Human responses to extreme altitudes

It is a strange coincidence that the highest point on Earthis very close to the limit of human tolerance to hypoxia. Thephysiological changes that allow humans to reach these extremealtitudes involve enormous alterations of their normal state.It is useful to contrast this response with two others to highaltitude. One is acclimatization that allows lowlanders to ascendto altitudes of up to 5000 m and remain there for an indefiniteperiod. The other is evolutionary adaptation which allows highlandersto live continuously over generations at altitudes up to 5000m. These two responses enable humans to survive for an indefiniteperiod at high altitude. By contrast, the changes that allowascent to extreme altitudes are not compatible with an extendedstay because of a poorly-understood process called high-altitudedeterioration. The most important physiological response toextreme altitude is extreme hyperventilation which, on the summitof Mt. Everest, drives the alveolar P_CO2 down to 7–8 mmHg.This is associated with a marked respiratory alkalosis withan arterial pH exceeding 7.7. Interestingly this alkalosis increasesthe oxygen affinity of hemoglobin, a response which the successfulclimber shares with many other animals in oxygen-deprived environments.The arterial P_O2 on the Everest summit is only about 30 mmHgand falls on exercise because of diffusion limitation of oxygenacross the blood-gas barrier. Maximal oxygen consumption onthe summit is just over 1 liter.min^–1. Anaerobic metabolismas measured by blood lactate levels is paradoxically reducedat extreme altitudes.