(1) 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 left atrium receives oxygenated blood from the lungs and pumps it down into the left ventricle which delivers it to the body.
Common Misspellings: atrum, autrium
(2) The top filling chamber of the heart. There are two atria - the left and the right, divided by a muscular wall, called the septum. The atrium contracts before the ventricle to allow optimal filling of the ventricle.