The circulatory system is a complex group of structures that includes the heart, lungs, and blood vessels. Its function is to carry nutrient-and oxygen-rich blood to every part of your body. At any given time, about five liters of blood are traveling through the circulatory system, propelled forward by the contractions of the heart. As the heart beats, it pushes blood through the aorta (ay-OAR-tuh) into the arteries, then into tiny vessels called capillaries. The blood then gathers up waste products such, as carbon dioxide, and travels into the veins on its way back to the heart. Muscles around your arm and leg veins contract to help push the blood along. As blood returns to the heart, it enters the upper right chamber called the right atrium. Blood is then pumped to the lower chamber, the ventricle, and from there it’s pushed to the lungs. The lungs filter out the carbon dioxide from the blood and re-introduce oxygen. This oxygen-rich blood returns to the heart and enters the upper left chamber, or left atrium, moves down into the left ventricle, and the cycle begins again. Assisting in the movement of blood are numerous valves which, when functioning properly, act like miniature swinging doors to keep blood from flowing backward.