Secretion of bicarbonate ions

Bicarbonate ions and water are secreted mainly by the epithelial cells of the ductules and ducts that lead from the acini

The Basic steps in the cellular mechanism for secreting sodium bicarbonate solution into the pancreatic ductules and ducts are following:

1. Carbon dioxide and water enter the cell and combine to
form carbonic acid under the influence of the
enzyme carbonic anhydrase

2. Hydrogen ions are actively secreted on the basal side of
the cell in exchange for sodium

3. Bicabonate ions are actively secreted on the apical or
lumen side of the cell in exchange for chlorine

4. Sodium and Water ions follow either passively through
'leaky' tight junctions, or through cell by the mechanism
described above in 'Water & Electrolyte secretions'