title: Gas-blood PCO2 gradients during avian gas exchange.
abstract: The avian respiratory system is a crosscurrent gas exchange system. One of the aspects of this type of gas exchange system is that end-expired PCO2 is greater than arterial PCO2, the highest possible value being equal to mixed venous PCO2. We made steady-state measurements of arterial, mixed venous, and end-expired PCO2 in anesthetized, spontaneously breathing chickens during inhalation of room air or 4-8% CO2. We found end-expired PCO2 to be higher than both arterial and mixed venous PCO2, the sign of the differences being such as to oppose passive diffusion. The observation that end-expired PCO2 was higher than arterial PCO2 can be explained on the basis of crosscurrent gas exchange. However, the observation that end-expired PCO2 exceeded mixed venous PCO2 must be accounted for by some other mechanism. The positive end-expired to mixed venous PCO2 gradients can be explained if it is postulated that the charged membrane mechanism suggested by Gurtner et al. (Respiration Physiol. 7: 173-187, 1969) is present in the avian lung.
mesh_terms: D000086:Acetazolamide / Q000494:pharmacology; D000818:Animals; D001784:Blood Gas Analysis; D002245:Carbon Dioxide / Q000097:blood*; D002257:Carbonic Anhydrase Inhibitors / Q000494:pharmacology; D002645:Chickens / Q000502:physiology*; D006863:Hydrogen-Ion Concentration; D008954:Models, Biological; D012119:Respiration / Q000187:drug effects
chemical_list: D002257:Carbonic Anhydrase Inhibitors; D002245:Carbon Dioxide; D000086:Acetazolamide