What is the function of a rat's circulatory system?

The general idea in having a circulatory system is as a transportation method to get needed materials to living cells, and then get waste material out. That's what a rat's blood and circulatory system do. The heart pumps, the vessels do the delivery work. We then attach lungs to do gas exchange in the blood (O2 in and CO2 out), a gastrointestinal tract to process food and get nutrients into the blood (dumping the waste out directly), and kidneys and a liver to get waste material out of the blood. That's it in a nutshell.