Explain what is meant by “double circulation” and why this is better than “single circulation”

Question

anonymous · Answer

In a double circulation system, the blood returns to the heart after being oxygenated by the lungs. This, in turn, allows the blood to be pumped again to reach the rest of the body. In a single circulation system, the pumping of the heart must provide enough force for the blood to travel through the lungs...and continue through the body, and get back to the heart. This only works well for fish - it wouldn't work so well for, say, humans. Check out this picture http://www.ideacenter.org/stuff/contentmgr/files/38ed8a0c6db7a4f1a37e405922c364bd/misc/hearts.jpg On the left, you have the single circulation (see how it's only one 'loop'?) Pretty inefficient, but works if you're a fish. Mammals, reptiles, and ampihbians have the double circulation system ...the blood from the lungs returns to the heart to be pumped out once more.