Travel to high altitude for the lowland dweller unmasks a complicated array of physiological mechanisms responsible for acclimatization (Dempsey 8c Forster, 1982). First, the reduction in both the barometric pressure and the arterial partial pressure for oxygen (PaO2) stimulates an increase in ventilation, the magnitude of which depends upon the individual ventilatory sensitivity to hypoxia. Then, the arterial partial pressure for carbon dioxide (PaCo2) is decreased due to hypoxic hyper-ventilation resulting in respiratory alkalosis. As time at altitude increases over the next 1-2 weeks, acid-base balance is normalized by renal excretion of bicarbonate, and Pao2 is improved by increases in the ventilatory sensitivity to hypoxia and polycythaemia. This simplified overview of altitude acclimatization highlights two major controlling factors for cerebral blood flow (CBF): (1) changes in arterial blood gases (i.e. Pao2 and -Paco2) and (2) changes inpH.
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