The birds and mammals have evolutionary advancement as far as structure of heart is concerned. They need more oxygen to live in terrestrial habitat.
In these animals, the blood received by left and right auricles is oxygenated and deoxygenated respectively. it passes towards the left and right ventricies and thus no oxygenated and deoxygenated blood is mixed. The ventricles pump oxygenated deoxygenated blood without mixing.
Thus. two separate circulatory pathways are found thus this type of blood circulation is called as double circulation, which include systemic and pulmonary circulation.
