How do spacesuits keep astronauts alive?

In the vacuum of space, an unprotected body would quickly lose consciousness because there is no air to breathe and no pressure to keep bodily fluids behaving normally. NASA's spacewalk suits are pressurised with pure oxygen at about 4.3 pounds per square inch, roughly 30% of sea-level pressure, which keeps them flexible enough to move in. Because of this lower pressure, astronauts breathe pure oxygen beforehand to remove nitrogen from their blood and avoid decompression sickness. Temperatures in Earth orbit can swing from well above 100°C in sunlight to below −100°C in shadow, so the suit has many insulating layers and an undergarment laced with tubes of cooling water. A backpack life-support system provides oxygen, removes carbon dioxide and powers the suit for spacewalks lasting up to about eight hours.