The separation of right and left sides of heart is useful to prevent oxygenated and deoxygenated blood from mixing. Such a supply is useful for animals which have to use energy constantly to maintain their body temperature like birds and mammals. The animals which do not use energy to maintain their body temperature and their body temperature varies with environmental temperature like, amphibians have three-chambered heart and tolerate some mixings of oxygenated and deoxygenated blood. Fishes have only two chambered heart. The deoxygenated blood is pumped to the gills, where it is oxygenated and directly passed to the body parts. Thus, in fishes blood is passed only once to the heart in one circulation. In human beings deoxygenated blood is passed to heart, then it is oxygenated in lungs and carried back again to heart. From heart oxygenated blood is passed to all body parts. Thus, blood flows twice in heart during each cycle. This is known as double circulation.