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?

Question

Accepted Answer

B. Aortic and pulmonary valves; end of ventricular systole. ## 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.

Answer

A. Mitral and tricuspid valves; end of ventricular diastole

Answer

C. Aortic and mitral valves; beginning of ventricular systole

Answer

D. Pulmonary and tricuspid valves; mid-ventricular systole

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

Timing and Mechanism

Physiological Basis

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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)