What causes the sound of a heartbeat?

The story behind the answer

The familiar “lub-dub” of a heartbeat comes from valves snapping shut, not from the heart squeezing or blood rushing out. The first sound (S1, “lub”) is the closure of the atrioventricular valves—the mitral and tricuspid—at the start of ventricular contraction (systole). The second sound (S2, “dub”) is the closure of the semilunar valves—the aortic and pulmonary—at the start of relaxation (diastole). These closures create vibrations in the heart walls and blood, producing the audible beats.

Memory tip: think of doors closing to direct traffic. “Lub” locks the inlet doors (AV valves) so blood moves out; “Dub” locks the outlet doors (semilunar valves) to prevent backflow. Extra whooshing (“murmurs”) often indicates valve problems.