Question

why there is complete separation of oxygenated and deoxygenated blood

Answer 1

It is necessary because mammals and birds are warm blooded. Mixing of theses two will decrease the efficency of blood to carry oxygen and our organs won't be able to work properly. Also ,mammals and birds need to maintain their body temperature (warm blooded) . Mixing of deoxygenated and oxygenated blood will increase their body temperature as the level of carbon dioxide will rise in blood. Mammals and birds have high energy requirements. Mixing of deoxygenated and oxygenated blood will decrease their stamina.

Answer 2

simple thing is that we take o2 for respiration otherwise we died so for flowing of o2 in body. the separation is held..