Which valves close to produce S1, and which valves close to produce S2?

• A. S1 is caused by closure of the AV valves (mitral and tricuspid) at the start of systole; S2 is caused by closure of the semilunar valves (aortic and pulmonary)
• B. S1 is caused by closure of the semilunar valves (aortic and pulmonary) at the start of systole; S2 is caused by closure of the AV valves
• C. S1 is caused by closure of the AV valves at the end of diastole; S2 by closure of the AV valves
• D. S1 is caused by closure of the AV valves at the start of systole; S2 is caused by closure of the semilunar valves

Answer: (A)

The sounds of the heart reflect valve closures and when they occur in the cardiac cycle. S1, the first sound, comes from the closure of the atrioventricular valves (the mitral and tricuspid) at the start of ventricular systole as the ventricles begin to contract and eject blood. S2, the second sound, comes from the closure of the semilunar valves (the aortic and pulmonary) at the end of systole, when the ventricles finish contracting and pressure falls, preventing backflow. So the combination that matches this timing is S1 from the AV valves at the start of systole and S2 from the semilunar valves, which is why this option is correct.