Which phase of the cardiac cycle does S1 represent?

S1, also known as the "lub" sound, occurs at the onset of systole, which is the phase of the cardiac cycle when the ventricles contract. This sound is produced by the closure of the atrioventricular valves (the mitral and tricuspid valves) as the ventricles prepare to eject blood into the aorta and pulmonary artery. The closing of these valves prevents the backflow of blood into the atria, marking the transition from diastole, where the heart fills with blood, to systole, where the heart pumps blood out to the body and lungs. Understanding the timing and significance of heart sounds is crucial in assessing cardiac function and recognizing potential pathologies.