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 bicarbonate ions in the plasma. 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 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

Mechanism Hco3 Secretion Pancreas

Figure 64-8

Secretion of isosmotic sodium bicarbonate solution by the pancreatic ductules and ducts.

Figure 64-8

Secretion of isosmotic sodium bicarbonate solution by the pancreatic ductules and ducts.

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 for sodium ions through the blood border of the cell by a secondary active transport process. This supplies the sodium ions (Na+) that are transported through the luminal border into 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.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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Responses

  • Iain
    How do ductal cells secrete bicarbonate ions?
    3 years ago
  • Kati
    Where are bicarbonate ions secreted?
    2 years ago
  • roma
    What are the steps of sodium bicarbonat in pancreatic juice?
    2 years ago

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