BICARBONATE AND THE RENAL REGULATION OF ACID BASE BALANCE

Abstract

The renal tubular reabsorption of bicarbonate in the dog has been assessed at plasma concns. ranging from 10-70 millimols/1. A renal threshold of roughly 25 milli-mols/1. has been defined. At plasma concns. above threshold the renal tubules reabsorb an avg. of 2.5 millimols/100 ml. of glomerular filtrate formed. Any excess is excreted in the urine. The reabsorptive capacity of the tubules is not fixed in the ordinary sense of Tm, for functional increases in filtration rate are accompanied by equivalent increases in tubular reabsorptive capacity. Thus the renal threshold remains constant. The thresholds for chloride and bicarbonate are interrelated in such a fashion as to maintain constant the sum of these 2 anionic components of the plasma. Thus an increase in plasma chloride reduces the renal threshold for bicarbonate and conversely an increase in plasma bicarbonate reduces the renal threshold for chloride. Evidence is presented that 2 functionally and morphologically distinct mechanisms are involved in the reabsorption of bicarbonate. A proximal tubular mechanism reabsorbs bicarbonate iso-hydrically; a distal, tubular mechanism reabsorbs it aniso-hydrically. The capacity of the proximal transfer mechanism is related to the rate of formation of glomerular filtrate. The capacity of the distal transfer mechanism is independent of the rate of formation of filtrate and of the rate of urine flow. The distal mechanism appears to be identical with that for acidifying the urine.