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from Guyton and Hall Textbook of Medical Physiology (4th South Asia Edition) write a proper 5 mark short note on Acclimatization to High Altitude-various changes,Acute Mountain Sickness

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Acclimatization to High Altitude and Acute Mountain Sickness

(Guyton and Hall, Textbook of Medical Physiology)
Background At high altitude the barometric pressure falls, so alveolar and arterial PO2 drop sharply (e.g., ~50 mm Hg at 15,000 ft). An unacclimatized person shows drowsiness, fatigue, headache, nausea, impaired mental proficiency (falls to ~50% of normal within an hour at 15,000 ft), and above ~18,000 ft may develop seizures, progressing to coma and death above ~23,000 ft.
Acclimatization is the process by which a person remaining at high altitude for days to years adapts so that the low PO2 causes progressively fewer deleterious effects, allowing greater work capacity and tolerance of still higher altitudes. The principal mechanisms are:
  1. Increased pulmonary ventilation - Hypoxia stimulates peripheral (carotid and aortic body) chemoreceptors, raising alveolar ventilation acutely to about 1.65 times normal. Over several days this rises to about 5 times normal. The initial rise blows off CO2, causing respiratory alkalosis that inhibits the brain stem respiratory center; this inhibition fades over 2-5 days as the kidneys excrete bicarbonate to correct the alkalosis, allowing full ventilatory response to hypoxia.
  2. Increased red blood cells and hemoglobin - Hypoxia stimulates renal erythropoietin secretion, raising erythropoiesis; hematocrit and hemoglobin concentration rise over weeks, increasing total O2-carrying capacity of the blood.
  3. Increased diffusing capacity of the lungs - due to increased pulmonary capillary blood volume, expanded pulmonary capillary bed, and increased lung volume.
  4. Increased vascularity of peripheral tissues (increased circulatory system capacity) - chronic hypoxia induces angiogenesis, increasing tissue capillary density.
  5. Cellular acclimatization - increased numbers of mitochondria and cellular oxidative enzymes, improving the ability of tissue cells to use O2 even at low PO2.
Natural acclimatization of native highlanders: People born and raised at high altitude (e.g., Andean and Himalayan natives) show acclimatization beginning in infancy - larger chest size, smaller body size, and even greater work capacity than lowlanders acclimatized over 10+ years, due to lifelong developmental adaptation.
Acute Mountain Sickness (AMS) Occurs in some unacclimatized people who ascend rapidly to altitudes above about 10,000-12,000 feet. Features:
  • Onset from a few hours up to about 2 days after ascent.
  • Symptoms: severe headache, nausea/vomiting, fatigue, irritability, insomnia.
  • Mechanism: attributed to mild-to-moderate cerebral edema, possibly related to local vasodilation of cerebral vessels caused by hypoxia, and to leaky capillaries with resultant brain swelling.
  • A small percentage of affected individuals are severely affected and can develop acute high-altitude pulmonary edema (HAPE), and may die if not treated with oxygen or rapidly moved to lower altitude.
  • Management: descent to lower altitude, supplemental O2; prevention by slow, graded ascent to allow physiological acclimatization.
Reference: Guyton and Hall, Textbook of Medical Physiology - "Chronic Breathing of Low O2 - The Phenomenon of Acclimatization," "Acclimatization to Low PO2," "Natural Acclimatization of Native People Living at High Altitudes," and "Acute Mountain Sickness and High-Altitude Pulmonary Edema" sections.
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