Ascent to High Altitude Is Accompanied by a Decrease in Inspired Oxygen Tension and Consequently by HypoxemiarWhich Leads to an Increase in Ventilation
The ventilatory response to the hypoxia of altitude varies, depending on whether it lasts for less than an hour or for several days. The acute hypoxia experienced on first ascending to high altitude causes hyperventilation mediated through activation of the peripheral chemoreceptors.
However, hyperventilation decreases Paco2 and increases pH, which dampens the response to hypoxia. After several hours to days, ventilation increases further and remains somewhat elevated for hours to days after the hypoxic stimulus is removed.This short-term acclimatization involves three mechanisms. First, it can partly be explained by readjustments in blood pH back toward normal as the kidneys compensate by eliminating HCO3. A second possible mechanism is readjustment of CSF pH toward normal. Third, the Chemosensitivty of the glomus cells to hypoxia increases.
Longer-term adjustment to hypoxia involves (1) production of more erythrocytes (increased hematocrit) under the influence of erythropoietin, (2) decreased affinity of hemoglobin for oxygen because of increased concentrations of 2,3-diphosphoglycerate (2,3-DPG), (3) an increase in the pulmonary surface area available for diffusion, and (4) increased capillary density in muscle.