What is the first heart sound caused by?

The first heart sound, often referred to as "S1", is primarily caused by the closure of the atrioventricular (AV) valves, which include the mitral and tricuspid valves. When the ventricles contract during ventricular systole, the pressure within the ventricles increases, leading to the closure of these valves. This closure creates vibrations in both the valves and the surrounding fluid, resulting in the sound we hear as S1.

Understanding this process is fundamental in distinguishing the mechanics of heart sounds. The AV valves closing marks the onset of ventricular contraction, while the sound produced is a result of both the physical closure of the valves and the rapid deceleration of blood flow as the ventricles start to pump.

In contrast to other options, such as the motion of the aortic semilunar valve or the action of blood rushing into the ventricles, these do not contribute to the first heart sound. The first sound specifically relates to the closure of the AV valves and the resulting vibrations, making this option accurate in description.