A double circulatory system refers to a system in which blood pumps through the heart twice during each trip around the body. The blood is first pumped into the lungs, where it becomes oxygenated, and is then pumped back into the heart, before it is finally pumped into the rest of the body. While most types of vertebrates have double circulatory systems, some have the more primitive single circulatory system.
In addition to its many other purposes, the blood of an animal must carry oxygen from the lungs to its body, and then transport the carbon dioxide from its body to its lungs, where it can be expelled with breath. Blood does this with the help of a molecule called hemoglobin. As evolution produced animals that were capable of high caliber performance, the double circulatory system evolved to move oxygen and carbon dioxide through bodies more efficiently.
Mammals, birds and reptiles all have double circulatory systems. However, most reptiles and amphibians have only a three-chambered heart, as opposed to the four-chambered heart of mammals. Fish have single circulatory systems and only two chambers in their hearts. Invertebrates usually have open circulatory systems, where their blood-like fluid called hemolymph flows freely.