Analysis of postcapillary pH changes in blood in vivo after gas exchange.

A quantitative description of the reaction and transport processes that take place in blood during and after gas exchange in capillaries is developed and used to interpret recently reported experimental results. Included in the computation are 1) CO2-H2CO3 hydration-dehydration reactions in plasma and erythrocytes, 2) CO2 reactions with hemoglobin, 3) O2 binding to hemoglobin, 4) buffering of H+ intra- and extracellularly, 5) HCO3- Cl- exchange across the red cell membrane, 6) diffusion of gases between alveolar gas and blood, and 7) transcellular movement of water. Ion and water fluxes are described assuming passive diffusion down their electrochemical potential gradients. Recent data on the magnitude of the Bohr and Haldane shifts and on carbamate formation in the presence of 2,3-diphosphoglycerate are used. The analysis is used to examine the direction, magnitude, and time course of plasma pH changes in blood leaving the pulmonary capillaries and is shown to preduct results that agree very closely with recently reported experimental measurements in vivo. The time computed for plasma pH equilibration after gas exchange when carbonic anhydrase activity is absent from plasma is so great that blood may never be in complete electrochemical equilibrium as it travels around the circulation in normal man.