Effects of Low Oxygen Pressure on the Body

Barometric Pressures at Different Altitudes. Table 43-1 gives the approximate barometric and oxygen pressures at different altitudes, showing that at sea level, the barometric pressure is 760 mm Hg; at 10,000 feet, only 523 mm Hg; and at 50,000 feet, 87 mm Hg. This decrease in barometric pressure is the basic cause of all the hypoxia problems in high-altitude physiology because, as the barometric pressure decreases, the atmospheric oxygen partial pressure decreases proportionately, remaining at all times slightly less than 21 per cent of the total barometric pressure—Po2 at sea level about 159 mm Hg, but at 50,000 feet only 18 mm Hg.

Alveolar PO2 at Different Elevations

Carbon Dioxide and Water Vapor Decrease the Alveolar Oxygen. Even at high altitudes, carbon dioxide is continually excreted from the pulmonary blood into the alveoli. Also, water vaporizes into the inspired air from the respiratory surfaces. These two gases dilute the oxygen in the alveoli, thus reducing the oxygen concentration. Water vapor pressure in the alveoli remains 47 mm Hg as long as the body temperature is normal, regardless of altitude.

In the case of carbon dioxide, during exposure to very high altitudes, the alveolar Pco2 falls from the sea-level value of 40 mm Hg to lower values. In the acclimatized person, who increases his or her ventilation about fivefold, the Pco2 falls to about 7 mm Hg because of increased respiration.

Now let us see how the pressures of these two gases affect the alveolar oxygen. For instance, assume that the barometric pressure falls from the normal sea-level value of 760 mm Hg to 253 mm Hg, which is the usual measured value at the top of 29,028-foot Mount Everest. Forty-seven millimeters of mercury of this must be water vapor, leaving only 206 mm Hg for all the other gases. In the acclimatized person, 7 mm of the 206 mm Hg must be carbon dioxide, leaving only 199 mm Hg. If there were no use of oxygen by the body, one fifth of this 199 mm Hg would be oxygen and four fifths would be nitrogen; that is, the Po2 in the alveoli would be 40 mm Hg. However, some of this remaining alveolar oxygen is continually being absorbed into the blood, leaving about 35 mm Hg oxygen pressure in the alveoli. At the summit of Mount Everest, only the best of acclimatized people can barely survive when breathing air. But the effect is very different when the person is breathing pure oxygen, as we see in the following discussions.

Table 43-1

Effects of Acute Exposure to Low Atmospheric Pressures on Alveolar Gas Concentrations and Arterial Oxygen Saturation*

Barometric PO2 in Pressure Air

Breathing Air

Arterial

Pco2 in Po2 in Oxygen

Alveoli Alveoli Saturation

Breathing Pure Oxygen

Arterial

Pco2 in Po2 in Oxygen

Alveoli Alveoli Saturation

Barometric PO2 in Pressure Air

Arterial

Pco2 in Po2 in Oxygen

Alveoli Alveoli Saturation

Arterial

Pco2 in Po2 in Oxygen

Alveoli Alveoli Saturation

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