Veins return blood from organs to the heart. In the circulatory system, oxygenated blood is pumped by the left ventricle through the arteries to organs and muscles. Nutrients and gases are exchanged at capillaries, entering the veins filled with cellular waste and carbon dioxide. The de-oxygenated blood is taken by veins to the right atrium of the heart, which transfers the blood to the right ventricle, where it is then pumped to the pulmonary arteries and eventually the lungs. In pulmonary circulation, the pulmonary veins (there are four pulmonary veins which extend from the left atrium to the lungs) return oxygenated blood from the lungs to the left atrium, which empties into the left ventricle, completing the circulation cycle.