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 upregulated 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.
Of course, the electrical activity of the heart discussed in the previous chapter is designed to subserve the heart’s primary physiologic role—to pump blood through the lungs, where gas exchange can occur, and thence to the remainder of the body (Clinical Box 30–1). 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); ventricular systole starts near the end of the R wave and ends just after the T wave. In this chapter, how these changes in contraction produce sequential changes in pressures and flows in the heart chambers and blood vessels, thereby propelling blood appropriately as needed by whole body demands for oxygen and nutrients, will be considered. 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
Late in diastole, the mitral (bicuspid) and tricuspid valves between 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 become distended and, especially when the heart rate is low, the cusps of the AV valves drift toward the closed position (Figure 30–1). The pressure in the ventricles remains low. About 70% of the ventricular filling occurs passively during diastole.
Divisions of the cardiac cycle: A) systole and B) diastole. The phases of the cycle are identical in both halves of the heart. The direction in which the pressure difference favors flow is denoted by an arrow; note, however, that flow will not actually occur if a valve prevents it. AV, atrioventricular.