The physiology of scaling Mt. Everest

The record for fastest ascent of Mount Everest is currently held by Sherpa Pemba Dorjie, who broke the world record for quickest Everest ascent on May 21, 2004, climbing in only 8 hours, 10 minutes. One expects that those living next to a river would be good swimmers and that those living next to a mountain would be skilled at mountain climbing. Nonetheless, researchers have sought to determine the physiological basis of this rock-climbing prowess. In an article published in Nature in 1965, Lahiri and Milledge examined the biology underlying this incredible feat.

To study the effects of acclimatization to the high-altitude environment, Lahiri and Milledge designed an elegant experiment where they compared the physiology of Sherpa high-altitude residents to that of lowlanders acclimatized to the same altitude.

The authors looked first at respiratory regulation. They measured the ventilatory response to carbon dioxide and hypoxia, examined at rest with respect to alveolar carbon dioxide pressure and oxygen pressure. Interestingly, high-altitude residents breathed less than recently acclimatized lowlanders both at rest and at different grades of exercise. However, high-altitude Sherpa residents breathed more than average subjects at sea-level.

The Sherpas also exhibited markedly lower sensitivity to hypoxia. Amazingly, the effect on Sherpas of reducing alveolar oxygen pressure from 200 to 40 mm Hg at constant CO2 pressure was to increase the ventilation by only 10% as compared to the increase observed in lowlanders.

The authors investigated blood pH as well. Sherpa subjects were able to maintain higher arterial O2 pressure at a given oxygen saturation, because of the Bohr effect on the oxygen dissociation curve. In the presence of carbon dioxide, the affinity of oxygen for respiratory pigments such as hemoglobin decreases. This is called the Bohr effect; the process means that an increase in blood carbon dioxide level or a decrease in pH causes hemoglobin to bind to oxygen with less affinity.

This study also examined muscular work, heart rate, maximum oxygen consumption and recovery, acid-base balance, arterial oxygen saturation, urinary steroids and Na+/K+ ratios, electrocardiograms, blood-hemoglobin values, and alveolar and mixed venous gas pressures.

Overall, the results showed that people residing at high altitude are less sensitive to the hypoxic stimulation of altitude, in comparison to well-acclimatized lowland counterparts. Sherpa subjects used a different combination of cardiovascular and respiratory functions than the acclimatized lowlanders to achieve the same physiological end: oxygen availability to the cell. The acclimatization undergone by anyone who wishes to scale Everest comprises short-term adjustments that allow effective performance at high-altitude conditions of low oxygen tension. In contrast, long-term changes in the bodies of those who live at high altitude include the reversal of some short-term changes once the initial crisis has been overcome. Further research is required to determine whether these differences in physiology are genetic or environmental in origin. But if the nature vs. nurture debate is any indication, the issue may continue to evade clear consensus. 

For more on the effects of low oxygen partial pressure, see: http://www.nature.com/scitable/topicpage/Environmental-Cues-Like-Hypoxia-Can-Trigger-Gene-41466