Numerical solution of partial differential equations for CO2 diffusion accompanying HCO3- shift in red blood cells.

A computer program for a numerical solution of the CO2 diffusion in the red blood cell including the HCO3- shift was developed. Comparing the calculated extracellular PCO2 curve with the experimental data on the primary and secondary CO2 diffusions, the permeabilities of CO2 and HCO3- across the red cell membrane, eta(CO2) and eta(HCO3-), were determined as eta(CO2)= 2.5 x 10(-6) cm/(sec.Torr), Inward eta(HCO3-), = 5 x 10(-4) cm/sec, Outward eta(HCO3-) = 7 x 10(-4) cm/sec. The dependencies of the CO2 diffusion rate on the hematocrit and initial intra- and extracellular HCO3- concentrations were examined using the above permeabilities. The half-time of the extracellular PCO2 change in a closed RBC suspension was obviously reduced as the hematocrit and initial extracellular HCO3- increased, while the effect of the initial intracellular HCO3- was not significant. The CO2 diffusion in vivo was simulated using pulmonary and tissue capillary models. The half-time of the extracellular HCO3- change was about 0.15 sec, while that of the intracellular HCO3- change ranged from 40 to 60 msec.