Fick’s law of diffusion

The rate of transfer of a gas through a sheet of tissue is proportional to the tissue area and the difference in gas partial pressures between the two sides, and inversely proportional to tissue thickness.

Vgas  alpha  frac{A}{T}  x  D  x  (P1-P2)

Where D is the diffusion constant (Graham’s law), stating that diffusion is proportional to solubility but inversely proportional to the square root of the molecular weight (or density).

D  alpha  frac{Solubility}{sqrt{MW}}

Carbon dioxide diffuses around twenty times more rapidly than oxygen does due to increased solubility despite a higher molecular weight.

Summary

The rate of diffusion through a tissue is proportional to the cross sectional area but inversely proportional to the thickness.

The rate of diffusion is proportional to the pressure differential between either side of the tissue.

The more soluble a substance is the more rapidly it will diffuse, but the larger it’s molecular weight the slower it will diffuse.

 

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