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.
• atrial septal