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What physical event directly produces the familiar "lub-dub" sound of a beating heart?

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Correct answer: The closing of the heart valves

The sounds you hear through a stethoscope are not the muscles contracting, but the abrupt snapping shut of the heart valves. The first sound ("lub") comes from the mitral and tricuspid valves closing, while the second ("dub") is the aortic and pulmonary valves shutting.

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Source: Wikipedia (CC BY-SA)

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