The upper heart compartments are called atria (vestibules).

They are separated from the ventricles by unidirectional valves.

The right atrium receives blood from the large vein system (Superior and Inferior Vena Cava) After having been collected from all body organs, this blood has low oxygen content. From the atrium it is transported through the Tricuspid valve to the right ventricle and from there, during the ventricular systole, to the Pulmonary artery for oxygenation.

The left ventricle receives oxygenated blood from the Pulmonary veins and it voids through the Mitral valve to the left ventricle and from there, during systole, to the Aorta.

The natural cardiac pacemaker, the Sinus, is located in the right atrium and from there electrical impulses are propagated to different sites in the heart.

Various conditions, such as valvular diseases, pulmonary diseases and more, can affect the atrial status.