Describe double circulation of blood in human beings. Why is it necessary?

Double circulation is a process during which blood passes twice through the heart during one complete cycle through the human body. First, the deoxygenated blood is send to lungs and oxygenated blood from lungs reaches heart. This circulation is known as pulmonary circulation. Oxygenated blood that reaches the heart is send to whole body and the deoxygenated blood from different body parts is again brought to heart. This is known as systemic circulation.

Human beings constantly use energy to maintain their body temperature. Therefore, they need a highly efficient supply of oxygen to the body. The mixing of oxygenated and deoxygenated blood lowers the efficiency to supply oxygen. Therefore, it is necessary to separate oxygenated and deoxygenated blood