Bicarbonate is absorbed -

Ans. is 'c' i.e., Indrectly in PCT o About 80-90 of filtered HCO2, is absorbed in PCT.o Absorption of HCO,' is indirect, which is due to secretion H+, i.e., secretion of H+ is coupled with absorption of HCO3-o Filtered HCO3' reacts with H+ to form H2CO, which then is dissociated into H2Oand CO2,.o CO. diffuses into epithelial cell and generate HCO2 which is then reabsorbed. In early tubular segmentso In proximal tubule, thick ascending limb of loop of Henle and early distal tubule, the H+ is secreted into tubular fluid by "Na+ - H+ exchanger", i.e., by 'sodium and hydrogen countertransport'o The energy is derived from Na+ Kf ATPase locate on the basolateral membrane. Therefore, it is a secondary active transport.o Filtered HCO3 is reabsorbed by interaction with these secreted H+ . Filtered HCO3- react with H' to form H2CO3o This H,CO. dissociates into CO. and H,0 by carbonic anhydrase, which is present in the brush border of proximal tubules, thick ascending limb and early distal tubules.o The CO, rapidly diffuses into the tubular cell where it reacts with water to form H2C03 under the influence of intracellular carbonic anhydrase.o This H,CO. in turn dissociates to form HCO. and H+; the HCO. then diffuses across the basolateral membrane into the interstitial fluid and is taken up by peritubular capillaries.Distal nephrono In the late distal tubule and collecting ducts, H+ ions are secreted by primary active transport, i.e., H+ ions are directly transported by hydrogen - transporting ATPase.o Primary active secretion of H+ occurs in special type of cell called the intercalated (I) cellsQ.o Hydrogen ion secretion in these cells is accomplished in two steps : (1) The dissolved CO2 in the cells combine withH2O to form H2CO3, and (2) The H2CO3. then dissociates into HCO3' which is reabsorbed into the blood, plus H-, which is secreted into the tubule.o For each H+ secreted, an HCO3 is reabsorbed similar to the process in the early segments.o The main difference is that H+ moves across the luminal membrane by primary active transport instead of by counter-transport as occurs in the early parts of the nephron.