The heart's natural pacemaker is located in which node?

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

Answer: (D)

The heart's natural pacemaker is the Sinoatrial node, located in the upper part of the right atrium near the opening of the superior vena cava. It contains pacemaker cells that automatically depolarize to threshold, generating regular electrical impulses and setting the heart’s rhythm—about 60–100 beats per minute in a healthy adult. This impulse swiftly spreads through the atria to cause contraction and then to the AV node, which provides a brief delay to ensure the ventricles fill properly before they contract. If the SA node isn’t firing, other tissues can take over at a slower rate (secondary pacemakers), but the SA node normally dictates the pace because it has the fastest intrinsic firing rate.