood pressure “hill” is spoken of as the blood pressure gradient. More precisely, the blood pressure gradi-ent is the difference between two blood pressures. The blood pressure gradient for the entire systemic circulation is the difference between the average or mean blood pressure in the aorta and the blood pres-sure at the termination of the venae cavae where they join the right atrium of the heart. The mean blood pressure in the aorta, given in Figure 12-16, is 100 mm of mercury (mm Hg), and the pressure at the termination of the venae cavae is 0. Therefore, with these typical normal fi gures, the systemic blood pressure gradient is 100 mm Hg (100 minus 0).

Why is it important to understand how blood pressure functions? The blood pressure gradient is vitally involved in keeping the blood fl owing. When a blood pressure gradient is present, blood circulates; conversely, when a blood pressure gra-dient is not present, blood does not circulate. For example, suppose that the blood pressure in the arteries were to decrease to a point at which it be-came equal to the average pressure in arterioles. The result would be that there would be no blood pressure gradient between arteries and arterioles, and therefore no force would be available to move blood out of arteries into arterioles. Circulation would stop, in other words, and very soon life it-self would cease. That is why when arterial blood pressure is observed to be falling rapidly, whether during surgery or in some other circumstance, emergency measures must be started quickly to try to reverse this fatal trend.

What we have just said may start you wonder-ing about why high blood pressure (meaning, of

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Basilic vein

Median cubital (basilic) vein

Ulnar vein

Radial vein

Brachial veins

Cephalic vein

Subclavian vein

Axillary vein

Brachiocephalicvein

Internal jugularvein

FIGURE 12-12

Main superfi cial veins of the arm.



course, high arterial blood pressure) and low blood pressure are bad for circulation. High blood pres-sure, or hypertension (hye-per-TEN-shun), is bad for several reasons. For one thing, if blood pres-sure becomes too high, it may cause the rupture of one or more blood vessels (for example, in the brain, as happens in a stroke). But low blood pres-sure can be dangerous too. If arterial pressure falls low enough, then blood will not fl ow through, or perfuse, the vital organs of the body. Circulation of blood and thus life will cease. Massive hemor-rhage, which dramatically reduces blood pressure, kills in this way.

Factors That Infl uence Blood PressureWhat causes blood pressure? What makes blood pressure change from time to time? Factors such as blood volume, the strength of each heart con-traction, heart rate, and the thickness of blood that help explain the answers to these questions are all discussed in the following paragraphs.

Blood VolumeThe direct cause of blood pressure is the volume of blood in the vessels. The larger the volume of blood in the arteries, for example, the more pres-

HEART

LUNGS

Arteries

Veins

Venules

Capillaries

Arterioles

Right atrium

Right ventricle

Pulmonaryartery

Venacava

Pulmonaryveins

Aorta

Veins of each organ

Venules ofeach organ

Arteries of each organ

Arterioles ofeach organ

Capillaries of each organ

Right AV valve

PulmonarySL valve

HEART

Left atrium

Left ventricle

Left AV valve

AorticSL valve

FIGURE 12-13

Diagram of blood fl ow in the circulatory system. Blood leaves the heart through arteries, then travels through arterioles, capillaries, venules, and veins before returning to the opposite side of the heart. Compare this fi gure with Figure 12-4.



sure the blood exerts on the walls of the arteries, or the higher the arterial blood pressure.

Conversely, the less blood in the arteries, the lower the blood pressure tends to be. Hemorrhage demonstrates this relation between blood volume and blood pressure. Hemorrhage is a pronounced loss of blood, and this decrease in the volume of blood causes blood pressure to drop. In fact, the major sign of hemorrhage is a rapidly falling blood pressure.

The volume of blood in the arteries is deter-mined by how much blood the heart pumps into the arteries and how much blood the arterioles drain out of them. The diameter of the arterioles plays an important role in determining how much blood drains out of arteries into arterioles.

Strength of Heart ContractionsThe strength and the rate of the heartbeat affect cardiac output and therefore blood pressure. Each time the left ventricle contracts, it squeezes a certain volume of blood (the stroke volume) into the aorta and on into other arteries. The stronger that each contraction is, the more blood it pumps into the aorta and arteries. Conversely, the weaker that each contraction is, the less blood it pumps. Suppose that one contraction of the left ventricle pumps 70 mL of blood into the aorta, and suppose that the heart beats 70 times a min-ute; 70 mL � 70 equals 4900 mL. Almost 5 L of blood would enter the aorta and arteries every minute (the cardiac output). Now suppose that the heartbeat were to become weaker and that

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Hepatic veins Inferior vena cava

Stomach

Gastric vein

Liver

Hepaticportal vein

Duodenum

Pancreas

Superiormesenteric vein

Ascending colon

Appendix

Small intestine

Inferiormesenteric vein

Descending colon

Gastroepiploicvein

Splenic vein

Pancreatic veins

Spleen

FIGURE 12-14

Hepatic portal circulation. In this very unusual circulation, a vein is located between two capillary beds. The hepatic portal vein collects blood from capillaries in visceral structures located in the abdomen and empties it into the liver. Hepatic veins return blood to the inferior vena cava. (Organs are not drawn to scale here.)



FIGURE 12-15

The fetal circulation.

Superior vena cava

Ascending aorta

Inferior vena cava

Liver

Hepatic portal vein

Fetal side of placenta

Maternal sideof placenta

Fetal umbilicus

Aortic arch

Leftlung

Pulmonary trunk

Abdominal aorta

Common iliac artery

Internal iliac

arteries

Foramen ovale

Ductus venosus

Umbilical vein

Umbilicalcord

Ductus arteriosus

Umbilical arteries

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L

I

R



each contraction of the left ventricle pumps only 50 mL instead of 70 mL of blood into the aorta. If the heart still contracts just 70 times a minute, it will obviously pump much less blood into the aorta—only 3500 mL instead of the more nor-mal 4900 mL per minute. This decrease in the heart’s output decreases the volume of blood in the arteries, and the decreased arterial blood volume decreases arterial blood pressure. In summary, the strength of the heartbeat affects blood pressure in this way: a stronger heartbeat increases blood pressure, and a weaker beat de-creases it.

Heart RateThe rate of the heartbeat also may affect arterial blood pressure. You might reason that when the heart beats faster, more blood enters the aorta,

and therefore the arterial blood volume and blood pressure would increase. This is true only if the stroke volume does not decrease sharply when the heart rate increases. Often, however, when the heart beats faster, each contraction of the left ventricle takes place so rapidly that it has little time to fi ll with blood, and therefore squeezes out much less blood than usual into the aorta. For example, suppose that the heart rate speeded up from 70 to 100 times per minute and that, at the same time, its stroke volume decreased from 70 mL to 40 mL. Instead of a cardiac output of 70 � 70 or 4900 mL per minute, the cardiac out-put would have changed to 100 � 40 or 4000 mL per minute. Arterial blood volume decreases un-der these conditions, and therefore blood pres-sure also decreases, even though the heart rate has increased.

Diastolicpressure

AortaLarge

arteriesSmall

arteries Arterioles

Capillaries

85 mm

35 mm

Venules

15 mm

Smallveins

Largeveins

Venaecavae

6 mm2 mm 1 mm

100 mm Hg

0 mm Hg

Flow rate � 1 L/min

mm Hg

120

100

80

60

40

20

0

Systolic pressure

FIGURE 12-16

Pressure gradients in blood fl ow. Blood fl ows down a “blood pressure hill” from arteries, where blood pressure is high-est, into arterioles, where it is somewhat lower, into capillaries, where it is still lower, and so on. All numbers on the graph indicate blood pressure measured in millimeters of mercury. The broken line, starting at 100 mm, represents the average pressure in each part of the circulatory system.



What generalization, then, can we make? We can say only that an increase in the rate of the heartbeat increases blood pressure, and a decrease in the rate decreases blood pressure. But whether a change in the heart rate actually produces a similar change in blood pressure depends on whether the stroke vol-ume also changes and in which direction.

Blood ViscosityAnother factor that we ought to mention in connec-tion with blood pressure is the viscosity of blood, or in plainer language, its thickness. If blood be-comes less viscous than normal, blood pressure de creases. For example, if a person suffers a hem-orrhage, fl uid moves into the blood from the inter-stitial fl uid. This dilutes the blood and decreases its viscosity, and blood pressure then falls because of the decreased viscosity. After hemorrhage, trans-fusion of whole blood or plasma is preferred to in-fusion of saline solution. The reason is that saline solution is not a viscous liquid and so cannot keep blood pressure at a normal level.

In a condition called polycythemia, the number of red blood cells increases beyond normal and thus increases blood viscosity. This in turn increases blood pressure. Polycythemia can occur when oxy-gen levels in the air decrease and the body attempts to increase its ability to attract oxygen to the blood, as happens in working at high altitudes.

Resistance to Blood FlowA factor that has a huge impact on local blood pres-sure gradients, and thus on blood fl ow, is any factor that changes the resistance to blood fl ow. The term peripheral (peh-RIF-er-al) resistance describes any force that acts against the fl ow of blood in a blood vessel. Viscosity of blood, for example, affects pe-ripheral resistance by infl uencing the ease with which blood fl ows through blood vessels.

Another factor that affects peripheral resistance is the tension in muscles of the blood vessel wall (Figure 12-17). When these muscles are relaxed, resistance is low and therefore blood pressure is low—thus blood may fl ow easily down its pres-sure gradient and into the vessel. When vessel wall muscles are contracted, however, resistance in-creases and therefore so does the blood pressure—thus the pressure gradient is reduced and blood will not fl ow so easily into the vessel. Such adjust-ment of muscle tension in vessel walls to control blood pressure, and therefore blood fl ow, is often called the vasomotor mechanism (vay-so-MOH-tor MEK-ah-niz-em).

Fluctuations in Blood PressureNo one’s blood pressure stays the same all the time. It fl uctuates, even in a perfectly healthy in-dividual. For example, it goes up when a person

(diameter = 1/2)

(diameter = 1)

Decreased resistance Increased resistance

Smooth musclecell

Smooth muscle relaxation Normal resting tone Smooth muscle contraction

(diameter = 2)

FIGURE 12-17

Vasomotor mechanism. Changes in smooth muscle tension in the wall of an arteriole infl uence the resistance of the vessel to blood fl ow. Relaxation of muscle results in decreased resistance; contraction of muscle results in increased resistance.



exercises strenuously. Not only is this normal, but the increased blood pressure serves a good pur-pose. It increases circulation to bring more blood to muscles each minute and thus supplies them with more oxygen and food for more energy.

A normal average arterial blood pressure is about 120/80, or 120 mm Hg systolic pressure (as the ventricles contract) and 80 mm Hg diastolic pressure (as the ventricles relax). Remember, how-ever, that what is “normal” varies somewhat among individuals and also varies with age.

The venous blood pressure, as you can see in Figure 12-16, is very low in the large veins and falls almost to 0 by the time blood leaves the venae cavae and enters the right atrium. The venous blood pres-sure within the right atrium is called the central venous pressure. This pressure level is important because it infl uences the pressure that exists in the large peripheral veins. If the heart beats strongly, the central venous pressure is low as blood enters and leaves the heart chambers effi ciently. However, if the heart is weakened, central venous pressure in-creases, and the fl ow of blood into the right atrium is slowed. As a result, a person suffering heart fail-ure, who is sitting at rest in a chair, often has dis-tended external jugular veins as blood “backs up” in the venous network.

Five mechanisms help keep venous blood mov-ing back through the circulatory system and into the right atrium. They include the following:

1. Continued beating of the heart, which pumps blood through the entire circulatory system.

2. Adequate blood pressure in the arteries, to push blood to and through the veins.

3. Semilunar valves in the veins that ensure continued blood fl ow in one direction (to-ward the heart).

4. Contraction of skeletal muscles, which squeezes veins, producing a kind of pump-ing action.

5. Changing pressures in the chest cavity dur-ing breathing that produce a kind of pump-ing action in the veins in the thorax.

PULSEWhat you feel when you take a pulse is an artery expanding and then recoiling alternately. To feel a pulse, you must place your fi ngertips over an

artery that lies near the surface of the body and over a bone or other fi rm base. The pulse is a valu-able clinical sign. It can provide information, for example, about the rate, strength, and rhythmicity of the heartbeat. It is also easily determined with little or no danger or discomfort. The nine major “pulse points” are named after the arteries over which they are felt. Locate each pulse point on Fig-ure 12-18 and on your own body.

FIGURE 12-18

Pulse points. Each pulse point is named after the artery with which it is associated.

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Superficialtemporal artery

Facial artery

Carotid artery

Axillary artery

Brachial artery

Radial artery

Femoral artery

Popliteal (posteriorto patella)

Dorsalis pedis



Three pulse points are located on each side of the head and neck: (1) over the superfi cial tempo-ral artery in front of the ear, (2) the common ca-rotid artery in the neck along the front edge of the sternocleidomastoid muscle, and (3) over the fa-

cial artery at the lower margin of the mandible at a point below the corner of the mouth.

A pulse is also detected at three points in the upper limb: (1) in the axilla over the axillary artery; (2) over the brachial artery at the bend of the elbow

Blood Pressure Readings

A device called a sphygmomanome-ter (sfi g-moh-mah-NAH-meh-ter) is often used to measure blood pres-sures in both clinical and home health-

care situations. The traditional sphygmomanometer is an inverted tube of mercury (Hg) with a balloon-like air cuff attached via an air hose. The air cuff is placed around a limb, usually the subject’s upper arm as shown in the fi gure. A stethoscope sensor is placed over a major artery (the brachial artery in the fi gure) to listen for the arterial pulse. A hand-operated pump fi lls the air cuff, increasing the air pressure and pushing the column of mercury higher. While listening through the stethoscope, the operator opens the air cuff’s outlet valve and slowly reduces the air pressure around the limb. Loud, tapping Korotkoff sounds suddenly begin when the cuff pressure measured by the mercury col-

Clinical Application

umn equals the systolic pressure—usually about 120 mm. As the air pressure surrounding the arm con-tinues to decrease, the Korotkoff sounds disappear. The pressure measurement at which the sounds dis-appear is equal to the diastolic pressure—usually 70 to 80 mm. The subject’s blood pressure is then expressed as systolic pressure (the maximum arterial pressure during each cardiac cycle) over the diastolic pressure (the minimum arterial pressure), such as 120/80 (read “one-twenty over eighty”). The fi nal read-ing can then be compared with the expected value, which is based on the patient’s age and various other individual factors. Mercury sphygmomanometers have been replaced in many clinical settings by nonmercury devices that similarly measure the maximum and minimum arterial blood pressures. In home health-care settings, patients are often taught how to monitor their own blood pressure.

Pressure cuff

Korotkoffsounds

No sound

Elbow

No sound

Sound first heard

Sound last heard

80 mm Hg

120 mm Hg



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along the inner or medial margin of the biceps bra-chii muscle, and (3) at the radial artery at the wrist. The so-called radial pulse is the most frequently monitored and easily accessible in the body.

The pulse also can be felt at three locations in the lower extremity: (1) over the femoral artery in the groin, (2) at the popliteal artery behind and just proximal to the knee, and (3) at the dorsalis pedis artery on the front surface of the foot, just below the bend of the ankle joint.

1. How does the blood pressure gradi-ent explain blood fl ow?

2. Name four factors that infl uence blood pressure.

3. Does a person’s blood pressure stay the same all the time?

4. Where are the places on your body that you can likely feel your pulse?

CardiologyWillem Einthoven (1860-1927)

Cardiology, the study and treatment of the heart, owes much to Dutch physiologist Willem Einthoven and his invention of the modern electrocardiograph in 1903. Einthoven’s fi rst major con-tribution was the invention

of a machine that could record electrocardiograms (ECGs) with far greater sensitivity than the crude machines of the 19th century. Then, with the help of British physician Lewis Thomas, Einthoven demon-strated and named the P, Q, R, S, and T waves and proved that these waves precisely record the elec-trical activity of the heart (see Figure 12-7). In 1905, he even invented a way that ECG data could be

Science Applications

sent from a patient over the telephone to his labora-tory where they could be recorded and analyzed—a technique now called telemetry. His detailed stud-ies of ECG recordings changed the practice of heart medicine forever. In fact, his invention was later ap-plied to the study of nerve impulses and led to breakthrough discoveries in the neurosciences.

Cardiologists today still use modern versions of Einthoven’s machine to diagnose heart disorders. Of course, biomedical engineers continue to develop refi nements to electrocardiograph equipment and to invent new machines to monitor heart function. In fact, engineers and designers have worked with car-diologists to develop artifi cial heart valves, artifi cial pacemakers, and even artifi cial hearts! Along with all of this medical equipment being used in cardiology, and medicine in general, are many technicians work-ing to keep it all in good repair.



OUTLINE SUMMARY

b. Consist of two atrioventricular, or AV, and two semilunar (SL) valves(1) Tricuspid—at the opening of the

right atrium into the ventricle(2) Bicuspid (mitral)—at the opening

of the left atrium into the ventricle(3) Pulmonary semilunar—at the be-

ginning of the pulmonary artery(4) Aortic semilunar—at the begin-

ning of the aortaC. Heart sounds

1. Two distinct heart sounds in every heartbeat or cycle—”lub-dup”

2. First sound (lub) is caused by the vibra-tion and closure of AV valves during contraction of the ventricles

3. Second sound (dub) is caused by the closure of the semilunar valves during relaxation of the ventricles

D. Blood fl ow through the heart (Figure 12-4)1. The heart acts as two separate pumps—

the right atrium and ventricle perform-ing different functions from the left atrium and ventricle

2. Sequence of blood fl ow: venous blood enters the right atrium through the supe-rior and inferior venae cavae—passes from the right atrium through the tricus-pid valve to the right ventricle; from the right ventricle it passes through the pul-monary semilunar valve to the pulmo-nary artery to the lungs—blood moves from the lungs to the left atrium, passing through the bicuspid (mitral) valve to the left ventricle; blood in the left ventricle is pumped through the aortic semilunar valve into the aorta and is distributed to the body as a whole

E. Blood supply to the heart muscle1. Blood, which supplies oxygen and nu-

trients to the myocardium of the heart, fl ows through the right and left coro-nary arteries (Figure 12-5); called coro-nary circulation

HEARTA. Location, size, and position

1. Triangular organ located in mediastinum with two thirds of the mass to the left of the body midline and one third to the right; the apex on the diaphragm; shape and size of a closed fi st (Figure 12-1)

2. Cardiopulmonary resuscitation (CPR)—the heart lies between the ster-num in front and the bodies of the tho-racic vertebrae behind; rhythmic com-pression of the heart between the sternum and vertebrae can maintain blood fl ow during cardiac arrest; if combined with artifi cial respiration procedure, it can be life saving

B. Anatomy1. Heart chambers (Figure 12-2)

a. Two upper chambers called atria (re-ceiving chambers)—right and left atria

b. Two lower chambers called ventricles (discharging chambers)—right and left ventricles

c. Wall of each heart chamber is com-posed of cardiac muscle tissue called myocardium

d. Endocardium—smooth lining of heart chambers—infl ammation of endocardium called endocarditis

2. Covering sac, or pericardiuma. Pericardium is a two-layered fi brous

sac with a lubricated space between the two layers

b. Inner layer called visceral pericardium or epicardium

c. Outer layer called parietal pericardium3. Heart action

a. Contraction of the heart is called systole

b. Relaxation is called diastole4. Heart valves (Figure 12-3)

a. Valves keep blood fl owing through the heart and prevent backfl ow

Continued



OUTLINE SUMMARY—cont’d

2. These tiny electrical impulses can be picked up on the surface of the body and transformed into visible tracings by a machine called an electrocardiograph

3. The visible tracing of these electrical signals is called an electrocardiogram, or ECG

4. The normal ECG has three defl ections or wavesa. P wave—associated with depolariza-

tion of the atriab. QRS complex—associated with de-

polarization of the ventriclesc. T wave—associated with repolariza-

tion of the ventricles

BLOOD VESSELSA. Types

1. Arteries—carry blood away from the heart

2. Veins—carry blood toward the heart3. Capillaries—carry blood from the arteri-

oles to the venulesB. Structure (Figure 12-9)

1. Arteriesa. Tunica intima—inner layer of endo-

thelial cellsb. Tunica media—smooth muscle

with some elastic tissue, thick in arteries; important in blood pres-sure regulation

c. Tunica externa—thin layer of fi brous elastic connective tissue

2. Capillaries—microscopic vessels with only one layer—tunica intima

3. Veinsa. Tunica intima—inner layer; valves

prevent retrograde movement of blood

b. Tunica media—smooth muscle; thin in veins

c. Tunica externa—heavy layer of fi -brous connective tissue in many veins

2. Blockage of blood fl ow through the cor-onary arteries is called myocardial infarc-tion (heart attack)

3. Angina pectoris—chest pain caused by inadequate oxygen to the heart

4. Coronary bypass surgery—veins from other parts of the body are used to by-pass blockages in coronary arteries (Fig-ure 12-6)

F. Cardiac cycle1. Heartbeat is regular and rhythmic—each

complete beat is called a cardiac cycle—average is about 72 beats per minute

2. Each cycle, about 0.8 seconds long, is subdivided into systole (contraction phase) and diastole (relaxation phase)

3. Stroke volume—volume of blood ejected from one ventricle with each beat

4. Cardiac output—amount of blood that one ventricle can pump each minute; average is about 5 L per minute at rest

G. Conduction system of the heart (Figure 12-7)1. Intercalated disks are electrical connec-

tors that join all the cardiac muscle fi -bers in a region together so that they re-ceive their impulse, and thus contract, at about the same time

2. SA (sinoatrial) node, the pacemaker—located in the wall of the right atrium near the opening of the superior vena cava

3. AV (atrioventricular) node—located in the right atrium along the lower part of the interatrial septum

4. AV bundle (bundle of His)—located in the septum of the ventricle

5. Purkinje fi bers—located in the walls of the ventricles

H. Electrocardiogram (Figure 12-8)1. Specialized conduction system struc-

tures generate and transmit the electri-cal impulses that result in contraction of the heart



OUTLINE SUMMARY—cont’d

c. Unique structures include the pla-centa, umbilical arteries and vein, ductus venosus, ductus arteriosus, and foramen ovale

BLOOD PRESSUREA. Defi ning blood pressure—push, or force, of

blood in the blood vessels1. Highest in arteries, lowest in veins (Fig-

ure 12-16)2. Blood pressure gradient causes blood to

circulate—liquids can fl ow only from the area where pressure is higher to where it is lower

B. Factors that infl uence blood pressure (Fig-ure 12-17)1. Blood volume2. Strength of ventricular contractions3. Blood viscosity,4. Resistance to blood fl ow

C. Fluctuations in blood pressure1. Blood pressure varies within normal

range2. Normal average arterial blood pressure

is 120/803. Venous blood pressure within right

atrium called central venous pressure4. Venous return of blood to the heart de-

pends on fi ve mechanisms—a strongly beating heart, adequate arterial blood pressure, valves in the veins, pumping action of skeletal muscles as they con-tract, and changing pressures in the chest cavity caused by breathing.

PULSEA. Defi nition—alternate expansion and recoil

of the blood vessel wallB. Nine major “pulse points” named after arter-

ies over which they are felt (Figure 12-18)

C. Functions1. Arteries—distribution of nutrients,

gases, etc., with movement of blood un-der high pressure; assist in maintaining the arterial blood pressure

2. Capillaries—serve as exchange vessels for nutrients, wastes, and fl uids

3. Veins—collect blood for return to the heart; low pressure vessels

D. Names of main arteries—see Figure 12-10 and Table 12-1

E. Names of main veins—see Figures 12-11 and 12-12 and Table 12-2

CIRCULATIONA. Basic plan of circulation—refers to the blood

fl ow through the vessels arranged to form a circuit or circular pattern (Figure 12-13)1. Systemic circulation

a. Carries blood throughout the bodyb. Path goes from left ventricle through

aorta, smaller arteries, arterioles, capillaries, venules, venae cavae, to right atrium

2. Pulmonary circulationa. Carries blood to and from the lungs;

arteries deliver deoxygenated blood to the lungs for gas exchange

b. Path goes from right ventricle through pulmonary arteries, lungs, pulmonary veins, to left atrium

B. Special circulatory plans1. Hepatic portal circulation (Figure 12-14)

a. Unique blood route through the liverb. Vein (hepatic portal vein) exists be-

tween two capillary bedsc. Assists with homeostasis of blood

glucose levels2. Fetal circulation (Figure 12-15)

a. Refers to circulation before birthb. Modifi cations required for fetus to

effi ciently secure oxygen and nutri-ents from the maternal blood



NEW WORDS

coronary bypass surgery

coronary circulationcoronary sinuscoronary veindiastolediastolic pressureductus arteriosusductus venosuselectrocardiogram

(ECG)endocarditisendocardiumepicardium (visceral

pericardium)foramen ovale

angina pectorisarteriolearteryatrioventricular (AV)

valveatrium (pl., atria)AV bundleAV nodebicuspid valve (also

called mitral valve)capillarycardiac outputcardiopulmonary

resuscitation (CPR)central venous pressurecoronary artery

hepatic portal circulation

mitral valvemyocardial

infarction (MI)myocardiumP wavepericarditispericardiumperipheral

resistancepulmonary

circulationpulsePurkinje fi bersQRS complex

semilunar valvesinoatrial node

(pacemaker)stroke volumesystemic circulationsystolesystolic pressureT wavetricuspid valveumbilicalvasomotor mechanismveinventriclevenule

REVIEW QUESTIONS

1. Describe the heart and its position in the body.

2. Name the four chambers of the heart.3. What is the myocardium? What is the

endocardium?4. Describe the two layers of the pericar-

dium. What is the function of pericardial fl uid?

5. Defi ne systole and diastole.6. Name and give the location of the four

heart valves.7. Trace the fl ow of blood from the superior

vena cava to the aorta.8. What is angina pectoris?9. Differentiate between stroke volume and

cardiac output.10. Trace the path and name the structures in-

volved in the conduction system of the heart.

11. Name and describe the main types of blood vessels in the body.

12. Name the three tissue layers that make up arteries and veins.

13. Describe both systemic and pulmonary circulation.

14. Name and briefl y explain the four factors that infl uence blood pressure.

15. List the fi ve mechanisms that keep the ve-nous blood moving toward the right atrium.

16. Name four locations in the body where the pulse can be felt.

CRITICAL THINKING

17. Explain how the traces on an ECG relate to what is occurring in the heart.

18. Explain hepatic portal circulation. How is it different from normal circulation, and what advantages are gained from this type of circulation?

19. Explain the differences between normal postnatal circulation and fetal circulation. Based on the environment of the fetus, ex-plain how these differences make fetal cir-culation more effi cient.

20. Explain why a pressure difference must exist between the aorta and the right atrium

CRITICAL THINKING



CHAPTER TEST

15. The __________ are the blood vessels that carry blood back to the heart.

16. The __________ are the blood vessels that carry blood away from the heart.

17. The __________ are the microscopic blood vessels in which substances are exchanged between the blood and tissues.

18. The innermost layer of tissue in an artery is called the __________.

19. The outermost layer of tissue in an artery is called the __________.

20. Systemic circulation involves the moving of blood throughout the body; __________ involves moving blood from the heart to the lungs and back.

21. The two structures in the developing fe-tus that allow most of the blood to bypass the lungs are the __________ and the __________.

22. The strength of the heart contraction and blood volume are two factors that infl u-ence blood pressure. Two other factors that infl uence blood pressure are __________ and __________.

23. Place the following structures in their proper order in blood fl ow through the heart by putting a 1 in front of the fi rst structure the blood would pass through and ending with a 10 in front of the last structure the blood would pass through.

1. __________ are the thicker chambers of the heart, which are sometimes called the discharging chambers.

2. The __________ are the thinner chambers of the heart, which are sometimes called the receiving chambers of the heart.

3. Cardiac muscle tissue is called __________.4. The ventricles of the heart are separated

into right and left sides by the __________.5. The thin layer of tissue lining the interior

of the heart chambers is called the __________.

6. Another term for the visceral pericardium is the __________.

7. Contraction of the heart is called __________.

8. Relaxation of the heart is called __________.9. The heart valve located between the right

atrium and right ventricle is called the __________ valve.

10. The term __________ refers to the volume of blood ejected from the ventricle during each beat.

11. The __________ is the pacemaker of the heart and causes the contraction of the atria.

12. The __________ are extensions of the atrioventricular fi bers and cause the con-traction of the ventricles.

13. The ECG tracing that occurs when the ven-tricles depolarize is called the __________.

14. The ECG tracing that occurs when the atria depolarize is called the __________.

Continued



CHAPTER TEST—cont’d

a. ____ left atriumb. ____ tricuspid valve (right atrioventricular valve)c. ____ right ventricled. ____ pulmonary veine. ____ aortic semilunar valvef. ____ mitral valve (left atrioventricular valve)g. ____ left ventricleh. ____ pulmonary arteryi. ____ right atriumj. ____ pulmonary semilunar valve



STUDY TIPS—cont’d

Continued from page 301 5. The structures of the heart can be learned with

fl ash cards. The location of the semilunar valves should be easy to remember because their names tell you where they are. It is harder to remember where the tricuspid and mitral valves are because the names don’t help. An easier way to remem-ber them is to use their other names, the right and left atrioventricular valves, respectively. This name tells you exactly where they are: between the atria and ventricles on the right or left side. Blood moves through the cardiovascular system in one direction, from the right heart, to the lungs, to the left heart, to the rest of the body and back to the right heart.

6. If you are asked to learn the sequence of blood fl ow through the heart, don’t forget the valves. Heart conduction may make more sense if you remember that atria contract from the top down, but ventricles must contract from the bottom up.

7. The letters that are used in an ECG trace don’t stand for anything; the P, Q, R, S, and T are arbitrary.

8. If you are asked to learn the names and loca-tions of specifi c blood vessels, use fl ash cards and the fi gures in this chapter.

9. Fetal circulation makes sense if you remember the environment in which the fetus is living. The blood is oxygenated and full of digested food, so it doesn’t have to go to the lungs or the liver.

10. A liquid moves from high to low pressure, so it is logical that the blood pressure in the cardio-vascular system is highest in the aorta and lowest in the vena cava.

11. In your study groups, bring photocopies of the fi gures of the heart and of the blood vessels if you need to learn them. Blacken out the labels and quiz each other on the name of each structure and its function.

12. Discuss the sequence of heart circulation, the parts of an ECG, the heart conduction system, the structure and function of the blood vessels, and the structures in fetal circulation.

13. Go over the questions at the end of the chap-ter and discuss possible test questions.


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The Circulatory System - coursewareobjects.com€¦ · 302 Chapter 12 The Circulatory System HEART Location, Size, and Position No one needs to be told where the heart is or what