Copyright © 2004, Elsevier Science (USA) All rights reserved.

KEY POINTS (continued)

gastrin from the G cells via gastrin-releasing peptide (GRP).

Pancreatic secretion consists of an aqueous component of water and HCO3 secreted from the ductule cells and an enzymatic component secreted from the acinar cells containing enzymes essential for the digestion of all three major foodstuffs.

The primary stimulant of the aqueous component of pancreatic juice is secretin, whose effects are potentiated by cholecystokinin (CCK) and acetylcholine, which are the major stimulants of enzyme secretion. The cephalic phase of pancreatic secretion is stimulated by acetylcholine from the vagus and results in a fluid with high concentrations of enzymes but little volume.

Bile is secreted by the liver and contains bile acids, phospholipids, cholesterol, bile pigments, water, and electrolytes.

Primary bile acids are synthesized by the hepatocytes from cholesterol. Some of these are dehydroxylated by the bacteria in the lumen of the gut to form secondary bile acids. Most bile acids are reabsorbed in the gut by active or passive processes and returned to the liver via the portal vein. Hepatocytes extract the bile acids from the blood and resecrete them into bile. This is the enterohepatic circulation of bile acids.

Bile acids are amphipathic and exist with cholesterol and phospholipids in a micellar solution that is stored and concentrated in the gall bladder between meals.

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