Why is the human circulatory system a double system?

The human circulatory system consists of the heart - a pump and a series of tubes - the blood vessels containing the blood. There is an atrium and a ventricle on each side of the heart and the two sides can be considered separately. The right side of the heart receives blood from the tissues and pumps blood to the lungs, while the left side receives from the lungs and pumps blood to the tissues. These happen together each time the heart contracts. In this way, blood passes through the heart twice before returning to the tissues.