Essentials of Anatomy and Physiology

veins, which empty blood into a large coronary sinus
that returns blood to the right atrium.
The purpose of the coronary vessels is to supply
blood to the myocardium itself, because oxygen is
essential for normal myocardial contraction. If a coro-
nary artery becomes obstructed, by a blood clot for
example, part of the myocardium becomes ischemic,
that is, deprived of its blood supply. Prolonged
ischemia will create an infarct, an area of necrotic
(dead) tissue. This is a myocardial infarction, com-
monly called a heart attack (see also Box 12–1: Coro-
nary Artery Disease).

CARDIAC CYCLE

AND HEART SOUNDS

The cardiac cycleis the sequence of events in one
heartbeat. In its simplest form, the cardiac cycle is the
simultaneous contraction of the two atria, followed a
fraction of a second later by the simultaneous contrac-
tion of the two ventricles. Systoleis another term for
contraction. The term for relaxation is diastole. You
are probably familiar with these terms as they apply to
blood pressure readings. If we apply them to the car-
diac cycle, we can say that atrial systole is followed by
ventricular systole. There is, however, a significant
difference between the movement of blood from the
atria to the ventricles and the movement of blood
from the ventricles to the arteries. The events of the
cardiac cycle are shown in Fig. 12–5. In this traditional

representation, the cardiac cycle is depicted in a circle,

because one heartbeat follows another, and the begin-

ning of atrial systole is at the top (12 o’clock). The size

of the segment or arc of the circle indicates how long

it takes. Find the segment for atrial systole and the one

for ventricular systole, and notice how much larger

(meaning “longer”) ventricular systole is. Do you

think this might mean that ventricular contraction is

more important than atrial contraction? It does, as you

will see. Refer to Fig. 12–5 as you read the following.

We will begin at the bottom (6 o’clock) where the atria

are in the midst of diastole and the ventricles have just

completed their systole. The entire heart is relaxed

and the atria are filling with blood.

Blood is constantly flowing from the veins into

both atria. As more blood accumulates, its pressure

forces open the right and left AV valves. Two-thirds

of the atrial blood flows passively into the ventri-

cles (which brings us to 12 o’clock); the atria then

contract to pump the remaining blood into the ven-

tricles.

Following their contraction, the atria relax and the

ventricles begin to contract. Ventricular contraction

forces blood against the flaps of the right and left AV

valves and closes them; the force of blood also opens

the aortic and pulmonary semilunar valves. As the

ventricles continue to contract, they pump blood into

the arteries. Notice that blood that enters the arteries

must all be pumped. The ventricles then relax, and at

the same time blood continues to flow into the atria,

and the cycle begins again.

278 The Heart

Aorta

Left coronary artery

Anterior

interventricular branch

Great cardiac

vein

Coronary sinus

Posterior

artery and

vein

Small

Right coronary artery cardiac vein

A and vein B

Figure 12–4. (A) Coronary vessels

in anterior view. The pulmonary

artery has been cut to show the left

coronary artery emerging from the

ascending aorta. (B) Coronary vessels

in posterior view. The coronary sinus

empties blood into the right atrium.

QUESTION:What is the function of

the coronary vessels?