Final answer:
The SA node is the primary pacemaker of the heart, responsible for establishing the normal cardiac rhythm by initiating the sinus rhythm.
Step-by-step explanation:
The primary pacemaker of the heart is the sinoatrial (SA) node. The SA node is a specialized clump of myocardial conducting cells located in the superior and posterior walls of the right atrium, in close proximity to the orifice of the superior vena cava. It has the highest inherent rate of depolarization, which establishes the normal cardiac rhythm by initiating the sinus rhythm. This rhythm is the normal electrical pattern followed by the contraction of the heart, where an electrical charge spontaneously pulses from the SA node causing the two atria to contract. The pulse then reaches the atrioventricular (AV) node, where it pauses before spreading to the walls of the ventricles, entering the Purkinje fibers and leading to ventricular contraction.