Left atrium

The left atrium is one of the four chambers of the heart in mammals and birds, along with the left ventricle, right ventricle and right atrium. The left atrium pumps newly oxygenated blood from the lungs to the left ventricle.

The right side of the heart collects oxygen-poor blood from the body and pumps it to the lungs, where it picks up oxygen and releases carbon dioxide. The left side of the heart collects the oxygen-rich blood from the lungs and pumps it to the body.

Blood enters the heart through two veins, the superior vena cava and the inferior vena cava, then goes to the right atrium, then the right ventricle and to the lungs. After the blood picks up oxygen in the lungs, it flows to the left atrium, to the left ventricle, then out the aorta to the body.

A thick, muscular wall, the septum, divides the right and left sides of the heart, keeping blood from passing between them.