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

Answer: 

• The birds and mammals are warm blooded animals who constantly maintain their body temperature according to the situation, maintain it colder when cold climate and keep it warmer when they are in hot climate. 
• Therefore, these species require more oxygen for cellular respiration to bring out more energy in order to maintain their body temperature at different climatic conditions. 
• The separation of oxygenated and deoxygenated blood in mammals and birds is necessary to produce more energy in order to maintain their body temperature at different climatic Conditions.