As a quick review, blood returns from the body and enters the right upper chamber of the heart, called the right atria. At this point, the blood is low in oxygen but high in carbon dioxide. It is then pumped from the atria through the right atrioventricular valve into the right ventricle . From this larger chamber, it is then forced on into the lung field through he pulmonary artery. This is the only artery in the body that carries non-oxygenated blood.
Carbon dioxide is a byproduct of body metabolism and is attached to the red blood cells.
In the lungs, carbon dioxide is replaced with oxygen. The reoxygenated blood then moves through the pulmonary vein back into the heart and enters the left atrium . This chamber pumps the blood through the mitral valve into the left ventricle, which is the largest, most heavily muscled chamber of the heart. While other chambers only move the blood a short distance, the left ventricle has the responsibility of forcing blood throughout the entire body. This completes the system, which allows blood to circulate throughout the body and then return to the heart.