Q.What is the significance of time gap in the passage of action potential from sino-atrial node to the ventricle?

The action potential reaches the atria first via the SA node, and only afterward is it carried onward to the ventricles through the AV node and AV bundle. This built-in time gap lets the atria complete their own contraction (atrial systole), pushing additional blood into the ventricles, before the ventricles themselves begin to contract. In this way, the delay ensures the ventricles are more fully filled with blood at the moment they start pumping, rather than the atria and ventricles contracting together …

The SA node fires an action potential that first stimulates both atria to contract together — atrial systole — pushing additional blood into the ventricles and raising the amount they receive by about 30 per cent. Only after this does the impulse travel onward, carried by the AV node and the AV bundle, to reach and stimulate the ventricles, triggering ventricular systole while the atria in turn relax. …

Method: Explaining the Functional Significance of a Physiological Time Delay

Use this approach for any "what is the significance of the delay/gap between X and Y" question in a physiological sequence.

Steps

Step 1: Identify what the delay separates

Name the two events the gap sits between -- here, atrial excitation (triggered directly by the SA node) and ventricular excitation (which only occurs after the impulse passes through the AV node and AV bundle).

Step 2: Ask what each side of the delay achieves while it lasts

During the gap, the atria complete their own contraction (atrial systole), pushing extra blood into the ventricles below -- the delay is not "wasted time," it is functional time.

Step 3: State what would go wrong WITHOUT the delay …