HUMAN BIOLOGY

124 Chapter 7

aortic valve Valve that
opens from the left ventricle
into the aorta.

atrioventricular valve
Valve through which blood
flows from an atrium to a
ventricle.

atrium Either of the upper
heart chambers, one above
each ventricle.

cardiac cycle The
sequence of contraction
and relaxation of the heart
chambers.

diastole Relaxation phase
of the cardiac cycle.

myocardium Cardiac
muscle tissue of the heart
wall.

pulmonary valve Valve
that opens from the right
ventricle into the pulmonary
artery.

systole Contraction phase
of the cardiac cycle.

ventricle Large heart cham-
ber below the atrium in each
side of the heart.

the heart: a Muscular double pump

n    The heart is a durable pump that consists mainly of

cardiac muscle.

n Links to Epithelium 4.1, Muscle tissue 4.3

Your heart is located more or less

in the center of your chest (Figure

7.4A). Its structure reflects its role as

a long-lasting pump. The heart wall

is mostly cardiac muscle tissue, the

myocardium (Figure 7.4B). A tough,

fibrous sac, the pericardium (peri 5

“around”), surrounds, protects, and

lubricates it. The heart’s chambers

have a smooth lining (endocardium)

composed of connective tissue and a

layer of epithelial cells. This kind of

epithelial cell layer, known as endo-

thelium, also lines blood vessels.

the heart has two halves

and four chambers

A thick wall, the septum, divides

the heart into two halves, right and

left. Each half has two chambers: an

atrium (plural: atria) located above a

larger ventricle. Flaps of membrane

separate the two chambers and

serve as a one-way atrioventricular

valve (AV valve) between them. The

AV valve in the right half of the

heart is called a tricuspid valve because its three flaps come

together in pointed cusps (Figure 7.4C). In the heart’s

left half, the AV valve consists of just two flaps; it is called

the bicuspid valve or mitral valve. Tough, collagen- reinforced

strands (chordae tendineae, or “heartstrings”) connect the AV

valve flaps to cone-shaped muscles that extend out from the

ventricle wall. When a blood-filled ventricle contracts, this

arrangement prevents the flaps from opening backward into

the atrium. Each half of the heart also has a valve between

the ventricle and the arteries leading away from it. The

pulmonary valve controls blood flow to the pulmonary

artery, and the aortic valve controls blood flow to the

aorta. Because both these valves are shaped like a half-

moon, they are also known as “semilunar” valves. During

a heartbeat, the valves open and close in ways that keep

blood moving in one direction, out of the heart.

Figure 7.4 Animated! The heart is divided into right and

left halves. A Location of the heart in the chest. B The heart’s

internal anatomy. Blue arrows represent blood flow into and out

of the right ventricle. Red arrows represent blood flow into and

out of the left ventricle. C The shapes of heart valves. (A: From

Frances Sienkiewicz Sizer; Eleanor Noss Whitney, Nutrition: Concepts and Controversies,

© 2002 Cengage Learning; B–C: © Cengage Learning)

in a “heartbeat,” the heart’s chambers

contract, then relax

Blood is pumped each time the heart beats. It takes less than

a second for a “heartbeat”—one sequence of contraction

and relaxation of the heart chambers. This sequence is the

cardiac cycle (Figure 7.5). It occurs almost simultaneously

in both sides of the heart. The contraction phase is called

systole (siss-toe-lee), and the relaxation phase is called

diastole (dye-ass-toe-lee).

During the cardiac cycle, the ventricles relax before the

atria contract, and the ventricles contract when the atria

relax. When the relaxed atria are filling with blood, the

7. 2

Right AV valve Left AV valve Aortic and

C pulmonary valves

A

diaphragm

pericardium

right lung left lung

superior vena cava

(flow from head, arms)

pulmonary valve

(closed)

right pulmonary

veins (from lungs)

right AV valve

(open)

inferior vena cava

(from trunk, legs)

trunk of pulmonary

arteries (to lungs)

left pulmonary

veins (from lungs)

left AV valve

(open)

Right ventricle

Right atrium Left atrium

Left ventricle

septum

aorta

aortic valve (closed)

cardiac muscle

(myocardium)

pericardium

B endocardium

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