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PART 4 Regulation and Maintenance

TABLE 20.2

Summary of the Events of the Cardiac Cycle

Abbreviation: AV = atrioventricular.

rise; however, because all the valves are closed, no blood flowsfrom the ventricles. This brief interval is called the period of iso-volumetric ( iso, same) contraction because the volume of bloodin the ventricles does not change, even though the ventricles arecontracting (figure 20.18, step 2 ). Ventricular contraction contin-ues, and ventricular pressures become greater than the pressuresin the pulmonary trunk and aorta. As a result, the semilunarvalves are pushed open, and blood flows from the ventricles intothose arteries. This time period, when blood moves from the

ventricles into the arteries, is called the period of ejection (figure20.18, step 3 ).As ventricular diastole begins, the ventricles relax, andventricular pressures decrease below the pressures in the pulmo-nary trunk and aorta. Consequently, blood begins to flow backtoward the ventricles, causing the semilunar valves to close(figure 20.18, step 4 ). With closure of the semilunar valves, all theheart valves are closed, and no blood flows into the relaxing ven-tricles during the period of isovolumetric relaxation .