Secretion of Bicarbonate Ions
Although the enzymes of the pancreatic juice are secreted entirely by the acini of the pancreatic glands, the other two important components of pancreatic juice, bicarbonate ions and water, are secreted mainly by the epithelial cells of the ductules and ducts that lead from the acini. When the pancreas is stimulated to secrete copious quantities of pancreatic juice, the bicarbonate ion concentration can rise to as high as 145 mEq/L, a value about five times that of bicarbon-ate ions in the plasma. This provides a large quantity of alkali in the pancreatic juice that serves to neutral-ize the hydrochloric acid emptied into the duodenum from the stomach.
The basic steps in the cellular mechanism for secret-ing sodium bicarbonate solution into the pancreatic ductules and ducts are shown in Figure 64–8. They are the following:
1.Carbon dioxide diffuses to the interior of the cell from the blood and, under the influence of carbonic anhydrase, combines with water to form carbonic acid (H2CO3). The carbonic acid in turn dissociates into bicarbonate ions and hydrogen
ions (HCO3_ and H+). Then the bicarbonate ions are actively transported in association with sodium ions (Na+) through the luminal border of the cell into the lumen of the duct.
2.The hydrogen ions formed by dissociation of carbonic acid inside the cell are exchanged forsodium ions through the blood border of thecell by a secondary active transport process. This supplies the sodium ions (Na+) that are transported through the luminal borderinto the pancreatic duct lumen to provide electrical neutrality for the secreted bicarbonate ions.
3.The overall movement of sodium and bicarbonate ions from the blood into the duct lumen creates an osmotic pressure gradient that causes osmosis of water also into the pancreatic duct, thus forming an almost completely isosmotic bicarbonate solution.
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