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CO2 equilibria in renal tissue.

Disequilibrium situations within the bicarbonate/CO2 buffer system have been observed in the kidney. On the one hand, a PCO2 difference between final urine and arterial blood has been detected. On the other, a disequilibrium pH was found in cortical renal tubules and attributed to either increased carbonic acid concentrations or to PCO2 levels above those of arterial blood. Recent methodological developments have yielded better insight into these disequilibrium situations. They include, besides the use of pH microelectrodes, the introduction of microcalorimetric total CO2 determinations and PCO2 microelectrodes. There is agreement concerning the finding of a PCO2 10--40 mmHg above that in arterial blood in renal cortical tubules; however, stellate vessel PCO2 equal to tubular PCO2 was found by only one group of investigators. According to their view, diffusion equilibrium exists between all cortical structures, a finding disputed by others using a similar methodology. CO2 permeability of tubular epithelium is also still controversial, with values ranging from one-half Microcalorimetric method) to one-twentieth (pH-equilibration method) of an equivalent water layer having been reported. Two positions emerge on the basis of recent experimental results: the existence of very high permeability to CO2 and consequent diffusion equilibrium in renal cortex, and the occurrence of CO2 diffusion limitation leading to the establishment of finite PCO2 gradients across renal tubular epithelium.