When a person from a region near sea level moves to a significantly higher elevation, several adjustments in respiratory function must be made to compensate for the decreased Po2 at the higher altitude. These adjustments include changes in ventilation, in hemoglobin affinity for oxygen, and in total hemoglobin concentration.
Reference to table 16.12 indicates that at an altitude of 7,500 feet, for example, the PO2 of arterial blood is 69 to 74 mmHg (compared to 90 to 95 mmHg at sea level). This table also indicates that the percent oxyhemoglobin saturation at this altitude is between 92% and 93%, compared to about 96% at sea level. The amount of oxygen attached to hemoglobin, and thus the total oxygen content of blood, is therefore decreased. In addition, the rate at which oxygen can be delivered to the cells (by the plasma-derived tissue fluid) after it dissociates from oxyhe-moglobin is reduced at the higher altitude. This is because the maximum concentration of oxygen that can be dissolved in the plasma decreases in a linear fashion with the fall in PO2. People may thus experience rapid fatigue even at more moderate elevations (for example, 5,000 to 6,000 feet), at which the oxyhemoglobin saturation is only slightly decreased. Compensations made by the respiratory system gradually reduce the amount of fatigue caused by a given amount of exertion at high altitudes.
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