Consider how you might feel if you drive your automobile to the top of Pikes Peak (14,109 ft above sea level, ASL) or if you ride a cable car to the top of the Zugspitze, the highest point in Germany (9718 ft ASL). If you have had the opportunity to visit either of these locations, you probably experienced the shortness of breath associated with breathing in environments with low oxygen pressure. (The percentage of oxygen does not vary much with the increase in altitude, but the partial pressure of oxygen diminishes.) Perhaps you even developed a headache after a short period. How you felt was dependent on how long it took to achieve the altitude, how long you remained, how well hydrated you may have been at the time, and a number of other potential factors.
Just as with normal respiration, at high altitude the driving force which helps to push oxygen into your blood is the partial pressure of oxygen. This partial pressure depends on both the barometric pressure and the relative percentage of air that consists of oxygen. Barometric pressure depends on the altitude above the earth's surface and varies approximately exponentially as shown in Fig. 3.13.
The equation for barometric pressure as a function of altitude depends on the density of the air at varying altitudes and therefore on air temperature. The equation for the standard atmosphere between sea level and 11 km above the earth's surface can be given by:
where Patm is the barometric pressure in millimeters of mercury, and 2 is the altitude above mean sea level in kilometers.
From Fig. 3.13, it is possible to see that on a standard day, the barometric pressure would decrease from 747 mmHg in Terre Haute, Indiana, to 625 mmHg in Denver, Colorado, to 360 mmHg at the summit of Mt. Kilimanjaro, and 235 mmHg at the summit of Mt. Everest. With the decrease in altitude and corresponding decrease in barometric pressure comes a decrease in the partial pressure of oxygen.
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