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. We have the systemic circulation and we have the pulmonary circulation. Systemic circulation simply means we carry the oxygenated blood from the heart into the tissues and then we take the de oxygenated blood from the tissues and bring it back to our heart. But pulmonary circulation means we take that deoxian blood from the heart and we move it into the lungs and then we oxygenate our blood in the lungs and we bring the oxygen blood back into the heart and we'll discuss these two systems in more detail towards the end of the lecture.