The SA node, or sinoatrial node, is a small cluster of cells located in the right atrium of the heart. It serves as the heart's natural 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 plays a crucial role in maintaining a regular heart rhythm. It responds to various factors, such as physical activity and stress, by adjusting the heart rate accordingly. This ensures that the body receives an adequate supply of oxygenated blood during different activities.