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CHAPTER

31

The Heart as a Pump

OBJECTIVES

After studying this chapter, you should be able to:

■

Describe how the sequential pattern of contraction and relaxation in the heart

results in a normal pattern of blood flow.

■

Understand the pressure, volume, and flow changes that occur during the cardiac

cycle.

■

Explain the basis of the arterial pulse, heart sounds, and murmurs.

■

Delineate the ways by which cardiac output can be up-regulated in the setting of

specific physiologic demands for increased oxygen supply to the tissues, such as

exercise.

■

Describe how the pumping action of the heart can be compromised in the setting

of specific disease states.

INTRODUCTION

Of course, the electrical activity of the heart discussed in the

previous chapter is designed to subserve the heart’s primary

physiological role—to pump blood through the lungs, where

gas exchange can occur, and thence to the remainder of the

body
(
Clinical Box 31–1
)

. This is accomplished when the

orderly depolarization process described in the previous

chapter triggers a wave of contraction that spreads through

the myocardium. In single muscle fibers, contraction starts

just after depolarization and lasts until about 50 ms after

repolarization is completed (see Figure 5–15). Atrial systole

starts after the P wave of the electrocardiogram (ECG); ven-

tricular systole starts near the end of the R wave and ends just

after the T wave. In this chapter, we will consider how these

changes in contraction produce sequential changes in pres-

sures and flows in the heart chambers and blood vessels, and

thereby propel blood appropriately as needed by whole body

demands for oxygen and nutrients. As an aside, it should be

noted that the term

systolic pressure

in the vascular system

refers to the peak pressure reached during systole, not the

mean pressure; similarly, the

diastolic pressure

refers to the

lowest pressure during diastole.

MECHANICAL EVENTS OF

THE CARDIAC CYCLE

EVENTS IN LATE DIASTOLE

Late in diastole, the mitral (bicuspid) and tricuspid valves be-
tween the atria and ventricles (atrioventricular [AV] valves)

are open and the aortic and pulmonary valves are closed.

Blood flows into the heart throughout diastole, filling the atria

and ventricles. The rate of filling declines as the ventricles be-

come distended, and, especially when the heart rate is low, the

cusps of the AV valves drift toward the closed position (Figure

31–1). The pressure in the ventricles remains low. About 70%

of the ventricular filling occurs passively during diastole.