For most mountaineers, some level of altitude sickness is inevitable. But Indigenous highlanders living on the Tibetan Plateau, known as Sherpas, have inhabited the high Himalaya long enough to have an evolutionary edge at tolerating elevation compared with lowlanders born and raised farther down. For a study in the Proceedings of the National Academy of Sciences USA, researchers compared Sherpa and lowlander blood samples during a Himalayan trek to investigate the Sherpas’ aptitude for altitude—and they found a crucial clue in the kidney.

The thinner atmosphere up high can lead to hypoxia, a dangerous lack of oxygen. This condition, which often occurs during medical events such as heart failure, can also cause acute altitude sickness; mountaineers can become nauseated, dizzy and disoriented, in severe cases developing deadly fluid buildup in the lungs and brain. Studying the physical responses of altitude-adapted people reveals how their bodies keep them healthy during hypoxia.

Hypoxic people breathe faster to bring more oxygen into their lungs. But extra breathing also empties the lungs of more carbon dioxide than usual, which in turn reduces the production of carbonic acid in the blood. And even tiny changes in acidity risk damaging the proteins and enzymes that keep our cells functioning. Once blood acidity shifts, “the only thing that can fix it is the kidneys,”