Which description correctly summarizes the interaction between the SA node and AV node?

• A. SA node initiates heartbeats and AV node delays conduction
• B. AV node initiates heartbeats and SA node delays conduction
• C. SA node delays conduction while AV node initiates
• D. Both nodes act independently without interaction

Answer: (A)

The timing of the heartbeat relies on the SA node setting the pace and the AV node shaping the timing by delaying the impulse. The SA node, the heart’s natural pacemaker, fires first and sends the impulse through the atria, causing atrial contraction. When that impulse reaches the AV node, it slows down the conduction, creating a short pause that allows the ventricles to fill with blood before they contract. This coordinated sequence matches the description that the SA node initiates heartbeats and the AV node delays conduction. The other ideas don’t fit: the AV node does not initiate heartbeat, the SA node does not primarily delay conduction, and the nodes do not operate completely independently—the AV node’s delay is a purposeful part of the pathway from SA node to ventricular contraction.