High Altitude Acclimatization: The Exception
Metabolic alkalosis does not occur during acclimatization to high altitude; instead, respiratory alkalosis develops initially, which is then partially compensated by renal mechanisms that create a compensatory metabolic acidosis, not alkalosis.
Physiological Changes During High Altitude Acclimatization
The body undergoes several well-characterized adaptations when exposed to hypobaric hypoxia at high altitude:
Respiratory Adaptations (Options 1 & 2 - These DO Occur)
- Increased pulmonary ventilation is a fundamental response to high altitude, driven by hypoxic stimulation of peripheral chemoreceptors in the carotid and aortic bodies 1, 2
- This hyperventilation increases both respiratory rate and tidal volume to compensate for reduced oxygen availability 1
- Increased sensitivity of peripheral chemoreceptors to hypoxia develops during acclimatization, allowing sustained hyperventilation even as pH changes 3
- Notably, the peripheral chemoreceptors (not central chemoreceptors as stated in the question) become more sensitive to hypoxia during acclimatization, which continues to drive ventilation even when pH becomes alkaline 3
Hematologic Adaptations (Option 3 - This DOES Occur)
- 2,3-DPG concentration in red blood cells increases during high altitude acclimatization 3
- This rightward shift of the oxygen-hemoglobin dissociation curve facilitates oxygen unloading to tissues despite lower arterial oxygen tensions
Acid-Base Changes (Option 4 - This Does NOT Occur)
The key distinction: High altitude exposure causes respiratory alkalosis, not metabolic alkalosis 1, 2
- Hyperventilation at altitude blows off CO₂, reducing PaCO₂ and creating respiratory alkalosis 1, 3
- The kidneys respond by excreting bicarbonate to partially compensate for the respiratory alkalosis 3
- This renal compensation creates a metabolic acidosis (decreased bicarbonate), not metabolic alkalosis 3
- During acclimatization, the internal milieu becomes alkaline initially, but the kidney works to restore acidity by eliminating bicarbonate 3
Clinical Pitfall to Avoid
The confusion likely stems from misunderstanding the compensatory mechanisms. While the initial response to altitude is respiratory alkalosis, the body does not develop metabolic alkalosis as an adaptive response. Instead, metabolic alkalosis is typically seen in clinical conditions involving acid loss (vomiting, diuretic use) or alkali accumulation 4, 5, 6, none of which are part of normal high altitude acclimatization.
Answer: Option 4 (metabolic alkalosis) is the change that does NOT occur during high altitude acclimatization.