The circulatory system in birds and mammals consists of oygenated and deoxydenated blood. However, they have a double circulation system where the oxygenated as well as deoxygenated blood remain separate. This is essential to maintain the body temperature efficiently over a wide range. As mammals and birds require to keep their body warm, they require a higher supply of oxygen. Thus, it is beneficial if the oxygenated blood remains separate as its mixing with deoxygenated blood can make the entire blood impure.