define double circulation system in human beings?

It is a circulation in which blood flows through the heart twice for each cycle of passage through the body. It consists of a systemic circulation and a pulmonary circulation.

 

(In man, blood passes through the heart twice; from the main circulation of the body to the heart, then to the lungs and back to the heart again, before the blood is pumped into the main circulation. Hence it is known as double circulation.)