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

Double circulation is the process of blood flow through heart twice. Deoxygenated blood from body is returned to heart from where it is pumped to lungs. Lungs purify the blood and the oxygenated blood from the lungs return back to heart and is pumped to the body via aorta.

Double circulation is necessary because it separates the oxygenated and deoxygenated blood, makes the circulatory system more efficient, maintains constant body temperature, increased pressure of blood flow, quick supply of oxygen etc.