The first heart sound (S1) and second heart sound (S2) are produced by the closing of which valves, respectively?

• A. Opening of AV valves
• B. Closing of AV valves and semilunar valves
• C. Closure of the chordae tendineae
• D. Atrial contraction

Answer: (D)

The sounds you hear are tied to valve closures. The first heart sound comes from the closure of the atrioventricular valves—the mitral and tricuspid valves—as the ventricles start to contract and push blood forward. That abrupt closing creates the characteristic “lub.” The second heart sound is produced by the closure of the semilunar valves—the aortic and pulmonic valves—at the end of systole when ventricular pressure drops, producing the “dub.” The chords tendineae themselves help prevent valve prolapse but don’t generate a separate sound. Opening of the AV valves happens during diastole and isn’t responsible for the first sound, and atrial contraction occurs during diastole and doesn’t produce these primary heart sounds. So, S1 reflects AV valve closure, and S2 reflects semilunar valve closure.