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

if the amt of CO2 increases in the blood there will be several probs like toxication etc.

some more detailed

please  ?

even i want the details

The partial pressure of O2 is very important for its diffusion from blood into tissues and from the lungs' air spaces into blood. Gases diffuse down their partial pressure gradient. Oxygenated blood has a high partial pressure of O2 and low partial pressure of CO2. Tissues have a low partial pressure of O2 and high partial pressure of CO2, so when blood flowes through capillaries of organs, O2 flows into the tissue and CO2 moves into the blood. Partial pressure of O2 in deoxygenated blood is low and CO2 is high. Partial pressure of O2 in the lungs is very high and CO2 is low. so when blood flows through the lung capillaries, O2 enters blood and CO2 leaves blood. 

If oxygenated and deoxygenated blood were allowed to mix in our body these pressure gradients would be much smaller, and the movement of O2 from lungs into blood and into tissues and CO2 from tissues into blood and into lungs would be much less effecient. The migh metabolic (energy) demarnds of the human nervous system (brain) require that O2 be readily accessable as it is in our circulatory system. Some lower life forms with less complex nervous systems (frogs) have a circulatory system which allows for mixing of blood. 

In humans when deoxy and oxy blood are allowed to mix, you see cyanosis (bluish hue to the skin) that indicates decreased oxygenavailability to tissues. This occurs in many congenital (birth) defects in the heart such as Tetralogy (aorta too big, pulmonary artery too small, hole between the 2 ventricles).

skin

Mammals and Aves are Warm Blooded. Since the Warm blooded animals have highenergyrequirement to maintain their body temperature, the deoxygenated blood must get oxygenated to produce ATP. If the oxygenated and deoxygenated blood gets mixed, the efficiency of producing more energy, i.e. ATP, will get reduced.

Four chambered heart in birds and mammals exhibit double circulation of blood. Double circulation system prevents mixing of oxygenated and deoxygenated blood with each other.

Birds and mammals are worm blooded. This worm body temperature helps to carry out body processes more efficiently. Maintaining the constant temperature inside the body requires huge amount of energy. Without double circulation system human would not be able to receive enough oxygen to produce adequate energy.