The blood from the circulation of all the body passes into the right atrium through the superior and inferior vena cava. The blood from the head and the arms goes in through the superior vena cava, whereas the inferior vena cava is in charge of receiving the blood coming from the inferior part of the body and the legs. This blood is deoxygenated because all of its oxygen and nutrients have been left in the body. It goes into the right atrium, then into the right ventricle and through the tricuspid valve, and all the way to the lungs through the pulmonary artery, until it passes through the pulmonary valve. This is the only artery that carries venous blood inside.

The pulmonary artery carries blood to the lungs in its left and right branches. Deoxygenated blood oxygenates again in the lungs (gas exchange; it exchanges carbon dioxide for oxygen, and thereby becomes enriched), then it returns to the left atrium through the four pulmonary veins (two on the left, and two on the right), recently oxygenated from the lungs. These pulmonary veins are the only veins in the body that carry blood with oxygen and nutrients. They go into the left ventricle, and then through the mitral valve, to then go to the aortic artery through the aortic valve. All the organs and tissues of the human body will be enriched by the aortic artery with this oxygenated blood.