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In the normal individual the concentration of bicarbonate in the extracellular fluid is maintained within limits of 24 and 28 mMper liter despite wide variations in the intake of acid and base forming foodstuffs. Stabilization of level depends on continuously operative renal mechanisms, for although carbonic acid is continously available, no large reserve depots of base exist upon which the body may draw when circulating stores are depleted, or to which the body may consign an excess in times of abundance. The renal problem of regulating the concentration of bicarbonate may be outlined in the following terms: 1, conservation of those stores which normally enter the renal tubules in the glomerular filtrate; 2, excretion of any excess present in the body; and 3, conversion of the salts of fixed acids into bicarbonate to replenish depleted body stores. These aspects of regulation have been studied in normal human subjects at plasma bicarbonate levels ranging from 13 to 39 mMper liter. It has been observed at plasma concentrations below 24 mMper liter that essentially all of the filtered bicarbonate is reabsorbed; negligible quantities are lost in the urine. Above 28 mMper liter a relatively constant quantity is reabsorbed, amounting on an average to 2.8 mMper 100 ml. of glomerular filtrate; any excess present in the filtrate is excreted. These properties of the reabsorptive mechanism would in themselves assure stabilization of plasma level were there no continuous drain on bicarbonate stores for purposes of neutralizing fixed metabolic acid. Because such a drain exists in the individual maintained on the usual acid ash diet, the renal tubules must split neutral salts of the glomerular filtrate, restoring the fixed base to the body as bicarbonate, and eliminating the unwanted anions either as free titratable acid or in combination with ammonia.

THE RENAL REGULATION OF ACID-BASE BALANCE IN MAN. III. THE REABSORPTION AND EXCRETION OF BICARBONATE 1