Figure 2 Excretion of acid in the collecting duct.

Bicarbonate reabsorption mainly occurs in the proximal convoluted tubule (PCT, Figure 1). Eighty-five per cent of filtered bicarbonate is reabsorbed at this site: 10% in the thick ascending limb of the loop of henle, the remainder being titrated to regulate total acid excretion in the collecting duct (Figure 2). As shown in the Figure, different mechanisms are involved at each tubular site. The enzyme carbonic anhydrase, on the luminal brush border of tubular cells, catalyzes the combination of filtered bicarbonate with H+, secreted by the apical H+-ATPase and Na+/H+ exchangers on tubular cells, to generate CO2. CO2 then diffuses into the tubular cells down its concentration gradient. Within the cell, carbonic anhydrase catalyzes the reverse reaction generating the production of H+ and HCO3. Hydrogen ions are then recycled to the tubular lumen and bicarbonate is secreted into the extracellular fluid (by baso-lateral anion exchangers or Na+—HCO3 cotransporters) passing into the extracellular fluid and blood. The tubule cells are also exposed to CO2 in the extracellular fluid and will continue to generate H+ even in the absence of filtered bicarbonate. This H+ is then buffered by other buffers in the glomerular filtrate including HPO43 and, to a lesser extent, creatinine. Strong acids (e.g., H2SO4) with low pKa values will dissociate in the urine (pH range 5-8) and are buffered, whereas weaker acids may be excreted intact. In the presence of alkalosis, the function of cellular transporters may be reversed so that H+ secretion occurs on the basolateral membrane and HCO3 secretion on the brush border of tubular cells resulting in alkaline urine.

Classically, the final mechanism by which the kidney can excrete H+ is by the generation of ammonium (NH4) from the metabolism of glut-amine by glutaminase (Figure 2), a process that is stimulated by low pH and increased PCO2. The excretion of H+ as part of ammonium accounts for around 70mmolday_1, increasing several-fold (albeit over a period of days) in the face of an acid load. Whether this is truly a urinary buffer is the subject of some debate as ammonium (NH4) is generated directly from glutamine rather than accepting additional protons. There are alternative mechanisms for the role of NH4 in overall acid-base homeostasis that involve the liver. After being pumped into the glomerular filtrate, NH4 may be reabsorbed by the tubule and used by the liver to synthesize urea, generating free H4 ions. Thus, there is no net loss of H4 and the overall role of NH4 in acid-base balance is dependent on the balance between tubular reabsorption of NH4 and the hepatic synthesis of urea. The latter function may also be directly influenced by extracellular pH.

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