The suits provide astronauts with a breathing supply of pure oxygen rather than the mixture of 78% nitrogen and 21% oxygen that is found in Earth's atmosphere. Before putting on a suit, the astronaut must eliminate all of the nitrogen from his or her system. Otherwise, bubles of nitrogen gas may form within body fluids, causing a painful condition know as "the bends." In addition to providing oxygen, the spacesuit deals with the carbon dioxide that astronauts breathe out, which can be dangerous in large amounts. Canisters in the spacesuit remove the gas so it does not build up inside the suit and harm the astronaut