3. Old stuff
          3.1. Old pharm stuff (pre 2009)
              3.1.5. Clinical measurements
                  3.1.5.1. Basic physics principles
 3.1.5.1.1. Diffusion 

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