Physiology · Temperature, exercise and metabolism
High altitude and acclimatization !
- Regulation
- Exam priority: high
At altitude air has the same share of oxygen but less pressure; the body breathes harder and makes more red blood cells.
Updated 2026-11-01
Also called: Hypobaric hypoxia, Acute mountain sickness
See it in 3D: Right lower lobe · Left lower lobe · Kidneys · Carotid arteries
What it is
As you climb, barometric pressure falls and inspired PO₂ falls with it, even though O₂ is still 21% of the air. In Bogotá (~2600 m, ≈560 mmHg) a normal PaO₂ is about 60–70 mmHg. The body adapts within hours (ventilation), days (kidney, 2,3-BPG) and weeks (red cell production); climbing too fast causes acute mountain sickness.
How it works, step by step
- Barometric pressure falls with altitude; inspired and alveolar PO₂ fall in proportion.
- PaO₂ below ≈60 mmHg activates the carotid chemoreceptors and increases ventilation.
- CO₂ is blown off: PaCO₂ falls (≈30–35 mmHg in Bogotá) and pH rises; the alkalosis initially brakes ventilation.
- The kidney excretes bicarbonate over days; pH normalizes and ventilation can rise more (ventilatory acclimatization).
- Hypoxia stabilizes HIF in the kidney: more erythropoietin, more red cells and more O₂ content.
- 2,3-BPG rises: the curve shifts right and eases O₂ delivery to tissues.
Oxygen–hemoglobin dissociation curve
P50 27.7 mmHgArterial blood 97 %Mixed venous blood 73 %O₂ given up to the tissues 24 %
Right shift (P50 27.7 mmHg): Hb has less affinity and gives up more O₂ in the tissues. Acid, CO₂, heat and 2,3-BPG cause it.
- Current curve
- Normal curve (pH 7.40 · 37 °C)
- Arterial blood (PO₂ 100)
- Mixed venous blood (PO₂ 40)
- O₂ given up to the tissues
Equations
- FiO₂
- O₂ fraction (0.21)
- PB
- barometric pressure
- PH2O
- water vapour at 37 °C (47 mmHg)
Bogotá: 0.21 × (560 − 47) ≈ 108 mmHg.
Approximate values (healthy adult, rest)
| Parameter | Value | Note |
|---|---|---|
| Barometric pressure in Bogotá | ≈560 mmHg | 760 mmHg at sea level |
| PaO₂ in Bogotá | ≈60–70 mmHg | ≈90–100 mmHg at sea level |
| PaCO₂ in Bogotá | ≈30–35 mmHg | 35–45 mmHg at sea level |
| SpO₂ in Bogotá | ≈90–95% | |
| Hemoglobin (adult man) in Bogotá | higher limits than at sea level | use local reference ranges |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Clinical notes
- Acute mountain sickness: headache, nausea, insomnia and fatigue 6–12 h after a fast ascent above ~2500 m.
- High-altitude pulmonary edema: uneven hypoxic vasoconstriction and high capillary pressure; breathlessness at rest and cough.
- High-altitude cerebral edema: ataxia and confusion; an emergency that requires descent.
- Chronic mountain sickness (Monge's disease): excessive polycythemia in high-altitude residents.
Test yourself
Think of the answer, then tap the question to see it.
Why doesn't the oxygen fraction of Bogotá's air explain its hypoxemia?
FiO₂ is still 0.21; what falls is barometric pressure, and with it inspired and alveolar PO₂.
Why does ventilation keep rising during the first days at altitude?
At first the respiratory alkalosis brakes the respiratory centre; once the kidney excretes bicarbonate and pH corrects, the brake is released.
A healthy Bogotá resident has PaO₂ 65, PaCO₂ 32, HCO₃⁻ 20 and pH 7.42. Is this abnormal?
No: it is the compensated chronic respiratory alkalosis of altitude, with a lower PaO₂ than at sea level. Read it against local reference values.
Why can altitude cause pulmonary edema without left heart failure?
Alveolar hypoxia constricts pulmonary arterioles unevenly; less-constricted areas get high flow and pressure, and their capillaries leak fluid.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- Physiology of the vital signs (including blood pressure measurement)
- Aviation and space physiology
- Exercise and training physiology
- Polycythemia: physiological causes
Related conditions
In Biochemistry
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 22.6 Modifications in Respiratory Functions (OpenStax (Rice University), 2022); 22.5 Transport of Gases (OpenStax (Rice University), 2022); Acute mountain sickness: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Respiratory alkalosis: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 44). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.