Doctors use stethoscope to hear the sounds produced during each cardiac cycle. The second sound is heard when
- AV node receives signal from SA node
- AV valves open up
- Ventricular walls vibrate due to gushing in of blood from atria
- Semilunar valves close down after the blood flows into vessels from ventricles.
Answer
(4) Semilunar valves close down after the blood flows into vessels from ventricles.
Solution
As per NCERT, two prominent sounds are produced during each cardiac cycle: the first heart sound (lub) is associated with the closure of the tricuspid and bicuspid (AV) valves, and the second heart sound (dub) is associated with the closure of the semilunar valves.
∵ The semilunar valves close at the end of ventricular systole, after blood has been pumped into the pulmonary artery and aorta.
∴ The second sound is heard when the semilunar valves close down.
Answer: (4)