Question

why we should separate oxygenated and deoxygenated blood

Answer 1

to maintain our body temperature and to get energy we need to separate blood

Answer 2

we should seprate oxygenated and deoxygenated blood because deoxygenated blood contains carbon dioxide which is exhaled by our body and oxygenated blood contain oxygen which must be inhaled by our body and also the the oxygenated blood is flown through all the body in each and every part by helping each cell for proper working.
deoxygenated blood contains co2 and also heat from our body