Question

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

• A. Semilunar valves close down after the blood flows into vessels from ventricles
• B. $AV$ valves open up
• C. $AV$ node receives signal from $SA$ node
• D. Ventricular walls vibrate due to gushing in of blood from atria

Answer 1

The correct option is A

Semilunar valves close down after the blood flows into vessels from ventricles

Heart sound

Heart produces two sounds during a cardiac cycle: Lub and dub, due to closure of the valves present in the heart.

Dub- Second heart sound

Dub, the second sound produced by the heart, is due to sharp closure of semilunar valves following ventricular systole.

$\bullet$ Closure of pulmonary semilunar valve occurs after deoxygenated blood from right ventricle is pumped into pulmonary artery for oxygenation.

$\bullet$ Closure of aortic valve $\left(\text{a semilunar valve}\right)$ occurs after oxygenated blood from left ventricle is pumped into aorta for supply to body tissues.

Valve closure is essential to prevent the backflow of blood as ventricular pressure starts decreasing during diastole.

Final answer

$\left(B\right)$ Semilunar valves close down after the blood flows into vessels from ventricles