Step 1: Understanding the Concept:
The heart's rhythmic contraction is controlled by a specialized conduction system. A "pacemaker" is the component that initiates the electrical impulse.
Step 2: Detailed Explanation:
1. SA Node (Sinoatrial Node): Located in the wall of the right atrium. It has the highest rate of spontaneous depolarization (auto-rhythmicity). It sets the heart rate to approximately 70–80 beats per minute. Therefore, it is the "primary natural pacemaker."
2. AV Node (Atrioventricular Node): Receives the impulse from the SA node and delays it slightly before passing it to the ventricles. It is a secondary pacemaker.
3. Bundle of His and Purkinje Fibers: These conduct the impulse rapidly throughout the ventricular muscle.
Step 3: Final Answer:
The SA node is the site where the heartbeat originates under normal physiological conditions.