atrium
human heart
An atrium is either of the two (right and left) upper chambers of the four-chambered heart. The left atrium receives oxygenated blood from the pulmonary veins; the right atrium receives deoxygenated blood from the venae cavae. The atria are comparatively thin-walled since they only pump blood down into the muscular ventricles. The term is also used for various other chambers in animals.