Which system primarily controls the heartbeat?

The cardiac conduction system is primarily responsible for controlling the heartbeat. This specialized network of tissues ensures the heart beats in a coordinated and rhythmical manner. It consists of key components such as the sinoatrial (SA) node, which acts as the natural pacemaker, the atrioventricular (AV) node, and the bundle of His, along with the Purkinje fibers. The SA node generates electrical impulses that initiate each heartbeat, causing the atria to contract and push blood into the ventricles. This electrical signal then travels to the AV node and through the conduction pathways, leading to the ventricular contraction.

While the nervous system can influence heart rate through various mechanisms, such as the autonomic nervous system's sympathetic and parasympathetic branches, it does not primarily control the rhythm of the heartbeat itself. The endocrine system does play a role in influencing heart rate through hormones like adrenaline, but it does not directly control the heartbeat. The muscular system, specifically the cardiac muscle, is responsible for the contraction of the heart, but it relies on the conduction system's electrical signals to contract in a synchronized manner.

Thus, the cardiac conduction system is the key player in setting the pace and rhythm of the heartbeat, making it the correct answer.