Why is it necessary to separate oxygenated and deoxygenated blood in mammals and birds
Mammals including Human and birds are a homothermal organism. That means they keep the temperature of the body constant.
This process takes a lot of energy. As a result, such organism need a continuous exchange of gases in large amount to meet the energy requirement.
Mixing of oxygenated blood with deoxygenated blood lowers the efficiency of gaseous exchange. So it becomes necessary for mammals and birds to have separate oxygenated and deoxygenated blood.
In human blood first oxygenates in the lung and then heart pump again to different organs. This we call double circulation as heart receive blood twice.