Which event does the closure of the AV valves indicate in the cardiac cycle?

The closure of the atrioventricular (AV) valves is a key event that marks the transition from diastole to systole in the cardiac cycle. When the ventricles contract during systole, the pressure within them rises significantly. This increase in pressure forces the AV valves (the mitral and tricuspid valves) to close to prevent the backflow of blood into the atria. The event is associated with the onset of ventricular contraction, which is crucial for pumping blood into the arteries.

When the AV valves close, it indicates that the ventricles are beginning to empty their contents into the aorta and the pulmonary artery, thus initiating systole. This event is also represented on the cardiac cycle graph with the first heart sound (S1), which is produced by the closure of these valves. In contrast, other events in the cardiac cycle, such as ventricular filling or end of diastole, occur at different times and are not characterized by the closure of the AV valves.