S2 occurs when which event happens?

S2 occurs when which event happens?

• A. End of systole; closure of pulmonic and aortic valves
• B. Beginning of systole; AV valve closure
• C. Opening of valves
• D. Atrial contraction

Answer: (A)

Second heart sound comes from the closure of the semilunar valves—the aortic and pulmonic valves—at the end of systole, as the ventricles have finished ejecting blood and diastole begins. That abrupt valve closure creates the characteristic “dub” sound. The other options describe events that produce the first heart sound (closure of the AV valves at the start of systole), valve opening, or atrial contraction, none of which generate S2.

Second heart sound comes from the closure of the semilunar valves—the aortic and pulmonic valves—at the end of systole, as the ventricles have finished ejecting blood and diastole begins. That abrupt valve closure creates the characteristic “dub” sound. The other options describe events that produce the first heart sound (closure of the AV valves at the start of systole), valve opening, or atrial contraction, none of which generate S2.