Renal acidosis

Renal acidification.The renal acidification process regulates acid-base balance by regulating the concentration of plasma bicarbonate [1].In normal man the renal acidification process operates to maintain plasma bicarbonate at physiologic concentrations.By way of accomplishing this, the acidification process a) reclaims all filtered bicarbonate and b) excretes an amount of acid equal to the amount of nonvolatile acid produced endogenously, approximately one mEq/kg of body weight per day in adults [2] and one to two mEq/kg of body weight per day in infants and young children [3].Both reabsorption of bicarbonate and excretion of acid appear to be mediated by a single operation of the renal tubule : the exchange of reabsorbed Na+ for secreted H+. At normal plasma bicarbonate concentrations and under physiologic conditions the proximal tubule secretes H+ at a rate that reclaims 85 to 90% of filtered bicarbonate [4, 5].For each mole of secreted H+ exchanged with luminal Na+, one mole of cellular HCO3- is generated and returned to the blood.Secreted H+ used in the titration of luminal HCO3- is not excreted in the urine as acid.Rather, the H2CO3 formed in the titration process dissociates to H2O and CO2, which equilibrates with peritubular CO2.By catalyzing this dissociation, carbonic anhydrase, located at the luminal border of the proximal tubule, acts to reduce the steady-state concentration of luminal H2CO3 and thereby to facilitate secretion of H+ [6].The acidification process of the proximal tubule can be considered a high-capacity (rate), low-gradient system [7], and accounts for the great preponderance of H+ secreted by the renal tubule. In the distal nephron the H+ secretory process titrates the remaining 10 to 15% of filtered bicarbonate.The distal H+ secretory process further titrates the major urinary buffers, Na2HPO4 and NH3 to NaH2PO4 and NH4+, a process that begins in the proximal nephron.“Titratable acid” usually is largely a measure of H+ excreted as NaH2PO4.In the collecting duct the secretory capacity for H+ is small, but a large gradient (lumen-peritubular) generating ability [6] enables the kidney to reduce urinary pH to values of five or less and to excrete ammonium and titratable acid at a combined rate equal to that of the endogenous production of nonvolatile acid.The combined excretion rates of titratable acid and NH4+ minus the normally negligible excretion rate of bicarbonate is termed “net acid excretion” [8].Net acid excretion normally