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.
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 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, 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.
Clinical Box 30–1
Heart failure occurs when the heart is unable to put out an amount of blood that is adequate for the needs of the tissues. It can be acute and associated with sudden death, or chronic. The failure may involve primarily the right ventricle (cor pulmonale), but much more commonly it involves the larger, thicker left ventricle or both ventricles. Heart failure may also be systolic or diastolic. In systolic failure, stroke volume is reduced because ventricular contraction is weak. This causes an increase in the end-systolic ventricular volume, so that the ejection fraction falls from 65% to as low as 20%. The initial response to failure is activation of the genes that cause cardiac myocytes to hypertrophy, and thickening of the ventricular wall (cardiac remodeling). The incomplete filling of the arterial system leads to increased discharge of the sympathetic nervous system and increased secretion of renin and aldosterone, so Na+ and water are retained. These responses are initially compensatory, but eventually the failure worsens and the ventricles dilate.
In diastolic failure, the ejection fraction ...
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