- The kidney filters approximately 4320 meq/day of HCO3- (24 meq/L ×180L/day). Under normal circumstances, the kidney is able to completely reabsorb all the filtered bicarbonate. This is vitally important, since any loss of bicarbonate in the urine would disturb acid base balance.
- The process of bicarbonate reabsorption occur predominantly in the proximal tubule (about 90%). The rest occur in the thick ascending limb and in the collecting tubule. All involve hydrogen ion secretion as shown in the diagram below. To completely reabsorb bicarbonate,the kidney must secrete 4320 meq/day of hydrogen ions in addition to the amount required to exrete the daily acid load.
- The primary step in proximal hydrogen secretion is the secretion of H+ by the Na+ - H+ antiporter in the luminal membrane. Hydrogen ions are generated by the intracellular breakdown of H20 to OH- and H+.
- Hydrogen ions secreted combine with filtered HCO3- ions to form carbonic acid and then CO2 + H2O, which are then passively reabsorbed.
- Technically, HCO3- ions reabsorbed in this process are not the same as the ones filtered. Note that a new HCO3- ion is generated from the intracellular breakdown of H20 to OH- and H+ and subsequent reaction of OH- with CO2 to form HCO3- . This new bicarbonate then crosses the basolateral membrane via a Na+ - 3HCO3- cotransporter.
- The net effect is one mol of bicarbonate ion returned to systemic circulation for every H+ ion that is secreted and reabsorption of virtually all filtered bicarbonate.
- Similar processes occur in the thick ascending loop of Henle and intercalating cells of the collecting duct. In contrast to the proximal tubule, hydrogen ion secretion in the collecting tubule is mediated by a H+ ATPase pump in the luminal membrane and a Cl-HCO3- exchanger in the basolateral membrane as shown in the diagram above. The H+ ATPase pump is influenced by aldosterone, which stimulates increased H+ secretion. Hydrogen ion secretion in the collecting tubule is the process primarily responsible for acidification of the urine, particularly during states of acidosis. The urine pH may fall as low as 4.0.
- Proximal reabsorption of bicarbonate can be affected by many factors, in particular, potassium balance, volume status and renin/angiotensin levels. Therefore these factors can have very significant effects on acid base balance. Their specific effects will be discussed later.
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