Question 15

College Answer

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.

Discussion

• Diffusion and perfusion are the two main processes which govern the rate of alveolar-capillary gas transfer
   
• 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.

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.