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.
MECHANICAL EVENTS OF THE CARDIAC CYCLE
Atrial systole starts after the p wave of the EKG; ventricular systole starts near the end of the R wave and ends just after the T wave. 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.
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.
Contraction of the atria propels some additional blood into the ventricles. Contraction of the atrial muscle narrows the orifices of the superior and inferior vena cava and pulmonary veins, and the inertia of the blood moving toward the heart tends to keep blood in it. However, despite these inhibitory influences, there is some regurgitation of blood into the veins.
At the start of ventricular systole, the AV valves close. Ventricular muscle initially shortens relatively little, but intraventricular pressure rises sharply as the myocardium presses on the blood in the ventricle (Figure 30–2). This period of isovolumetric (isovolumic, isometric) ventricular contraction lasts about 0.05 s, until the pressures in the left and right ventricles exceed the pressures in the aorta (80 mm Hg; 10.6 kPa) and pulmonary artery (10 mm Hg), ...