Double circulation refers to the presence of two separate circulatory pathways in birds and mammals. In this system, oxygenated and deoxygenated blood are received by the left and right atria respectively and pass on to the ventricles of the same sides. The ventricles pump out the blood without any mixing up.

The two pathways are: pulmonary circulation (deoxygenated blood is pumped by the right ventricle to the lungs where it is oxygenated and returned to the left atrium) and systemic circulation (oxygenated blood is pumped by the left ventricle to the aorta and carried to all body tissues, while deoxygenated blood is returned to the right atrium).

The significance of double circulation is that it ensures complete separation of oxygenated and deoxygenated blood without any mixing. This is more efficient than single circulation in fishes or incomplete double circulation in amphibians and reptiles where blood gets mixed up in the single ventricle.