well the respiratory system brings oxygen to every part of the body, and it takes carbon dioxide (CO2) from the cells and delivers oxygen, (O2) to them. as it does this it also takes away all the wastes from the cells and then brings them to the kidneys to get filtered. the kidneys are where your liquid waste gets filtered out of your blood. Then they go to the heart, which pumps it to the lungs, and they will exchange the carbon dioxide for oxygen at the lungs. this exchange happens at the aveoli, which are little nodules that are covered on the surface with small blood vessels called capileries. these blood vessels are so small they can only fit a single blood vessel at a time. then it all happens again.
P.S i left a lot of stuff out because i either think that its more advanced than what you're asking, or its not really relavent to your question.
hope i helped.