Down on the surface of Earth and on up to about 60-75 miles altitude where Space begins, our planet's atmosphere is 20 percent oxygen and 80 percent nitrogen. On the way up, at around four miles altitude, the atmosphere is half as dense as it is on the ground. At eight miles above the surface of Earth, the air is so thin and the amount of oxygen so small you must wear a pressurized oxygen mask. Above twelve miles altitude, the total air pressure is no longer sufficient to keep body fluids from boiling. So, humans must wear spacesuits that not only supply oxygen for breathing, but also maintain a pressure around the body to keep body fluids liquid.
Spacesuits provide atmospheric pressure.
International Space Station spacesuits are to be pressurized to 8.3 psi. The gas in a spacesuit is 100 percent oxygen, so the wearer has more oxygen to breathe than he or she would at sea level without the spacesuit. Before going outside a shuttle to work in space, an astronaut has to spend hours breathing pure oxygen. Space station spacesuits will be pressurized to 8.3 psi to shorten the amount of time spent in pre-spacewalk breathing of pure oxygen. If you skip to NASA's Spacesuits Page you will learn more about spacesuits worn by shuttle astronauts.