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Question

Assertion [A]: Both the semilunar valves are thicker than the atrioventricular valves.

Reason [R]:These valves are subjected to greater pressure and have to be stronger than the AV valves.


  1. Assertion [A] is True and Reason [R] is True and is a correct explanation to [A].

  2. Assertion [A] is True and Reason [R] is True but is not a correct explanation to [A].

  3. Assertion [A] is True and Reason [R] is False.

  4. Reason [R] is True and Assertion [A] is False.


Solution

The correct option is A

Assertion [A] is True and Reason [R] is True and is a correct explanation to [A].


Valves between the ventricles and blood vessels exiting in pulmonary artery or aorta; are known as the semilunar valves. Valves between the atria and ventricles are known as the atrioventricular (AV) valves. The atrioventricular valves (tricuspid and mitral) prevent backflow of blood from the ventricles into the atria. This is more likely to occur when the ventricles contract (systole). The tight seal created by these atrioventricular valves ensures that the blood in the ventricles are forced out into the pulmonary artery or aorta. The semilunar valves (pulmonic and aortic) perform the same function as the other heart valves – it prevents the backflow of blood but in this case from the arteries into the ventricles when the ventricles relax (diastole). These valves are subjected to greater pressure and have to be stronger than the AV valves. Therefore, they are thicker in build. Also, the mechanism of action of the semilunar valves, is different from that of the atrioventricular valves. They do not work on supporting muscles strength. They work on negative pressure gradient. These valves require backflow of blood to close shut. Without backflow, these valves remain open.

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