Diffusion

Fick's law of diffusion

Rate of gas transfer = A x D x (P1-P2) / T

• A = area available for transfer
• T = tissue thickness
• P1-P2 = partial pressure gradient
• D = diffusion constant
  * D = k/(square root of MW)
  * D = k x gas solubility in tissue

Alternatively,

Rate of transfer = DL x (P1-P2)

where DL = diffusion capacity

• an all-encompassing term, taking into account of effects of
  - tissue thickness
  - diffusion characteristics of gas
  - diffusion characteristics of tissue
• aka "transfer factor"

Graham's law of diffusion

Relative rate of diffusion are inversely proportional to the square root of the gas molecular weight.

NB:

• MW of O2 = 32
• MW of CO2 = 44
• But CO2 still diffuse faster because of higher solubility