F7iii / 21A15: Explain perfusion-limited and diffusion-limited transfer of gases

21A15: Exam Report

Explain perfusion limited and diffusion limited transfer of gases in the alveolus.

36% of candidates passed this question.

This question required detail on those factors affecting gas exchange at the level of the alveolus. A description of the components of the Fick equation was expected – and how this related to oxygen and carbon dioxide transfer at the alveolar capillary membrane. The rapid rate of equilibration (developed tension) was the limiting factor in of blood/alveolar exchange that rendered some gases perfusion
limited (examples – N2O, O2 under usual conditions but not all) and the slower rate of others diffusion limited (examples CO and O2 under extreme conditions e.g., exercise, altitude). Estimates of time taken for each gas to equilibrate relative to the time taken for the RBC to travel across the interface was also expected for full marks. CO2 despite rapid equilibration and higher solubility was correctly described as perfusion limited (unless in disease states). Better answers described CO2 as ventilation limited. Some answers also correctly included the component of interaction with the RBC and haemoglobin. Ventilation/perfusion inequalities over the whole lung were not asked for and scored no marks.

F7iii / 21A15: Explain perfusion-limited and diffusion-limited transfer of gases

Gas Transfer in Alveoli

  • Gas is transferred between the alveoli and pulmonary capillary blood by diffusion
    • Gas moves down a partial pressure gradient – in either direction
  • Factors that affect the rate of diffusion are outlined by Fick’s Law of Diffusion:
  • \(\text{Rate of diffusion = K ×} \normalsize \frac{\text{ Surface area × partial pressure difference }}{\text{Membrane thickness}}  \)
    • K = Krogh’s diffusion coefficient. This takes into account the mass of the molecules (\(\text{rate of diffusion ∝ } \normalsize \frac{\text{1}}{\sqrt{\text{molar mass}}}  \)), temperature, solubility of the gas, viscosity of the fluid. K is different for each gas.
  • Other factors affecting the amount of gas transferred include:
    • Blood flow rate/capillary transit time – normally ~0.75s, but can decrease with increased blood flow/cardiac output
    • Gas-protein binding (e.g. O2 and Hb) – maintains a low partial pressure (only dissolved gas exerts a partial pressure)

Perfusion Limited Gas Exchange

  • The total amount of gas exchange is dependent on the blood flow to the alveolus.
  • Occurs when there is rapid diffusion between the alveoli and pulmonary capillary blood
    • The partial presssure equalises early in the blood transit through the pulmonary capillary – diffusion stops. The only way to transfer more gas is by delivering more blood -> maintains the partial pressure gradient.
    • Gas exchange is complete before blood reaches the end of the pulmonary capillary
  • Examples
    • Oxygen
      • Equilibrates rapidly – occurs in ~0.25s (1/3 of blood transit)
      • Hb increases the total amount of O2 exchange by maintaining a low PaO2 until most Hb saturated
    • CO2
      • Equilibrates extremely rapidly (much faster than O2)– high solubility, small partial pressure difference
      • CO2 transfer limited is more limited by ventilation – this controls the partial pressure gradient at the alveolus
    • N2O

Diffusion Limited Gas Exchange

  • The total amount of gas exchange is dependent on the rate of diffusion of the gas.
  • Occurs when diffusion is slow – Partial pressures do not equilibrate along the pulmonary capillary and diffusion can continue along the whole length of the capillary
  • Examples
    • Carbon Monoxide
      • Slow diffusion, avid Hb binding maintains a very low partial pressure in blood – diffusion continues along the entire capillary length without equilibration of partial pressures.
    • Oxygen under extreme conditions
      • When capillary transit time decreases – oxygen transfer may not complete before the end of the capillary (partial pressure gradient remains). Occurs in very high cardiac output (e.g. intense exercise, when capillary transit time is < 0.25s), increased alveolar membrane thickness (e.g. pulmonary oedema), or
Aveolar Partial Pressure
      • when the partial pressure gradient is low (e.g. low PAO2 at high altitude). Partly due to Hb-O2 binding.

Images from Deranged Physiology

Author: Joshua McLarty