What does S1 mark the onset of?

The first heart sound, known as S1, marks the onset of systole, which is the phase of the cardiac cycle when the heart muscle contracts to pump blood out of the ventricles. S1 is primarily produced by the closure of the mitral and tricuspid valves as the ventricles begin to contract, creating a high-pressure environment that forces blood into the aorta and pulmonary artery. This sound is a key indicator that the heart is transitioning from the filling phase (diastole) to the contraction phase (systole), highlighting its role in the mechanical function of the heart during the circulation of blood. Understanding this phase is essential in the assessment of cardiac function during an echocardiogram and contributes to a comprehensive evaluation of heart health.