If you hold a stethoscope to your chest, you can hear the familiar "lub-dupp" sound of your heart in action. The "lub" you hear is

Question

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The "lub" sound, also known as the first heart sound (S1), is primarily caused by the closing of the atrioventricular (AV) valves, which are the mitral and tricuspid valves, at the beginning of ventricular systole. This event prevents the backflow of blood into the atria.

If You Hold a Stethoscope to Your Chest, You Can