A stethoscope is used by doctors for cardiac auscultation. The second sound of each cardiac cycle is heard when

The correct option is: Semilunar valves close down after the blood flows into vessels from ventricles

To understand why this is the correct answer, let us break down the mechanism of heart sounds during a cardiac cycle step-by-step:

1. Understanding Heart Sounds (Auscultation):
During cardiac auscultation using a stethoscope, doctors typically hear two primary sounds during each cardiac cycle, often described phonetically as "lub-dub". These sounds are produced by the closure of the heart valves, which causes turbulence in the blood flow and vibrations in the surrounding tissues.

2. The First Heart Sound (S₁ or "Lub"):
The first heart sound is associated with the closure of the atrioventricular (AV) valves (the tricuspid and bicuspid/mitral valves) at the beginning of ventricular systole (contraction). When the ventricles begin to contract, the pressure inside them rises, forcing the AV valves to shut to prevent the backflow of blood into the atria.

3. The Second Heart Sound (S₂ or "Dub"):
The second heart sound is associated with the closure of the semilunar valves (the aortic and pulmonary valves) at the beginning of ventricular diastole (relaxation).
After the ventricles contract, blood is pumped out of the ventricles into the major blood vessels (the aorta and pulmonary artery). As the ventricles start to relax, the pressure inside the ventricular chambers drops below the pressure in the aorta and pulmonary artery.
To prevent this ejected blood from flowing back into the ventricles, the semilunar valves snap shut. The sudden closure of these valves creates the second heart sound.

4. Conclusion:
Therefore, the second heart sound of each cardiac cycle is heard precisely when the semilunar valves close down after the blood flows into the vessels from the ventricles.