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Proximal Reabsorption of Bicarbonate

figureIntracellular H2CO3, derived from hydration of intracellular CO2, provides a readily available source of H+ and HCO3- ions. The H+ ions are secreted by both primary and secondary active mechanisms into the proximal tubular lumen. The HCO3- ions are actively transported across the basolateral membrane, mainly by a Na+, HCO3- symporter, into the renal ISF where they are associated with reabsorbed Na+ ions. In the lumen the secreted H+ combines with filtered HCO3- to form H2CO3 which is then dehydrated to form H2O and CO2. The CO2 formed in the lumen rapidly enters the cell where it is hydrated to form H2CO3 which dissociates to provide additional H+ and HCO3- ions.

The hydration and dehydration reactions are catalyzed by carbonic anhydrase (CA) located both intracellularly and in the brush border of the luminal membrane. The end result of this cycle of events is the reabsorption of filtered HCO3-. Similar mechanisms are employed throughout the nephron for further reabsorption of HCO3- and for the generation of new HCO3-. The role of HCO3- transport in the regulation of acid base balance will be discussed in greater detail in the chapter on acid-base balance.