It's no secret that scaling Mount Everest tests the limits of human survival; more than 200 people have died trying to reach its summit. Today we have new information about just how seriously climbers push their bodies on the world's highest peak: Those who manage to stay alive do so on an amount of oxygen that is so minute it would only be seen, at sea level, in people who were in cardiac arrest or dead.

Four doctors from University College London (U.C.L.) trekked up to Everest's 29,029-foot (8,848-meter) summit.  They then descended to 27,559 feet (8,400 meters), where it was warmer and more sheltered from the high winds. There, they drew one another's blood and handed it off to a sherpa named Pafang, who took it down to a blood-gas analyzing machine at 20,997 feet (6,400 meters) to measure the oxygen levels in it.
 
The results, published in this week’s The New England Journal of Medicine, showed there was about a quarter less oxygen in the mountaineers' blood than is normal in people at sea level. There's about a third less oxygen on Everest's summit than on the ground.

"The oxygen levels were extremely low—lower than we expected and lower than have been measured in humans before," says study co-author Mike Grocott, a senior lecturer in critical care medicine at U.C.L. "We don’t know how people can survive at these low levels, but we believe there are cellular and molecular changes that allow people to metabolize low oxygen. It emphasizes that we're on the very edge of the physiological envelope."

As climbers acclimatize, or adjust to the thin mountain air, they may "turn down" their cells' housekeeping activities to preserve oxygen, Grocott says. They may also improve the efficiency of proteins that regulate how much oxygen is used to produce adenosine triphosphate (ATP), the body's energy currency.

The findings from U.C.L.'s Caudwell Xtreme Everest program, which studies Everest climbers to figure out how to better treat oxygen-deprived heart and lung patients on the ground, are just the latest in a growing body of medical research on Mount Everest. And while they're not entirely surprising—pulmonary edema, or fluid in the lungs that can prevent air sacs from absorbing oxygen, is a factor in most high-altitude deaths—the results shed light on the specifics of what's happening in the climbers' bodies, says R. Douglas Fields, chief of nervous system development and plasticity at the National Institutes of Health (NIH). Less than 4 percent of climbers are able to scale Everest without supplemental oxygen.

"It’s not easy putting up with arterial sticks while climbing," quips Fields. "The climbers' bod[ies] showed remarkable acclimatization to be able to function at such low oxygen levels.” The amount of hemoglobin (an oxygen-carrying protein in the blood) increased, he says, “so, together with many other biological changes, their blood was able to capture more of the limited oxygen in the air. This is already very well known, but the study shows it nicely.

"If you go slow enough, your body will acclimatize, but you can't stay there long—no civilization has ever lived above 17,000 feet," Fields adds. "When you go up there it's borrowed time—you have to go back down. "

Image of Mike Grocott, courtesy of Caudwell Xtreme Everest