The influence of changes in pCO2 on the fractional packed cell volume of whole blood.

In order to investigate the influence of changes in pCO2 on the fractional packed cell volume (FPCV, hematocrit) of whole blood, a device for measuring the conductivity was developed. This method allows an instantaneous and continuous determination of the FPCV, because the erythrocyte membrane has insulating properties, and, consequently, the resistance of blood depends on the relative cell volume. The steady state and transient relationships between FPCV and acid-base levels were investigated by combining this method with simultaneous recordings of pCO2. The experiments showed that addition of CO2 caused an increase in the resistance of whole blood, whereas the resistance of separated plasma decreased slightly and the resistance of true plasma remained almost constant. The change in the FPCV (delta H) can be described by a linear function of pH or log pCO2 (formula: see text). The transient response of the resistance, after a stepwise increase in the CO2 content, was found to be the slowest process in attaining an acid-base equilibrium. In blood with acetazolamide, the time courses of changes in pH and pCO2 were retarded, whereas the time course of the resistance change reflecting the swelling of the erythrocytes was nearly the same (T 50 approximately equal to 4 s). This may indicate a rate-limited water shift due to a slight water permeability of the erythrocyte membrane.