S2 heart sound occurs when the...

• A. The closing of the semilunar valves (aortic and pulmonic)
• B. The closing of the mitral and tricuspid valves
• C. Atrial contraction
• D. The start of ventricular systole

Answer: (A)

The second heart sound comes from the sudden closure of the semilunar valves—the aortic and pulmonic valves—at the end of ventricular systole, marking the start of diastole. When these valves snap shut, the backflow into the valves is halted, producing that brief, sharp sound. You can think of it as two components: A2 from the aortic valve and P2 from the pulmonic valve, which may be heard as a single sound or as a split with inspiration. This is distinct from the first heart sound, which results from the closure of the atrioventricular valves at the start of ventricular contraction. The closing of the AV valves occurs at the beginning of systole, not the end, and atrial contraction happens during diastole and does not generate a separate heart sound. The start of ventricular systole is associated with the first heart sound, not the second.