Physiological adaptation to high altitude involves rapid responses in respiration and slower responses in nervous, muscular, and cardiovascular system. An excellent criterion of adaptation is measurement of the capacity for supplying oxygen to tissues, oxygen consumption (Vo2 maximum). Such measurements reveal four stages of response. At 10,000 feet, for example, stage 1 is reached in minutes; Vo2 max declines 10%. In one to three days, stage 2, it declines another 10%; this is the stage of unpleasant subjective responses. In a few weeks, stage 3, performance approaches the level of stage 1. Red blood cell volume increases in stage 4, reaching its maximum after a year or more. Performance improves pari passu: Eventually sea-level performance can be achieved at 13,200 feet. The above course of events is idealized. Individuals vary widely in rate of adaptation; the amount of exercise plays a role, and age is a factor. Above
Dill DB. Physiological Adjustments to Altitude Changes. JAMA. 1968;205(11):747–753. doi:10.1001/jama.1968.03140370049010
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