How Bicarbonate (HCO3-) Ions Secrete? - QS Study
QS Study

Bicarbonate ions secretion occurs from the duct and ductless acini of an epithelial cell during the pancreatic enzymes are secreted by acini of the pancreatic juice. This provides a large quantity of alkali in the pancreatic juice that serves to neutralize the hydrochloric acid emptied into the duodenum from the stomach. The basic steps in the cellular mechanism for secreting NaHCO3 solution into pancreatic ductules and ducts are the followings –

(i) CO2 diffuses to the interior of the cell from the blood and combines with water under the influence of carbonic anhydrase to form H2CO3. The H2CO3, in turn, dissociates into bicarbonate ions and hydrogen ions (HCO3 and H+). The HCO3 ions are actively transported in association with Na+ through the luminal border of the cell into the lumen of the duct.

Fig: Bicarbonate Ions Secrete

(ii) The hydrogen ion (H+) formed by dissociation of carbonic add inside the cell are exchanged for sodium ions (Na+) through brush border of the cell by secondary active transport process. This supplies the N+ ions that are transported through the luminal border into the pancreatic duct lumen to provide electrical neutrality for the secreted bicarbonate ions.

(iii) The movements of Na+ and HCO3 ions from the blood into the duct lumen create an osmotic gradient that causes osmosis of water also into the pancreatic duct. Thus forming an almost completely iso-osmotic bicarbonate solution.