Ganong's Review of Medical Physiology, 23rd Edition

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-
MECHANICAL EVENTS OF
THE CARDIAC CYCLE
EVENTS IN LATE DIASTOLE
Late in diastole, the mitral (bicuspid) and tricuspid valves between
the atria and ventricles (atrioventricular [AV] valves)
C H A P T E R
31
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 pressures
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.
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 become
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.
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