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What causes the lub-dub sounds of a heartbeat?

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Illustration of a human heart cross-section showing four chambers-left and right atria and ventricles-with labeled major blood vessels. An inset highlights cardiac muscle tissue, showing myocytes and intercalated disks. — human cardiovascular system The human heart is both a pump and a muscle. Its four chambers move blood efficiently, while cardiac muscle cells joined by intercalated disks synchronize each contraction.

The "lub-dub" sounds of a heartbeat are caused by the closing of the heart's valves, which triggers vibrations in the walls of the heart and its major vessels.12

The "Lub" (First Sound): This low-pitched sound occurs when the mitral and tricuspid valves (the atrioventricular valves) close as the ventricles begin to contract. The contraction forces blood backward, closing the valves and causing them to bulge. The elasticity of these valves causes blood to bounce back into the ventricles, setting the ventricular walls into vibration.12
The "Dub" (Second Sound): This sharper, higher-pitched sound is produced when the pulmonary and aortic valves (the semilunar valves) close. This action causes blood to reverberate between the walls of the arteries and the valves, creating vibrations in the walls of the aorta and pulmonary artery.12

These vibrations travel to the chest wall, where they are transformed into the audible sound waves detected by a stethoscope. The opening of the valves is silent.12