Question

why double circulation is necessary for human beings

Answer 1

Hey!!

Double circulation is necessary in humans to oxygenate the deoxygenated blood and hence purify it.
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Hope it helps!

Answer 2

double circulation prevents mixing up of blood oxygenated and deoxgenated because if impure blood mix with the pure blood then there will be problem in blood circulation and the life will be ruin as it happen when some nerves got damage