Final answer:
The heart's internal pacemaker beats due to its intrinsic electrical activity, initiated by the sinoatrial node and then transmitted through the heart's conduction system to cause a coordinated contraction.
Step-by-step explanation:
The heart's internal pacemaker beats due to the intrinsic electrical activity of the cells that comprise the cardiac conduction system. This system includes the sinoatrial (SA) node, which is the heart's primary pacemaker, initiating electrical impulses that cause the atria to contract. Following the SA node's action, the electrical impulse travels to the atrioventricular (AV) node where there is a brief delay, allowing the ventricles to fill with blood. The impulse then moves through the bundle of His, the right and left bundle branches, and the Purkinje fibers, causing the ventricles to contract. This entire sequence creates the rhythmical heartbeat known as sinus rhythm. This autorhythmicity is intrinsic to the pacemaker cells and occurs independently of external stimuli, although it can be modulated by the autonomic nervous system.