What is the primary function of the SA node?

• A. Ventricular contraction
• B. Pacemaker, atrial contraction
• C. Delay, ventricular filling
• D. Ventricular relaxation

Answer: (B)

The SA node acts as the heart’s natural pacemaker, generating regular electrical impulses that set the pace of the heartbeat. This spontaneous activity triggers depolarization of the atrial myocardium, producing atrial contraction. The impulse then travels to the AV node, which introduces a brief delay to allow the ventricles time to fill, before the signal proceeds to the ventricles to cause their contraction. So the main function is to start the rhythm and drive atrial activity, establishing the overall heart rate. Ventricular contraction and relaxation occur later in the sequence after the signal has passed through the AV node and into the ventricles, which is why those options don’t describe the SA node’s role.