Which heart sound marks the onset of systole?

The sound that marks the onset of systole is S1, which is also known as the first heart sound. The S1 heart sound is generated primarily by the closure of the mitral and tricuspid valves at the beginning of ventricular contraction. This event signals the start of systole, the phase of the cardiac cycle during which the heart muscle contracts and pumps blood out of the ventricles.

S1 is typically described as a "lub" sound, and it is crucial for assessing the timing and function of the heart during an examination. It represents the transition from diastole, when the heart fills with blood, to systole, when blood is ejected into the aorta and pulmonary artery. The clarity and timing of the S1 sound can provide valuable information about valve function and the overall efficiency of the cardiac cycle.

Understanding S1 and its significance helps clinicians evaluate cardiac function and diagnose various heart conditions.