Which structure acts 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 pace comes from the Sinoatrial node, a cluster of pacemaker cells in the right atrium near the superior vena cava. These cells automatically generate impulses at their own steady rate, a property called automaticity. Because the SA node fires the fastest among the heart’s conduction tissues, it sets the usual rhythm of the heartbeat—roughly 60–100 beats per minute in a healthy adult.

From there, the impulse travels to the Atrioventricular node, which deliberately delays the signal to allow the atria to finish contracting before the ventricles beat. The Bundle of His and the Purkinje fibers then carry the impulse rapidly to the ventricles for coordinated contraction. If the SA node fails, the AV node can take over, but at a slower rate (about 40–60 bpm), which is why the SA node is considered the natural pacemaker under normal conditions.