Which heart sound corresponds to closure of the semilunar valves?

• A. S1
• B. S2
• C. S3
• D. S4

Answer: (B)

The closing of the semilunar valves happens at the end of ventricular systole, when the ventricle has finished ejecting blood and its pressure falls below that of the aorta and pulmonary artery. That rapid valve closure produces the second heart sound, the familiar “dub” in the sequence lub-dub. This sound is distinct from the first heart sound, which comes from the closure of the AV valves at the start of systole. If heard, the third and fourth heart sounds are related to different events during diastole (rapid ventricular filling for the third, atrial contraction against a stiff ventricle for the fourth), not semilunar valve closure. Sometimes the second heart sound splits into two components because the aortic valve closes before the pulmonary valve, especially with inspiration.