What is the primary function of the sinoatrial (SA) node in cardiac conduction?

• A. It initiates ventricular contraction directly.
• B. It acts as the natural pacemaker by generating rhythmic impulses that set heart rate and initiate atrial contraction.
• C. It delays impulses to coordinate ventricular filling.
• D. It stores blood during diastole.

Answer: (B)

The primary function tested here is that the sinoatrial node acts as the heart’s natural pacemaker, generating rhythmic electrical impulses that set the heart rate and initiate atrial contraction. Located in the right atrium, its pacemaker cells depolarize spontaneously, reaching threshold without external input, which establishes the intrinsic heart rate. The impulse then spreads through the atria to the AV node and onward to the ventricles, coordinating the heartbeat. Ventricular contraction is ultimately driven by the downstream conduction system, not the SA node itself. The brief delay that helps optimize ventricular filling occurs at the AV node, not at the SA node. The SA node doesn’t store blood; its role is electrical pacing, while autonomic input can modulate its rate up or down as needed.