In the single circulation the deoxygenated blood from the whole body of the animal spammed into the heart. From the heart it goes to the gills and the oxygenated blood is pumped back into the body. This kind of circulation takes place in fishes. Whereas in double circulation the deoxygenated blood is collected from the body, goes to the lungs for the process of oxygenation and then this oxygenated blood comes back to the heart. The heart pumps back this oxygenated blood into the body tissues. In this type of circulation, as the blood passes through the heart two times, it is known as double circulation. It takes place in birds & mammals like human beings.