What heart sound marks the start of diastole?

The heart sound that marks the start of diastole is S2. This sound, also known as the "dub," occurs when the aortic and pulmonary valves close at the end of ventricular systole. The closure of these valves signals the transition from the contraction phase of the heart, during which blood is pumped out of the ventricles, to the relaxation phase, or diastole, where the heart chambers fill with blood.

This phase is critical as it allows the heart to refill with blood before the next contraction begins. Understanding this helps in interpreting the cardiac cycle and recognizing the importance of sound timing in the functioning of the heart. Other heart sounds, such as S1, S3, and S4, occur during different phases of the cardiac cycle: S1 marks the beginning of systole, S3 is associated with rapid ventricular filling, and S4 is related to atrial contraction. However, S2 specifically indicates the end of systole and the commencement of diastole.