Journal of Applied Physiology, Vol 42, Issue 3 326-328, Copyright © 1977 by American Physiological Society
Can alveolar pCO2 exceed pulmonary end-capillary CO2? No
R. E. Forster
The exchanges and chemical reactions of CO2 within the blood are not
complete during its transit time through the alveolar capillaries, so that
theoretically alveolar PCO2 cannot exactly equal end-capillary blood PCO2.
However, as a practical matter, the technical errors in determining
alveolar and peripheral arterial PCO2 are so large in comparison with the
difference between pulmonary arterial PCO2 and pulmonary venous PCO2, owing
to the large effective solubility of CO2 in blood, that for this reason
alone any difference between alveolar and end-capillary PCO2 can be
neglected. The hypothetical mechanism that was originally proposed by
Gurtner, Song, and Farhi for the production of an alveolar PCO2 greater
than end-capillary PCO2 during rebreathing and depends on the existence of
a radial gradient of H+ X HCO3- in the capillary blood near the wall
appears unlikely because a) it would require a significant separation of
charged ions over a large proportion of the capillary length and for a
large proportion of the blood transit time and b) it would require a large
radial gradient of PCO2 within the blood which on thermodynamic grounds
would in turn require an unreasonable amount of energy to maintain.
