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What causes the first and second heart sounds?

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Silhouette diagram of the human body highlighting the heart and major blood vessels, with labeled arteries in red and veins in blue to show the structure of the cardiovascular system. — Circulatory system Human circulatory system.

The first and second heart sounds are produced by the vibration of the heart walls and major vessels following the closure of the heart valves.12

First Heart Sound: This low-pitched, prolonged sound occurs when the mitral and tricuspid 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 then causes blood to bounce back into the ventricles, setting the ventricular walls into vibration.12
Second Heart Sound: This sharper, higher-pitched sound is caused by the sudden closure of the pulmonary and aortic semilunar valves. This action creates vibrations in the walls of the aorta, the pulmonary artery, and the ventricles as blood reverberates between the vessel walls and the valves.12

These vibrations are transmitted to the chest wall, where they can be detected as audible sound waves using a stethoscope. The opening of the heart valves is silent.12