In the electrical conduction system of the heart, which structure is considered the primary pacemaker that normally initiates each heartbeat?

• A. Atrioventricular (AV) node
• B. Bundle of His (AV bundle)
• C. Purkinje fibers
• D. Sinoatrial (SA) node

Answer: (D)

Verified step by step guidance

1. Understand that the heart's electrical conduction system controls the heartbeat by generating and conducting electrical impulses.
2. Identify the main components of this system: the Sinoatrial (SA) node, Atrioventricular (AV) node, Bundle of His, and Purkinje fibers.
3. Recognize that the primary pacemaker is the structure that normally initiates the electrical impulse causing the heart to contract.
4. Recall that the SA node, located in the right atrium, has the highest intrinsic firing rate and thus sets the pace for the heart's rhythm.
5. Conclude that the SA node is the primary pacemaker because it initiates each heartbeat under normal conditions.