Bile Pathways and the Enterohepatic Circulation, Animation

Bile composition, synthesis pathways, primary and secondary bile acids and bile salts, and enterohepatic recirculation.

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Voice by: Ashley Fleming
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Bile is a yellowish-green fluid secreted by the liver. Bile contains bile acids or bile salts, cholesterol, phospholipids, bilirubin, electrolytes and water. Bile acids are essential for digestion and absorption of fats and fat-soluble vitamins in the small intestine. Bile is also the major route by which cholesterol, as well as bilirubin, a byproduct of red blood cell breakdown, are eliminated from the body.
Bile acids are synthesized from cholesterol via several pathways. The synthesis occurs mainly in the liver. The primary bile acids produced by the liver include cholic acid, CA, and chenodeoxycholic acid, CDCA. These primary bile acids are then combined with the amino acids glycine or taurine, forming conjugated bile acids, which are secreted into bile ducts.
Bile flows into the small intestine to aid with digestion. Outside mealtimes, it is diverted to the gallbladder for concentration and storage. Following food intake, the presence of fats and proteins in the duodenum - the first segment of the small intestine - stimulates the secretion of the enteric hormone cholecystokinin. Cholecystokinin induces the gallbladder to contract and release bile to the small intestine.
In the intestine, microorganisms of the gut microbiota chemically modify the primary bile acids, converting them into secondary bile acids. The bacteria also de-conjugate some of the bile acids.
While passing through the intestine, bile acids help with digestion of fats. They act as a detergent, breaking large globules of fat into smaller micelles, making them more accessible to lipase. Bile acids are also critical for transport and absorption of the fat-soluble vitamins.
About 95% of the bile acids delivered to the duodenum are reabsorbed back into blood. Some unconjugated and uncharged bile acids are passively absorbed along the intestine, but most of the conjugated bile salts are actively transported through the cells of the ileum - the last segment of the small intestine. The small amount that is excreted in feces is compensated for by newly synthesized bile acids in the liver.
Intestinal cells (enterocytes) send the reabsorbed bile acids back to the liver via the portal vein. In the liver, most of the bile acids are extracted from the hepatic circulation for reuse. Only a small portion escapes to the systemic circulation. Thus, bile acids can be recycled many times a day between hepatocytes in the liver and enterocytes in the intestine. This recycling circuit is known as enterohepatic circulation, or recirculation.