Which heart structure acts as the natural pacemaker under normal rhythm?

• A. Atrioventricular node
• B. Sinoatrial node
• C. Bundle of His
• D. Purkinje fibers

Answer: (B)

The sinoatrial node is the natural pacemaker because it has the highest intrinsic pace among the heart’s electrical tissues, so it sets the rhythm under normal conditions. It spontaneously depolarizes and generates impulses that start each heartbeat, causing the atria to contract and then pass the signal to the atrioventricular node. The AV node introduces a brief delay to allow filling, and then the impulse travels through the bundle of His and Purkinje fibers to stimulate the ventricles. While the AV node and the Purkinje system can take over if the SA node fails, their inherent rates are slower, so they don’t determine the rhythm during normal function.