Double circulation has two components, pulmonary circulation and systemic circulation.- Pulmonary Circulation :
It is movement of blood from heart to the lungs and back. Deoxygenated blood of the body enters the righ auricle, passes into righ ventricle which pumps it into pulmonary arch, With the help of two separate pulmonary arteries the blood passes into the lungs. Systemic Circulation :
It is the circulation of blood between heart and different parts of the body except lungs. Oxygenated blood received by left auricle passes into left ventricle. The left ventricle pumps it into aorta for supply to different body parts including walls of the heart with the help of arteries.
Double circulatin is an improvement over single circulation as the heart pumps both the types of bloods (oxygenated and deoxygenated) forcefully through the body. In amphibians and reptiles, the double circulation is incomplete due to mixing of the blood. It is complete in birds and mammals.