Atrium

Blood returning from the body has given up much of its oxygen. It enters the right atrium through the vena cava, moves into the right ventricle, and is pumped through the pulmonary artery to the lungs. There, it gains oxygen and loses carbon dioxide.

The vena cava brings deoxygenated blood from the body into the right atrium. It passes through a valve into the right ventricle, which pumps it through the pulmonary artery to the lungs. Now oxygenated, it returns through the pulmonary vein to the left atrium.

Deoxygenated blood from the body enters the right atrium through the vena cava.

There are four chambers: the right atrium and left atrium above, and the right ventricle and left ventricle below. The atria receive blood returning to the heart and pass it into the ventricles. The ventricles have the main job of pumping blood out.