Carbonic Acid and Bicarbonate in Urine

In the course of some studies of acid excretion undertakenin this laboratory it became desirable to learn something if possible, of the factors which determine the elimination of carbonic acid in urine. Although carbonic acid may be regarded as only incidentally a urinary acid, the bulk of its production leaving the body by way of the lungs, it is, nevertheless, regularly present in urine, both free and bound as bicarbonate. Appraisement of the extent to which it may enter the urine is, therefore, necessary from the point of view of an understanding of the factors controlling the reaction of the urine, and is also required in quantitative studies of the acid-base content of urine. To explain the presence of H&03 in urine several conjectures are permissible: (a) that H&03 enters the urine from the blood plasma at a regulated rate, i.e. is secreted by the kidney; (b) that BHCOS1 entering the urine reacts with acid phosphate and liberates H&03 to such an extent as will be determined by the phosphate concentration; and (c) that H&O, diffuses readily through renal epithelium with the result that the concentration of H&OS in the urine is determined by that obtaining in the blood plasma. Considering the inappreciable part of the total H&O3 production conveyed from the body in the urine, its volatile character, and the fact that its concentration in the plasma is regulated by the respiratory mechanism, an active control of its elimination by the kidney seems unlikely. That the renal epithelium is no barrier