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[Physiology](https://durgical.com/en/physiology/) · [Temperature, exercise and metabolism](https://durgical.com/en/physiology/#s-integrative)

# 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](https://durgical.com/en/anatomy/inferior-lobe-of-right-lung/) · [Left lower lobe](https://durgical.com/en/anatomy/inferior-lobe-of-left-lung/) · [Kidneys](https://durgical.com/en/anatomy/kidney/) · [Carotid arteries](https://durgical.com/en/anatomy/common-carotid-artery/)

## Key points

- PIO₂ = FiO₂ × (PB − 47): ≈150 mmHg at sea level, ≈108 mmHg in Bogotá, ≈70 mmHg on Nevado del Ruiz (~5300 m).
- First response: hypoxemia stimulates the carotid bodies → hyperventilation → respiratory alkalosis.
- Over 2–3 days the kidney excretes bicarbonate and partly corrects pH, allowing ventilation to rise further.
- Renal EPO rises within hours; red cell mass grows over weeks: higher hemoglobin reference ranges in Bogotá.
- Alveolar hypoxia constricts pulmonary arterioles (hypoxic vasoconstriction): pulmonary hypertension and risk of high-altitude pulmonary edema.

## 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 mmHg **Arterial 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

After days at altitude 2,3-BPG rises and the curve moves right. Compare with «Hyperventilation».

## Equations

**Inspired PO₂**PIO2 = FiO2 × (PB − PH2O)

**FiO₂**
O₂ fraction (0.21)

**P B**
barometric pressure

**P H2O**
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.

## Memory aid

«Hours: breathe; days: pee out bicarbonate; weeks: build blood».

## 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.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/physiology/high-altitude-and-acclimatization/)

## See it in 3D

The structures of Durgical's 3D anatomy where it happens.

- [Right lower lobe](https://durgical.com/en/anatomy/inferior-lobe-of-right-lung/)
- [Left lower lobe](https://durgical.com/en/anatomy/inferior-lobe-of-left-lung/)
- [Kidneys](https://durgical.com/en/anatomy/kidney/)
- [Carotid arteries](https://durgical.com/en/anatomy/common-carotid-artery/)

## Related topics

- [Physiology of the vital signs (including blood pressure measurement)](https://durgical.com/en/physiology/physiology-of-the-vital-signs/)
- [Aviation and space physiology](https://durgical.com/en/physiology/aviation-and-space-physiology/)
- [Exercise and training physiology](https://durgical.com/en/physiology/exercise-and-training-physiology/)
- [Polycythemia: physiological causes](https://durgical.com/en/physiology/polycythemia-physiological-causes/)

## Related conditions

- [Respiratory alkalosis](https://durgical.com/en/conditions/respiratory-alkalosis/)

## In Biochemistry

- [Hemoglobin and 2,3-BPG](https://durgical.com/en/biochemistry/enzymes/hemoglobin-and-2-3-bpg/)

[← All of physiology](https://durgical.com/en/physiology/)

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](https://openstax.org/books/anatomy-and-physiology-2e/pages/22-6-modifications-in-respiratory-functions) (OpenStax (Rice University), 2022); [22.5 Transport of Gases](https://openstax.org/books/anatomy-and-physiology-2e/pages/22-5-transport-of-gases) (OpenStax (Rice University), 2022); [Acute mountain sickness: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000133.htm) (MedlinePlus, US National Library of Medicine, 2026); [Respiratory alkalosis: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000111.htm) (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.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/high-altitude-and-acclimatization/