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

Explanation

## Physiology of S2 **Key Point:** S2 is a composite sound produced by the near-simultaneous closure of the aortic (A2) and pulmonary (P2) valves at the end of ventricular systole (beginning of diastole). ### Timing and Mechanism 1. **End of ventricular systole** — ventricular pressure falls below aortic and pulmonary artery pressures 2. **Valve closure** — aortic valve closes first (higher pressure gradient), followed by pulmonary valve 3. **Sound generation** — sudden tensioning of valve leaflets and associated structures produces the audible sound ### Physiological Basis | Feature | Details | | --- | --- | | **Aortic component (A2)** | Closes when LV pressure < aortic pressure (~80 mmHg) | | **Pulmonary component (P2)** | Closes when RV pressure < pulmonary artery pressure (~15 mmHg); delayed relative to A2 | | **Normal splitting** | Physiologic split of 10–20 ms (audible during inspiration due to increased RV stroke volume) | | **Valve involved** | Semilunar valves only (aortic and pulmonary) | **High-Yield:** S2 marks the **end of systole and beginning of diastole**. This is the key temporal landmark for auscultation and hemodynamic interpretation. **Clinical Pearl:** In inspiration, increased venous return to the RV prolongs RV ejection time, delaying P2 closure and widening the A2–P2 split. This physiologic split is normal and disappears in expiration.