Countercurrent Considerations Relating to Renal Medullary pCO₂ and Bicarbonate Concentrations

Summary

In mieropuneture samples from vasa rectum collecting duct, and the renal artery, pH, pCO₂ and bicarbonate concentrations were determined.

During bicarbonate infusion the CO₂ pressure in vasa recta blood was the same as in animals infused with saline but about 10 mm Hg above that of renal artery samples. Inhibition of carbonic anhydrase with diamox enlarged the CO₂ pressure difference between renal artery and renal papilla to about 40 mm Hg. So it is concluded that increased medullary tissue pressure is mainly due to delayed dehydration. However, the CO₂ pressure in collecting duct samples in both conditions was significantly higher than in vasa recta samples, indicating delayed dehydration in collecting duct urine. When, in contrast, carbonic anhydrase is infused to remove delayed dehydration along the total nephron, the pCO₂ in collecting duct samples and vasa recta samples became equal. The difference between pCO₂ in renal artery and renal medulla under these circumstances was not significant.

The final high pCO₂ of alkaline urine is obtained in part during its passage through the renal papilla. But at the time the urine leaves the papilla the dehydration of urinary carbonic acid is not in its equilibrium state and so the final urinary pCO₂ is only attained in the urinary tract.

In animals infused with saline, there was no difference in bicarbonate concentration in vasa recta and arteria renalis samples. This suggests that bicarbonate is not involved in the general concentrating process of the medulla. The bicarbonate concentration in vasa recta did not rise before the bicarbonate concentration in collecting ducts reached high values. This was interpreted as a diffusion of bicarbonate from collecting duct to vasa recta.

Methodical details and further results of this investigation are published in Pflügers Arch. ges. Physiol. 303, 31 (1968).