Which of the following best describes the first sound (S1) heard during the cardiac cycle?
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Correct answer: Closure of the mitral and tricuspid valves
S1 (“lub”) is produced by closure of the atrioventricular valves (mitral and tricuspid) at the beginning of ventricular systole when pressure in ventricles rises. This marks the end of ventricular filling and start of isovolumetric contraction.
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