Which valves closure produces the 1st heart sound?

The first heart sound, often described as "lub," is produced primarily by the closure of the atrioventricular (AV) valves during the early phase of ventricular contraction (systole). The atrioventricular valves include the mitral valve, located between the left atrium and left ventricle, and the tricuspid valve, positioned between the right atrium and right ventricle.

When the ventricles contract, pressure rises within them, leading to the closure of these valves to prevent backflow of blood into the atria. The sound generated from this closure is low-frequency and is best heard with a stethoscope, contributing to the characteristic "lub" sound of the heartbeat.

In contrast, the closure of the semilunar valves, which includes the pulmonary and aortic valves, occurs later in the cardiac cycle and produces the second heart sound, often referred to as "dup." Therefore, the correct answer emphasizes the role of the atrioventricular valves in generating the first heart sound, which is a fundamental concept in understanding cardiac physiology and the mechanics of the heart during the cardiac cycle.