Double circulation describes the fact that our oxygenated and deoxygenated blood is kept separate in our hearts. In fish (two-chambered heart) and amphibians (three-chambered heart), the oxygenated and deoxygenated blood is allowed to mix in the ventricles, in mammals and birds (four-chambered heart), the oxygenated and deoxygenated blood is kept separate. Fish and amphibians are cold-blooded, meaning that they conform to the temperature of their environment. Mammals and birds are warm blooded. They maintain a constant internal temperature, despite the environment. Maintaining a constant internal temperature allows mammals and birds to carry-out body processes more efficiently, but it requires more energy. Without double circulation, the body tissues would not receive enough oxygen to maintain a constant temperature, and would die.

In response to the above answer, some amphibians have lungs. Having lungs does not mean an organism has a double circulatory system

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