Question 6

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

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College Answer

This question required an explanation of the factors that influence perfusion and diffusion and a detailed description of the behaviour of specific gases including perfusion limited oxygen and carbon dioxide and diffusion limited carbon monoxide. Factors influencing perfusion include flow/cardiac output, resistance (radius and length) and viscosity whereas diffusion is influenced by the characteristics of the gas (MW and solubility), surface area of diffusing surface and the pressure/concentration gradient. Perfusion and/or diffusion limited characteristics of different gases included how quickly if at all equilibrium is reached and why this occurs with each specific gas

Discussion

  • Diffusion -limited gas exchange:
    • Exchange where the rate of gas uptake in the capillary is determined by the rate of diffusion across the blood-gas barrier
      • The rate of diffusion from alveolus to blood is very slow
      • For all of the length of the capillary, the gradient between the alveolus and the blood remains high
      • An increase in the capillary blood flow rate will have minimal effect on gas uptake
      • An increase in the partial pressure gradient between the alveolus and the capillary will increase the rate of difffusion
      • An example of a diffusion-limited gas is carbon monoxide
         
  • Perfusion-limited gas exchange
    • Exchange where the rate of gas uptake in the capillary is determined by capillary blood flow:
      • The rate of gas diffusion into the capillary is very rapid
      • Equilibration between the alveolus and capillary occurs shortly after blood enters the alveolar capillary
      • For most of its length, the capillary blood is fully saturated with the gas
      • Increasing the blood flow rate will increase the rate of total gas uptake, until the capillary transit time is faster than the gas diffusion time
      • Increasing the partial pressure gradient between the alveolus and capillary does not significantly increase the rate of gas uptake into the blood if the blood flow remains the same (even more so in the case of oxygen)
      • Oxygen, carbon dioxide and nitrous oxide are perfusion-limited.

References

Kanthakumar, Praghalathan, and Vinay Oommen. "A simple model to demonstrate perfusion and diffusion limitation of gases." Advances in physiology education 36.4 (2012): 352-355.

Kobayashi, H., et al. "Diffusion and perfusion limitation in alveolar O2 exchange: shape of the blood O2 equilibrium curve." Respiration physiology 83.1 (1991): 23-34.

Piiper, Johannes, and Peter Scheid. "Blood-gas equilibration in lungs." Pulmonary gas exchange 1 (1980): 131-171.