When do S1 and S2 heart sounds occur?

• A. S1 at the End of Systole; S2 at the Start of Diastole
• B. S1 at the Start of Systole; S2 at the End of Systole
• C. S1 During Diastole; S2 During Systole
• D. S1 at the Start of Diastole; S2 at the End of Diastole

Answer: (B)

The timing reflects which valves are closing as the heart cycles through systole and diastole. The first heart sound comes from the closing of the atrioventricular valves (mitral and tricuspid) as the ventricles begin to contract, marking the start of systole. The second heart sound comes from the closing of the semilunar valves (aortic and pulmonary) as ventricular ejection ends, signaling the start of diastole. So the best answer is that S1 occurs at the start of systole and S2 occurs at the end of systole.