Double circulation is the process by which the cardiovascular systems of many vertebrates such as mammals and birds circulate blood throughout their bodies. In this system, the heart pumps the blood twice to perform its function. The first pumping sends the blood to be circulated though the lungs, and the second pumping circulates the blood throughout the body.
Humans, like most other mammals, have a highly developed circulation system. In humans, the heart is divided into the right and left halves, which are actually two separate pumping and flow systems. The right side of the heart accepts blood that has been circulated throughout the body and pumps it at a relatively low pressure to the lungs. In the lungs, the blood is replenished with oxygen.
Once the blood has been completely circulated through the lungs, it returns to the heart. Arriving at the much larger left side of the heart, the blood is then pumped out to the entire body. As this side must push the blood though a greater amount of tissue, it squeezes much harder than the right, creating a greater amount of pressure. After the blood has been fully circulated throughout the body, it returns to the right side of the heart, where the process is repeated.
Though the blood is considered to be fully circulated upon its return to the heart, each blood cell does not actually visit each part of the body. In double circulation systems, there are many different paths called arteries that the circulating blood may take to reach different parts of the body. There are also many different paths called veins that the blood uses to return to the heart. The blood is delivered in the quantities needed by each portion of the body and then returned to the heart to be sent for reoxygenating in the lungs. Generally, the more blood needed by a specific part of the body, the more direct the path the blood takes to and from the heart.
Though most highly developed animals have double circulation systems, not all do. Fish, for example, have a single circulation system that lacks the double-sided hearts of mammals. In fish, the blood circulates throughout the body until it reaches the heart. Once it returns there, the blood is pumped again, passes through the fish’s gill system, and then circulates on through the body again.