The sinoatrial (SA) node is a small cluster of specialized cells located in the right atrium of the heart. It serves as the primary pacemaker, generating electrical impulses that initiate each heartbeat. These impulses spread through the heart, causing the atria to contract and pump blood into the ventricles. The SA node is influenced by the autonomic nervous system, which can adjust the heart rate based on the body's needs. For example, during exercise, the sympathetic nervous system increases the heart rate, while the parasympathetic nervous system slows it down during rest.