Which structure is the primary pacemaker of the heart?

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

Answer: (C)

The heart’s natural pace is set by the sinoatrial node, which acts as the primary pacemaker. Its pacemaker cells have the fastest intrinsic rate and automatically generate impulses, typically about 60 to 100 beats per minute in adults. This impulse starts here and initiates the heartbeat, spreading through the atria to cause atrial contraction, then reaching the atrioventricular node where a brief delay occurs to allow the ventricles to fill. From there, the impulse travels down the conduction system via the Bundle of His and Purkinje fibers to produce coordinated ventricular contraction. The other structures are crucial parts of the conduction pathway or can take over if the SA node fails, but they normally do not set the pace on a healthy heart.