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The first heart sound "Lub" is caused by the closure of the atrioventricular (AV) valves.
Ref: NCERT Class 11 Biology Chapter 15: Body Fluids and Circulation Human Heart - Structure and Cardiac Cycle
The AV valves (tricuspid and bicuspid) prevent backflow into the atria when the ventricles contract.
Ref: NCERT Class 11 Biology Chapter 15: Body Fluids and Circulation Human Heart - Structure and Cardiac Cycle
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