Blood flows to the heart in vessels known as veins. They contain valves which prevent the backwards flow of blood. Body movements help keep blood circulating in veins also. When the upper and lower vena cavas return the blood to the heart, it enters the right atrium. This chamber receives the deoxygenated blood and sends it down to the right ventricle. This is the pumping chamber which pumps the blood out to the lungs through the pulmonary arteries. In the lungs, the blood loses its carbon dioxide and gains oxygen becoming oxygenated blood. This blood returns via the pulmonary veins to the left atrium of the heart. The left atrium sends blood to the left ventricle. This contains more muscle than the right ventricle and pumps the blood out of the heart to the aorta. This is the biggest artery in the body, which has branches to all the organs to supply them with oxygenated blood. The direction of blood flow in the arteries is away from the heart. In between arteries and the veins are tiny blood vessels called capillaries. It is through the walls of the capillaries that diffusion occurs and an exchange of oxygen and carbon dioxide can occur--oxygen will enter the cells from the circulating blood and carbon dioxide will leave the cells to be transported by the blood to the lungs. Also, nutrients can enter the cells and wastes can leave the cells to be carried by the blood to the kidneys to be filtered out and excreted.