The 2nd heart sound is caused by the closure of which valves?

The second heart sound, often denoted as S2, arises from the closure of the semilunar valves, which consist of the aortic and pulmonary valves. During the cardiac cycle, after the ventricles contract and eject blood into the aorta and pulmonary artery, these semilunar valves close to prevent backflow of blood into the heart. The closing of these valves produces the characteristic "dup" sound associated with S2, indicating the end of systole (the contraction phase of the heart) and the beginning of diastole (the relaxation phase).

Understanding this mechanism is crucial to interpreting heart sounds and recognizing the functionality of the heart's anatomy. The other options involve different valves; the atrioventricular valves (such as the tricuspid and mitral valves) close during ventricular contraction, producing the first heart sound (S1). Since the question specifically asks about the second heart sound, the closure of the semilunar valves is indeed the correct response.