What causes the heart sound S2?

The heart sound S2 is primarily generated by the closure of the semilunar valves, which are the aortic and pulmonic valves. During the cardiac cycle, S2 occurs at the end of ventricular systole when the ventricles empty their blood into the aorta and pulmonary artery. As the ventricles relax, the higher pressure in these great vessels causes the semilunar valves to close, resulting in the characteristic "dub" sound of S2. This sound provides important information about the end of the systolic phase of the heart's pumping action.

The closure of the atrioventricular valves occurs earlier in the cycle, contributing to the heart sound S1, which represents the onset of ventricular contraction. Opening of the valves, whether atrioventricular or semilunar, does not generate heart sounds, as sound is produced by closure rather than opening of the valves. Understanding this distinction is crucial in differentiating the various heart sounds and their significance in assessing cardiac function.