Journal of Applied Physiology, Vol 39, Issue 3 405-410, Copyright © 1975 by American Physiological Society
Gas-blood PCO2 gradients during avian gas exchange
D. G. Davies and R. E. Dutton
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.
