right atrium

Think of the right atrium as the heart's loading dock for tired, oxygen-poor blood. Blood that has already delivered its oxygen to the body drains back here before being passed along to be sent to the lungs for a refill.

Anatomically, the right atrium is the upper right chamber of the heart. It receives deoxygenated blood from the whole body through two large veins — the superior vena cava (from the head, neck, and arms) and the inferior vena cava (from the trunk and legs) — as well as from the heart's own circulation via the coronary sinus. When it contracts, it pushes blood through the tricuspid valve into the right ventricle.

The right atrium also houses the heart's natural pacemaker, the sinoatrial node, which sits in its upper wall and sets the pace of every heartbeat. Because its walls are relatively thin, the right atrium works at low pressure; it is a filling and forwarding chamber rather than a powerful pump.