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Doctors use stethoscope to hear the sounds produced during each cardiac cycle. The second sound is heard when:

1. Ventricular wall vibrate due to gushing in of blood from atria.
2. Semilunar valves close down after the blood flows into vessels from ventricles.
3. AV node receives signal from SA node.
4. AV valves open up.

Correct answer: Semilunar valves close down after the blood flows into vessels from ventricles.

Solution

The second heart sound (S2) is produced when the semilunar valves close at the end of ventricular systole. This prevents blood from flowing back from the aorta and pulmonary artery into the ventricles.

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