It's a closed circuit. And what that basically means is all that blood travels throughout the blood vessels and it never actually leaves those blood vessels. Now that's not to say that the blood isn't actually filtered. In fact our kidneys are responsible for filtering our blood but our blood is reabsorbed back into the body in our kidneys and only our waste products are essentially excreted to the outside portion of the body. So the cardiovascular system forms a closed circuit which means that the blood remains inside the circuit, inside the blood vessels the entire time. Now if we take this entire loop, entire closed circuit we can divide the closed circuit into two systems.