Normal cardiac rhythm is primarily determined by the:

The sinoatrial node (SA node) is considered the primary pacemaker of the heart, responsible for initiating the electrical impulses that regulate the heart's normal rhythm. Located in the right atrium, the SA node generates electrical signals at a regular pace, which spread through the atria, causing them to contract and push blood into the ventricles. This rhythmic generation of impulses is crucial for maintaining a consistent heartbeat and a coordinated contraction of the heart chambers.

While other components of the cardiac conduction system, such as the atrioventricular (AV) node and the bundle of His, play important roles in conducting and relaying electrical impulses, they do not initiate the rhythm themselves. The AV node acts as a gatekeeper, allowing the impulses from the SA node to pass into the ventricles after a slight delay, while the bundle of His transmits impulses through the ventricles, facilitating their contraction.

The coronary arteries, meanwhile, are responsible for supplying oxygenated blood to the heart muscle itself, but they do not influence the heart's electrical rhythm directly. Thus, the sinoatrial node stands out as the key structure in establishing the heart's normal rhythm, making it the correct choice.