Final answer:
The coordinating structure responsible for an intrinsic heartbeat is the Sinoatrial Node (SA Node), which is the pacemaker of the heart and initiates the sinus rhythm.
Step-by-step explanation:
The coordinating structure responsible for an intrinsic heartbeat is the Sinoatrial Node (SA Node). The SA Node, often called the pacemaker of the heart, is responsible for establishing the normal cardiac rhythm. This is a specialized clump of myocardial conducting cells located in the superior and posterior walls of the right atrium near the orifice of the superior vena cava. The SA Node sets the pace for the heart by initiating the sinus rhythm due to its highest inherent rate of depolarization. It signals the two atria to contract and then the impulse reaches the Atrioventricular Node (AV Node) where it pauses briefly, ensuring the atria have emptied their contents into the ventricles before the ventricles begin to contract.