Why does double circulation only happens in mammals?
Birds and mammals need a lot more oxygen than animals with a single circulatory system e.g. fish because they have to move without water and maintain a constant body temperature. A double circulatory system sends blood at high pressure to carry lots of oxygen to active cells of the body, it also gets rid of waste very quickly too. The 2 circulation systems are important as they mean that oxygenated and deoxygenated blood do not mix so that tissues are as oxygenated as possible. Because the oxygenated blood is pumped hard and sent around the body at high pressure it reaches all the tiny capillaries between body cells to provide oxygen for an active way of life.
