What is the role of bile in digestion?
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Bile acids are derivatives of cholesterol synthesized in the hepatocyte. It has various components
like water, cholesterol, bile pigments, anions of the bile acids, phospholipids, bicarbonate and
other ions.
Cholesterol is converted into the cholic acid and chenodeoxycholic acids, which are then
conjugated to an amino acid (glycine or taurine) that is actively secreted into cannaliculi.
Bile acids act as detergents; their main function is lipid emulification and solubilization.
Bile acid contains both a hydrophobic and hydrophilic end. In aqueous solution they aggregate to
form micelles.
The dispersion of food fat into micelles provide a largely increased surface area for the action of
the enzyme pancreatic lipase, which actually digests the triglycerides, and is able to reach the
fatty core through gaps between the bile salts.
A triglyceride is broken down into two fatty acids and a monoglyceride, which are absorbed by
the villi on the intestine walls. Without bile salts, most of the lipids in the food would be passed
out in feces, undigested.
Since bile increases the absorption of fats, it is an important part of the absorption of the fatsoluble substances, such as the vitamins D, E, K and A.
How is bile produced?
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Bile is produced by hepatocytes in the liver
During this process, the epithelial cells add a watery solution that is rich in bicarbonates that
dilutes and increases alkalinity of the solution.
Bile then flows into the common hepatic duct→ cystic duct →common bile duct→ pancreatic
duct → duodenum.
If the sphincter of Oddi is closed, bile is prevented from draining into the intestine and instead
flows into the gallbladder, where it is stored and concentrated to up to 5 times its original
potency between meals (concentration occurs through the absorption of water and small
electrolytes)
When food is released by the stomach into the duodenum in the form of chyme, the duodenum
releases cholecystokinin, which stimulates the contraction of the gallbladder and relaxation of
the sphincter of Oddi so that bile enters the duodenum.
How is it circulated through the body?
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The human liver can produce close to one litre of bile per day (depending on body size).
About 95% of the salts secreted in bile are reabsorbed in the terminal ileum and re-used.
Blood from the ileum flows directly to the hepatic portal vein and returns to the liver where the
hepatocytes reabsorb the salts and return them to the bile ducts to be re-used, sometimes two
to three times with each meal (enterohepatic circulation of bile acids).