Transport of oxygen in points

• Oxygen enters the blood in the lungs:

It is necessary to separate the oxygenated blood from mixing with deoxygenated blood as mammals and birds have high energy needs because they constantly require energy to maintain their body temperature.
Amphibians and many reptiles have three chambered heart. They can tolerate some mixing of the oxygenated and de-oxygenated blood streams. The fish has a two chambered heart. Oxygenation of the blood takes place in the gills. The oxygenated blood from the gills is supplied to the body parts of the fish where oxygen is utilized and carbon dioxide enters into it making it deoxygenated. The deoxygenated blood returns to the heart to be pumped into gills again.

Thus, single circulation is a type of circulatory system in which the blood passes through the heart only once in one complete cycle of the body.
Double circulation is a circulatory system in which the blood travels twice through the heart in one complete cycle of the body.