Can alveolar pCO2 exceed pulmonary end-capillary CO2? No.

Abstract

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.