Definition: The human circulatory system has two separate circuits through which blood moves around the body - it is a dual circulatory system. The first system contains blood that is high in oxygen and low in carbon dioxide. Blood high in oxygen and low in carbon dioxide will start its journey in the lungs, where red blood cells absorb oxygen from the air that we breathe in and carry it off into the bloodstream to the heart. The heart then pumps this blood around the body via the aorta delivering oxygen to the body's cells and tissue. These cells need oxygen for respiration which allows them to create energy. The other part of the circulatory system contains blood that is low in oxygen but high in carbon dioxide. The process of respiration depletes the amount of oxygen in the blood and produces carbon dioxide, which the body has no need for, so the blood cells carry the carbon dioxide molecules, and the heart pumps it back to the lungs, where it is released when we breathe out.