For one thing, they're well trained. The burly Sherpas on most high-mountain expeditions have spent lots of time carrying heavy loads at high altitude--as much as 80 pounds at altitudes of 18,000 feet. And growing up at altitude has given their bodies ample time to adapt to the hypoxic environment.
But their strength comes from more than just practice. "They have high resting ventilation rates compared to lowlanders, and that's one way they offset the lower oxygen concentrations in the blood," says anthropologist Cynthia Beall of Case Western Reserve University. That finding doesn't mean that Sherpas sit around trying to breathe at a faster rate. Ventilation rates are controlled subconsciously, and Beall cautions against trying to artificially increase your ventilation rate--such a strategy will just lead to hyperventilation. Beall's team has also found evidence of a gene that allows Tibetans to maintain higher oxygen levels in their blood at high altitudes.