It is a type of blood circulation system in which the blood flows through the heart twice. In this type of circulation, the pulmonary circulation is separate from the systemic circulation.
In this circulation, the heart consists of: the right side that pumps deoxygenated blood to the lungs and back to the left side of the heart. The oxygenated blood is then pumped to the body and back via a network of blood vessels. Thus the blood travels through the heart twice on each complete journey around the body. There is no mixing of the oxygenated and deoxygenated blood. A double circulation system maintains the high blood pressure needed for efficient transport of materials around the body.
This circulation is common among mammals and birds.