The heart is the central organ of the circulatory system. We distinguish the pulmonary circulation and the systemic circulation. Oxygen deprived blood from the vena cava enters the right atrium of the heart and flows through the tricuspid valve (right atrioventricular valve) into the right ventricle, from which it is then pumped through the pulmonary semilunar valve into the pulmonary artery to the lungs. Gas exchange occurs in the lungs, whereby CO2 is released from the blood, and oxygen is absorbed. The pulmonary vein returns the now oxygen-rich blood to the heart. From here the oxygenated blood is pumped away from the heart, to the rest of the body, and returns oxygen-depleted blood back to the heart.