How do the kidney reabsorb bicarbonate ions?

Location
  • Around 80% reabsorbed in the proximal convoluted tubule (PCT)
  • Remaining HCO3- reabsorbed in the distal convoluted tubule and collecting ducts
Process
  • CO2 combines with water to form carbonic acid, catalysed by carbonic anhydrase
  • The H2CO3 then ionizes to H+ and bicarbonate
  • The hydrogen ions are secreted into the tubular lumen in exchange for Na+ ions
  • Bicarbonate ions combine with H+ to form H2CO3 which dissociates into water and CO2
  • CO2 diffuses into the cell which again combines with water to form H2CO3
  • As carbonic anydrase catalyses the reaction to H+ and HCO3-, the bicarbonate is reabsorbed into the blood along with Na+ ions
  • Prevents the loss of bicarbonate in the urine
Result
  • Reabsorption of filtered bicarbonate - urine usually free of bicarbonate
  • No net generation of bicarbonate or excretion of H+