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minor medical illnesses, known collectively as acute mountain sickness (AMS) or major life threatening illnesses (i.e., HAPE or HACE) (see Cymerman, Chapter 16 in this volume).

The challenging environmental conditions at high altitudes include hypobaric hypoxia, dry air, and extreme variations in external temperature. Each of these conditions elicits physiological responses that appear to allow the individual to remain conscious and functional during the first hours and days at high altitude (Cymerman and Rock, 1994).

The governing biophysical factor at high altitudes is the decreasing barometric pressure that characterizes increasing altitude (Cymerman and Rock, 1994). Although the atmospheric concentration of oxygen remains at a constant 20.93 percent at all terrestrial altitudes, the partial pressure of oxygen (pO2 = 0.2093 × barometric pressure) falls along with the decline in barometric pressure. As altitude increases, the lowered oxygen pressure (pO2) in pulmonary alveolae causes a declining saturation of hemoglobin in arterial blood, and a lower oxygen pressure gradient throughout the body, especially at the level of the capillaries, where the pO2 may be close to zero. With low pO2 the blood flow is too rapid to allow appropriate gaseous exchange, resulting in unfavorable conditions for oxyhemoglobin dissociation. Exercise of any kind becomes difficult. Respiratory rate and heart rate increase in response to chemoreceptor activity with resultant modest improvement in oxygen delivery.

The dry air found in high environments adds to the problem of oxygen delivery at altitude. Because air must be moistened to protect the respiratory epithelium, water is added to the air inspired at each breath. As a result, alveolar pO2 is further reduced. This process adds about 47 mm Hg of water vapor pressure to the alveolar gasses. Further exacerbating the alveolar ''crowding" is expired CO2, which contributes 40 mm Hg with normal breathing and increases transiently at altitude. With the higher respiratory rates characteristic of high altitudes, the concentration (and thus partial pressure) of alveolar CO2 declines somewhat with time, allowing alveolar pO2 to increase somewhat with length of exposure (Milledge, 1992; Buskirk and Mendez, 1967). The increased rate of respiration increases fluid loss through the lungs, creating the potential for dehydration if fluid intake is not maintained.

The oxygen dissociation curve for hemoglobin is another important physiological factor at high altitudes. The curve "breaks" at about 14,110 ft (4,300 m), at which point hemoglobin saturation is already decreased to 85 percent of that seen at sea level. At even higher altitudes, small declines in the pO2 of alveolar air result in large declines in arterial blood and hemoglobin saturation, making even the act of breathing hard work.

As altitude increases, ambient temperature decreases at a rate of approximately 3.6°F/984 ft (2°C/300 m) rise in elevation. However, work at high altitudes has the potential for creating extremes of temperature for the individual, with sweating one minute as a result of strenuous work in heavy

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