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