F7iii: Explain perfusion limited & diffusion limited transfer of gases


  • Diffusion = passive movement of particles from area of high [ ] to area of low [   ] across a semi-permeable membrane
    • In lungs, gases move down a partial pressure gradient
    • Rate of diffusion is predicted by FICK’S LAW:

Diffusion Capacity

  • The rate of gas transfer per min/partial pressure difference of the gas
  • Used to measure diffusion in lung because measuring A & T is impractical

Perfusion Limitation

  • RBC transit time in pulmonary caps is 0.75secs
  • O2 combines with Hb
  • ↑partial P of O2
  • ∴O2 diffusion is PERFUSION LIMITED
  • It requires more blood to maintain the partial pressure gradient
  • Normally O2 ↑partial pressure by 0.25secs

Diffusion Limitation

  • If alveolar membrane is thickened e. fibrosis
  • PaO2 does not equilibrate with PA2
  • CO has greater (x250) affinity for Hb cf. O2
  • ∴binds so avidly its partial pressure almost never rises
  • Since partial pressures never equilibrate, it is a DIFFUSION LIMITED gas

NOTE: partial pressure of O2 is never 0