Normal Biliary Physiology

Bile, which is formed in the hepatic lobules, is secreted into the canaliculi, small bile ductules, and larger bile ducts that drain it into portal tracts. Interlobular bile ducts join to form larger septal bile ducts that coalesce to form the right and left hepatic ducts, which in turn join to form the common hepatic duct. The common hepatic duct is joined by the cystic duct of the gall bladder to form the common bile duct, which enters the duodenum (often after joining the main pancreatic duct) through the ampulla of Vater.

The largest bile components are water (82%), bile acids (12%), lecithin and other phospholipids (4%), and unesterified cholesterol (0.7%). The total daily basal secretion of hepatic bile is approximately 500-600 ml. The primary bile acids, cholic acid and chenodeoxycholic acid, are synthesized from cholesterol in the liver, conjugated with glycine or taurine, and excreted into the bile. Secondary bile acids, including deoxycholate and lithocholate, are formed in the colon as bacterial metabolites of the primary bile acids. The normal bile acid pool size is approximately 2-4 g. Bile salts play an important role not only in facilitating the biliary excretion of cholesterol but also in intestinal absorption of dietary fats. They are absorbed passively in the entire gut and actively absorbed by the terminal ileum. The bile acid pool circulates approximately 5-10 times daily.

Cholesterol is poorly soluble in water, and its solubility in bile depends on its lipid concentration and the quantity of bile acids and lecithin. Usually, cholesterol is solubilized and forms mixed micelles. Supersaturation of cholesterol provokes the precipitation of cholesterol crystals in bile.

Cholecystokinin (CCK) is the most powerful stimulator of gall bladder contraction. It is released from the duodenal mucosa in response to the ingestion of fats and amino acids. CCK plays its role through contraction of the gall bladder, reducing resistance of the sphincter of Oddi, increasing hepatic secretion of bile, and therefore enhancing flow of biliary contents into the duodenum.

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