Examination of the mixing hypothesis as an explanation for elevated urinary CO₂ tensions

¹ Department of Internal Medicine, The University of Texas Southwestern Medical School, Dallas, Texas

To examine the adequacy of the mixing hypothesis as an explanation for high urinary CO₂ tensions, urine-plasma pCO₂ gradients were examined in normal subjects receiving NaHCO₃ infusions in whom urinary buffer concentration was minimized by solute and water diuresis, antecedent carbohydrate diet and glucose infusions. U-P gradients varying from 23 to 78 mm Hg were generated in the face of minimal urinary buffer concentrations which varied from 1.40 to 2.95 mEq/l. Only by the most extreme assumptions, i. e. maximal heterogeneity and no back-diffusion of CO₂, could the mixing hypothesis account for the observed gradients. Since these assumptions were highly improbable, it was concluded that the mixing hypothesis is not a tenable explanation of the high CO₂ tension of alkaline urine.