Which statement correctly describes the S1 heart sound?

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S1 marks the start of ventricular systole and is generated as the atrioventricular valves (mitral and tricuspid) close, producing the familiar “lub” sound. As the ventricles contract and pressure rises, these AV valves snap shut, while the semilunar valves (aortic and pulmonic) are pushed open to allow blood to flow into the aorta and pulmonary arteries. That combination—AV valves closing and semilunar valves opening—best describes what you hear with the first heart sound. The alternatives describe the opposite timing (semilunar valves closing at S1, which is S2) or AV valves opening, which isn’t what produces S1.