An abnormal enlargement of a chamber or part of the heart which may be one of the two smaller chambers of the heart or both of them (atria).

The atria consist of open spaces with recessed walls. The right atrium (space) 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 and the left atrium receives oxygenated blood from the lungs and pumps it down into the left ventricle which delivers it to the body.