ABSTRACT
The essence of high altitude is hypoxia, reducing arterial Po2. This chapter deals mainly with how pulmonary gas exchange is modified by altitude. Both rest and exercise are considered, and O2 transport is analyzed in terms of (1) effects of reduced inspired Po2, (2) diffusion limitation of O2 exchange, and (3) ventilation/ perfusion relationships. Acute altitude exposure and prolonged, acclimatized situations are described separately. Hypoxia is of course the result of barometric pressure (Pb) decreasing with altitude. While air at any altitude remains approximately 21% O2, the virtually exponential fall in Pb with altitude reduces ambient Po2. When it is further remembered that as soon as air is inhaled into the large airways it becomes warmed to body temperature (37°C usually) and saturated with water vapor [contributing a partial pressure of 47 mmHg (at 37°C) at all altitudes], it is clear that inspired Po2 in the airways (Pio2) is rapidly reduced by ascent to altitude. Table 1 indicates pertinent values derived from the U.S. Standard atmosphere relating Po2 to altitude.
