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

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Abstract
The renal re-absorption and excretion of bicarbonate has been studied in three normal subjects over a range of plasma concn. of 13 to 38 m[image]. per liter. At plasma levels below 24 m[image], per liter essentially all of the bicarbonate entering the renal tubules in the glomerular filtrate is reabsorbed. Above 28 m[image]. per l. a relatively constant quantity, averaging 2.8 m[image], per 100 ml. of glomerular filtrate, is reabsorbed; the excess is eliminated in the urine. Under the conditions of these expts. the renal threshold is 25 to 27 m[image]. of bicarbonate per 1. of plasma. Evidence is presented that bicarbonate is reabsorbed by 2 dissimilar mechanisms, a proximal and a distal one. The latter mechanism appears to be the same as that which converts buffer salts of the tubular urine into free titratable acid. The properties of the proximal and distal tubular reabsorptive mechanisms of man are similar to those previously described in the dog.

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