Analysis of PCO2 differences during rebreathing due to slow pH equilibration in blood.

A quantitative analysis of the reaction and transport processes that occur in blood during and after gas exchange has been used to investigate mechanisms that might account for positive alveolar-mixed venous (A-V) and alveolar-arterial (Aa) PCO2 differences during rebreathing. The analysis was used to determine PCO2 changes that take place in blood as it travels from veins to arteries under conditions in which no CO2 is exchanged in the lung. The predicted A-V and Aa PCO2 differences are all positive and lie within the range of reported measured values. The differences are due to disequilibrium of [H+] between plasma and red blood cells, and to disequilibrium of the reactions CO2 in equilibrium HCO3- + H+ in plasma, as blood leaves the tissue and/or lung capillaries. The differences are increased with exercise and with continued O2 uptake in the lung, the latter due to the Haldane shift. We conclude that the two disequilibria and the Haldane shift contribute to the reported PCO2 differences in rebreathing animals but may not fully account for them. These mechanisms cannot explain any PCO2 differences that might exist during net CO2 elimination from blood in the lung.