Which principle explains the alternation of systole and diastole in the heart's pumping action?

The alternation of systole and diastole in the heart's pumping action is best understood through the concept of the cardiac cycle. The cardiac cycle refers to the sequence of events that occur from the beginning of one heartbeat to the beginning of the next, encompassing both systole, which is the phase of contraction where the heart pumps blood out to the arteries, and diastole, the phase of relaxation where the heart fills with blood.

During systole, the ventricles contract, and blood is ejected into the arteries; concurrently, during diastole, the heart chambers relax, allowing them to fill with blood in preparation for the next contraction. This rhythmic contraction and relaxation are driven by the electrical signals generated in the heart that coordinate the timing of these phases.

The heart's mechanics and the regulation of cardiac output are related elements but do not fundamentally define the alternating phases of contraction and relaxation. Electrophysiological conduction specifically pertains to the pathway of electrical impulses that initiate the contractions and regulate the heart rate but does not fully capture the entire sequence of the cardiac cycle and its systematic alternation. Thus, the cardiac cycle serves as the correct framework to explain the rhythmic alternation of systole and diastole in the heart's pumping action.