What is the natural pacemaker of the heart?

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

Answer: (B)

The natural pacemaker is the sinoatrial node because its cells have the highest rate of spontaneous depolarization, so it regularly initiates the electrical impulses that set the heart’s rhythm. Located in the right atrium near the superior vena cava, it generates impulses that pace the heart at about 60–100 beats per minute in a healthy adult and start the heartbeat by causing atrial contraction. From there, the impulse travels to the atrioventricular node, which slows conduction to allow the ventricles time to fill, then moves through the bundle of His and Purkinje fibers to coordinate ventricular contraction. The other parts of the conduction system can take over if the SA node fails, but they do so at slower intrinsic rates, so they don’t normally set the pace. Hence, the sinoatrial node is the natural pacemaker.