What causes the sound of a heartbeat?

The sound of a heartbeat is caused by the closure of the heart valves. When the heart contracts, the atrioventricular (AV) valves close to prevent the backflow of blood from the ventricles to the atria. This closure produces the first heart sound, "lub." Similarly, when the ventricles relax, the semilunar valves close to prevent blood from flowing back into the ventricles. This closure generates the second heart sound, "dub." These valve closures create the characteristic rhythmic "lub-dub" sound of the heartbeat that can be heard through a stethoscope.