What makes the lub sound of our heartbeat?

The "lub" sound of our heartbeat, known as the first heart sound (S1), is primarily caused by the closure of the atrioventricular (AV) valves—the mitral and tricuspid valves—during ventricular contraction (systole). When the ventricles contract, blood is pumped out of the heart, and the sudden closure of these valves creates a distinct sound. This sound is essential for indicating that the heart is properly pumping blood and is part of the normal cardiac cycle.