Physiology · Cell and membrane
Diffusion and Fick's law !
- Concept
- Exam priority: medium
Molecules move from where there are more to where there are fewer; Fick's law says how fast.
Updated 2026-11-01
Also called: Fick's law, Permeability coefficient
What it is
Diffusion is the net movement of molecules down their concentration gradient, driven only by thermal motion. Fick's law sums up what sets the rate: it rises with the gradient, the area and lipid solubility, and falls with barrier thickness and molecular size. The same law governs gas exchange in the lung, the placenta and the tissues.
Key facts
- Diffusion works over short distances (microns); longer ones need bulk flow (blood, ventilation).
- Diffusion time grows with the square of distance: twice the distance takes about four times as long.
- The permeability coefficient bundles lipid solubility, diffusion coefficient and thickness: P = D × K / Δx.
- In a normal pulmonary capillary, the red cell equilibrates with alveolar O₂ within the first third of its transit; there is reserve.
Equations
- J
- net flux
- P
- permeability coefficient
- A
- area
- C1 − C2
- concentration difference
P = D × K / Δx, with K = partition coefficient (lipid solubility) and Δx = thickness.
- A
- area
- T
- thickness
- D
- ∝ solubility / √molecular weight
- P1 − P2
- partial pressure difference
The basis of the lung diffusing capacity (DLCO).
What changes the diffusion rate
| Factor | Effect | Clinical example |
|---|---|---|
| Concentration or pressure difference | Bigger gradient, more flux | Supplemental oxygen raises alveolar PO₂ |
| Area | More area, more flux | Emphysema: loss of alveolar walls |
| Barrier thickness | Thicker, less flux | Pulmonary fibrosis, interstitial oedema |
| Lipid solubility | More soluble, more flux | O₂, CO₂ and anaesthetics cross the bilayer |
| Molecular size | Larger, less flux | Proteins do not cross; water does (also via aquaporins) |
Clinical notes
- Pulmonary fibrosis: the thickened barrier lowers DLCO; desaturation shows first on exercise, when red cells pass faster.
- Emphysema: less exchange area and a low DLCO; in pure asthma DLCO is usually normal.
- Altitude (Bogotá, ≈2600 m): alveolar PO₂ is lower and so is the diffusion gradient; mild lung disease shows earlier than at sea level.
Test yourself
Think of the answer, then tap the question to see it.
Why does saturation fall on walking but not at rest in pulmonary fibrosis?
At rest there is enough time to equilibrate despite the thick barrier; on exercise cardiac output rises, red cells pass faster and diffusion cannot finish.
If O₂ and CO₂ have similar weights, why does CO₂ diffuse about 20 times faster?
It is far more soluble in water and tissue; in Fick's law the diffusion coefficient depends mostly on solubility.
What happens to flux if thickness doubles and area halves?
It falls to a quarter: flux is proportional to area and inversely proportional to thickness.
Why is diffusion enough inside a cell but not to carry O₂ from lung to foot?
Diffusion time grows with the square of distance: microns take milliseconds; metres would take years. That is why circulation exists.
Related topics
- Diffusion across the respiratory membrane and DLCO
- Membrane transport: diffusion, channels and carriers
- Osmosis, osmolarity and tonicity
- Starling forces at the capillary
- High altitude and acclimatization
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: 3.1 The Cell Membrane (OpenStax (Rice University), 2022); 22.4 Gas Exchange (OpenStax (Rice University), 2022); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 4). 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.