What structure serves as the natural pacemaker of the heart?

• A. Atrioventricular (AV) node
• B. Sinoatrial (SA) node
• C. Bundle of His
• D. Purkinje fibers

Answer: (B)

The heart’s natural pacing source is the sinoatrial node because its pacemaker cells have the highest rate of spontaneous depolarization, so they set the pace for the entire heart. Located in the right atrial wall near the superior vena cava, the SA node fires regular impulses that initiate each heartbeat and trigger atrial contraction. From there, the impulse travels through the atrioventricular node and onward via the conduction pathways to coordinate ventricular contraction.

Other parts of the conducting system—the AV node, Bundle of His, and Purkinje fibers—can take over if the SA node stops functioning, but their intrinsic firing rates are slower, leading to a slower heart rate. The AV node also provides a brief delay to ensure the ventricles fill with blood before they contract.