Atrium: One of the two smaller chambers of the heart. Each atrium consists of an open space with recessed walls. The plural of atrium is atria.

The right atrium receives deoxygenated blood from the body through the vena cava and pumps it down into the right ventricle which then sends it to the lungs.

The left atrium receives oxygenated blood from the lungs and pumps it down into the left ventricle which delivers it to the body.

The atrium was an open area in the center of an ancient Roman home. The atria of the heart are also called the auricles (the Latin auricula means little ear) because they resemble the floppy ears of a dog.