Question

The figure represents total period of one cardiac cycle i.e., $0.8$ sec and $A, B$ and $C$ represent its stages. Identify $A, B$ and $C$ and select the correct statement regarding them.

• A. During $A$, tricuspid and bicuspid valves open and blood flows from atria into the ventricles
• B. During $B$, bicuspid and tricuspid valves close producing first heart sound
• C. During $C$, the semilunar valves close producing second heart sound
• D. During $B$, the atria contract due to -a wave of contraction by $SA$ node

Answer 1

Answer: (D)

In the given figure, A is atrial and ventricular diastole (complete cardiac diastole), $B$ is artrial systole and $C$ is ventricular systole.
During atrial systole, the atria contract due to wave of contraction, stimulated by the $SA$ node. The blood is forced into the ventricles as the bicuspid and tricuspid valves are open. At the beginning of ventricular systole, the ventricles begin to contract due to a wave of contraction, stimulated by the $AV$ node. The bicuspid and tricuspid valves close immediately producing part of the first heart sound. With complete ventricular systole, the ventricles complete their contraction and the blood flows into the pulmonary trunk and aorta as the semilunar valves open. At the beginning of ventricular diastole, the ventricles relax and the semilunar valves are closed. This causes the second heart sound. During complete ventricular diastole, the tricuspid and bicuspid valves open as the pressure in the ventricles falls and blood flows from the atria into the ventricles. Contraction of the heart does not cause this blood flow. It is due to the fact that the pressure within the relaxed ventricles is less than that in the atria and veins.