The sinus node produces the electrical impulses that control the heartbeat. It is located in the upper part of the atrium, the heart's right upper chamber. Medical scientists sometimes refer to the node as the heart's natural pacemaker, the sinoatrial node or SA node.
The sinus node consistently charges and resets with each beat of the heart. In a normal, resting heart, the node fires between 60 and 80 times per minute. When people run or otherwise exert themselves, the node fires more frequently, which causes the heart to pump more blood around the body.
The electrical charge travels from the sinoatrial node to the atrioventricular node, located in the center of the heart between the ventricles and the atria, the vessels that carry blood into and out of the heart. Called the AV node for short, these cells slow down the current, preventing the atria and ventricles from contracting at the same time. The current goes from the AV node to the ventricles via fibers in the walls of the heart's lower chamber.
The sinus node's firing is governed by the autonomic nervous system, which automatically increases the heart rate when needed. When people have abnormal heart rhythms, called arrhythmias, a pacemaker is sometimes needed to get the heart's electrical system back on track.