Question

Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) : The conduction in the Atrio-ventricular (AV) node is slow i.e. 0.05 m/sec, there is a delay of approx. 0.1 second before excitation spreads to the ventricles.
Reason (R) : AV node delay enables the atria to empty the blood present within them into the ventricles before these contract.
In the light of the above statements, choose the most appropriate answer from the options given below :

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A)
• B. Both (A) and (R) are correct but (R) is not the correct explanation of (A)
• C. (A) is correct but (R) is not correct
• D. (A) is not correct but (R) is correct

Hint:

Think of the AV node as an electrical "speed bump" or "traffic light" in the heart. It deliberately pauses the electrical signal (delay) so the atria can finish their job of topping off the ventricles with blood before the main pumping action begins.

Answer 1

The Correct Option is A

Step 1: Concept:
This question focuses on the electrophysiology of the cardiac conduction system, specifically highlighting the physiological significance of the decreased conduction velocity at the Atrio-ventricular (AV) node.

Step 2: Step-by-step Explanation:

• Analyzing Assertion (A): Electrical impulses generated by the Sinoatrial (SA) node travel rapidly through the atrial syncytium and reach the AV node. The AV node fibers have smaller diameters and significantly fewer gap junctions compared to other cardiac tissues. This structural difference causes a marked decrease in conduction velocity (down to about 0.05 m/sec), resulting in a physiological delay of roughly 0.09 to 0.1 seconds. The assertion is factually accurate.

• Analyzing Reason (R): If the electrical impulse traveled instantly from the atria to the ventricles, the entire heart would contract simultaneously. This would be highly inefficient, as the ventricles would attempt to pump blood while still mostly empty.

• The critical 0.1-second delay imposed by the AV node ensures that atrial systole (contraction) is fully completed, allowing the atria to pump their remaining blood volume into the ventricles (active ventricular filling) before ventricular systole initiates. The reason is factually correct.

• Because the necessity of complete ventricular filling is the exact biological evolutionary reason why the AV node slows down the electrical signal, Reason (R) perfectly explains Assertion (A).

Step 3: Final Answer:
Both statements are true, and (R) correctly explains (A).