S1 heart sound is caused by the closing of which valves?

• A. The mitral and tricuspid valves
• B. The aortic and pulmonary valves
• C. The atrioventricular valves closing during systole
• D. The ventricles contracting

Answer: (A)

S1 is produced when the atrioventricular valves close as the ventricles begin to contract. The mitral valve on the left and the tricuspid valve on the right shut to prevent blood from flowing back into the atria as ventricular pressure rises, creating the characteristic “lub” sound. The second heart sound, S2, comes from the closure of the semilunar valves (aortic and pulmonary) at the end of systole, not from the AV valves. Saying AV valves close during systole is true, but the specific valves responsible for S1 are the mitral and tricuspid valves.