The second heart sound (S2) is produced by the closure of which valve(s), and at what point in the cardiac cycle does this occur?

Explanation

## Physiology of S2 **Key Point:** S2 is a composite sound produced by near-simultaneous closure of the aortic (A2) and pulmonary (P2) valves at the END of ventricular systole, marking the transition into ventricular diastole. ### Mechanism 1. During ventricular systole, the aortic and pulmonary valves remain open as blood is ejected into the aorta and pulmonary artery. 2. As ventricular pressure falls below aortic and pulmonary artery pressures at the end of systole, these semilunar valves snap shut. 3. The closure of these valves generates S2. ### Timing in Cardiac Cycle - **S2 marks the end of ventricular systole** and the beginning of ventricular diastole. - In a normal cardiac cycle: S1 (systole begins) → S2 (systole ends) → diastole → S1 again. - The interval between S2 and the next S1 represents the diastolic period. ### Physiological Split of S2 **High-Yield:** During inspiration, P2 (pulmonary valve closure) occurs slightly AFTER A2 (aortic valve closure) due to: - Increased venous return to the right heart → prolonged RV ejection time - Decreased venous return to the left heart → shortened LV ejection time - This creates an audible split: A2-P2 (best heard at the left upper sternal border) **Clinical Pearl:** In expiration, A2 and P2 fuse into a single sound because the timing difference narrows. ### Contrast with S1 | Heart Sound | Valves Involved | Timing | Cardiac Phase | |---|---|---|---| | **S1** | Mitral and tricuspid (AV valves) | Beginning of ventricular systole | Systole starts | | **S2** | Aortic and pulmonary (semilunar valves) | End of ventricular systole | Diastole begins | [cite:Harrison 21e Ch 227]