Gas laws of breathing: partial pressure, solubility and water vapour
- Concept
- Exam priority: medium
Each gas in air pushes with its own share of the pressure. How much dissolves in blood, and how fast it moves, depends on that push and on the gas.
Also called: Dalton's law, Henry's law, Graham's law
See it in 3D: Trachea
What it is
Gas exchange is driven by partial pressures, not by percentages. Dalton's law gives each gas its share of total pressure; Henry's law links that pressure to how much gas dissolves in a liquid; diffusion speed depends on solubility and molecular weight. In the airway, inhaled air is warmed and saturated with water vapour, which takes 47 mmHg of the total pressure at body temperature and dilutes the other gases.
Equations
- Fgas
- fractional concentration (dry gas)
- Ptotal
- barometric pressure
In the body, subtract water vapour first.
- PB
- barometric pressure, mmHg
- 47
- water vapour pressure at 37 °C, mmHg
Sea level: 0.21 × (760 − 47) ≈ 150 mmHg.
- C
- dissolved gas concentration
- α
- solubility coefficient
- P
- partial pressure
Dissolved O₂ in plasma ≈ 0.003 mL/dL per mmHg.
- MW
- molecular weight
Net transfer also scales with area and pressure difference and inversely with thickness (Fick's law).
Reference values (approximate)
| Parameter | Value |
|---|---|
| Barometric pressure, sea level | 760 mmHg |
| Barometric pressure, Bogotá (≈ 2600 m) | ≈ 560 mmHg |
| Water vapour pressure at 37 °C | 47 mmHg |
| Inspired PO₂, sea level (humidified) | ≈ 150 mmHg |
| Inspired PO₂, Bogotá (humidified) | ≈ 108 mmHg |
| CO₂ vs O₂ diffusion rate | ≈ 20 : 1 |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Which law explains what
| Law | In one line | Where it shows up in breathing |
|---|---|---|
| Dalton | Each gas exerts its own share of the total pressure | Low inspired PO₂ at altitude with FiO₂ still 0.21 |
| Henry | Dissolved amount rises in proportion to partial pressure | N₂ loading in divers; dissolved O₂ under hyperbaric O₂ |
| Graham / Fick | Diffusion rises with solubility and falls with √MW and with thickness | CO₂ crosses a thick membrane far more easily than O₂ |
| Water vapour | Saturated gas at 37 °C holds 47 mmHg of H₂O | Humidification dilutes inspired O₂ and N₂ in the trachea |
Clinical notes
- At altitude, PiO₂ falls with barometric pressure while FiO₂ stays 0.21; residents of Bogotá have a lower normal PaO₂ than at sea level.
- Divers breathe gas at high total pressure, so more N₂ dissolves (Henry's law); rising too fast lets it come out as bubbles (decompression sickness).
- Hyperbaric oxygen raises dissolved O₂ in plasma enough to supply tissues even without hemoglobin.
Test yourself
Think of the answer, then tap the question to see it.
What is the humidified inspired PO₂ in Bogotá, where barometric pressure is about 560 mmHg?
0.21 × (560 − 47) ≈ 108 mmHg, against about 150 mmHg at sea level.
Why does the water vapour correction of 47 mmHg not change with altitude?
Water vapour pressure depends only on temperature; airway gas is fully saturated at 37 °C, so it is 47 mmHg at any barometric pressure.
Why does thickening of the alveolar membrane cause hypoxemia long before hypercapnia?
CO₂ diffuses about 20 times faster than O₂ because of its much higher solubility, so CO₂ still equilibrates when O₂ no longer does.
According to Henry's law, what happens to dissolved O₂ in plasma when PaO₂ rises from 100 to 600 mmHg?
It rises six-fold, from about 0.3 to about 1.8 mL/dL, because dissolved gas is proportional to partial pressure.
Connections
See it in 3D1
The structures of Durgical's 3D anatomy where it happens.
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: Acute mountain sickness: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2025); Arterial Blood Gas (ABG) Test: MedlinePlus Medical Test (MedlinePlus, US National Library of Medicine, 2025).
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Representative model, for educational purposes only.