Why is it necessary to separate oxygenated and deoxygenated blood in mammals and birds?

Birds and mammals are warm blooded animals. They maintain a constant body temperature by cooling and warming their bodies in warmer and cooler environment respectively. To maintain this constant temperature, they require large amount of oxygen for cellular respiration. So, it is necessary to separate oxygenated and deoxygenated blood because the circulatory system becomes more efficient and supplies more amount of oxygen needed to maintain this constant body temperature.