What event is indicated by the S1 heart sound?

The S1 heart sound is primarily associated with the closing of the atrioventricular (AV) valves, which include the mitral and tricuspid valves. This event occurs at the beginning of ventricular contraction (systole) when the ventricles are filled with blood and begin to contract, creating pressure that forces the AV valves to close. The sound generated by this closure is a low-pitched "lub" sound, which is the first heart sound heard during auscultation.

This sound marks the transition from diastole, where the heart is relaxed and filling with blood, to systole, where the heart is actively pumping blood into the lungs and the systemic circulation. Understanding the significance of the S1 sound is critical for assessing normal heart function and identifying potential cardiac issues. Therefore, recognizing that the S1 heart sound signals the closing of the AV valves is foundational in both anatomy and the physiology of the cardiovascular system.