Bile

Bile (yellow material) in a liver biopsy in the setting of bile stasis, i.e. cholestasis. H&E stain

Action of bile salts in digestion

Bile or gall is a bitter-tasting, dark green to yellowish brown fluid, produced by the liver of most vertebrates, that aids the process of digestion of lipids in the small intestine. In many species, bile is stored in the gallbladder and upon eating is discharged into the duodenum. Bile is a composition of the following materials: water (85%), bile salts (10%), mucus and pigments (3%), fats (1%), inorganic salts (0.7%) and cholesterol (0.3%).

In the medical theories prevalent in the West from Classical Antiquity up to the Middle Ages, the body's health depended on the equilibrium between four "humors" or vital fluids: blood, phlegm, "yellow bile" (or choler) and "black bile". Excesses of the last two humors were supposed to produce aggression and depression, respectively; and the Greek names for them gave rise to the English words "cholera" and "melancholia". Those same theories explain the derivation of the English word "bilious" from "bile", and the meaning of "gall" in English as "exasperation" or "impudence".

Physiological functions

Bile acts to some extent as a surfactant, helping to emulsify the fats in the food. Bile salt anions have a hydrophilic side and a hydrophobic side, and therefore tend to aggregate around droplets of fat (triglycerides and phospholipids) to form micelles, with the hydrophobic sides towards the fat and hydrophilic towards the outside. The hydrophilic sides are positively charged due to the lecithin and other phospholipids that compose bile, and this charge prevents fat droplets coated with bile from re-aggregating into larger fat particles. Ordinarily, the micelles in the duodenum have a diameter of around 14-33 μm.

The dispersion of food fat into micelles thus 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. After being transferred across the intestinal membrane, fatty acids are reformed into triglycerides, then absorbed into the lymphatic system through lacteals. 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 fat-soluble substances, such as the vitamins D, E, K and A.

Besides its digestive function, bile serves also as the route of excretion for bilirubin, a byproduct of red blood cells recycled by the liver. Bilirubin derives from haemoglobin by glucuronidation.

The alkaline bile also has the function of neutralizing any excess stomach acid before it enters the ileum, the final section of the small intestine. Bile salts also act as bactericides, destroying many of the microbes that may be present in the food.

Bile soap

Bile from slaughtered animals can be mixed with soap. This mixture, called bile soap, can be applied to textiles a few hours before washing and is a traditional and rather effective method for removing various kinds of tough stains.

Abnormal conditions associated with bile

• The cholesterol contained in bile will occasionally accrete into lumps in the gallbladder, forming gallstones. Cholesterol gallstones are generally treated through surgical removal of the gallbladder. However, they can sometimes be dissolved by increasing the concentration of certain naturally occurring bile acids, such as chenodeoxycholic acid and ursodeoxycholic acid.

• On an empty stomach – after repeated vomiting, for example – a person's vomit may be green or dark yellow, and very bitter. The bitter and greenish component is commonly mistaken as bile.^{[citation needed]} (The color of bile is often likened to "fresh-cut grass", unlike components in the stomach that look greenish yellow or dark yellow.)

• In the absence of bile, fats become indigestible and are instead excreted in feces, a condition called steatorrhea. Feces lack their characteristic brown color and instead are white or gray, and greasy. Steatorrhea can lead to deficiencies in essential fatty acids and fat-soluble vitamins. In addition, past the small intestine (which is normally responsible for absorbing fat from food) the gastrointestinal tract and gut flora are not adapted to processing fats, leading to problems in the large intestine.

Principal bile acids

• 

  Cholic acid

• 

  Chenodeoxycholic acid

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  Glycocholic acid

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  Taurocholic acid

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  Deoxycholic acid

• 

  Lithocholic acid

See also

• Bile acid sequestrant
• Bile bear
• Intestinal juice
• Ox gall

References

Notes

Bibliography

• Bowen, R. (2001-11-23). "Secretion of Bile and the Role of Bile Acids In Digestion". Colorado State Hypertextbook article on Bile. http://www.vivo.colostate.edu/hbooks/pathphys/digestion/liver/bile.html. Retrieved 2007-07-17. 
• Krejčí, Z; Hanuš L., Podstatová H. & Reifová E (1983). "A contribution to the problems of the pathogenesis and microbial etiology of cholelithiasis". Acta Universitatis Palackianae Olomucensis Facultatis Medicae 104: 279–286. PMID 6222611. 
• Maton, Anthea; Jean Hopkins, Charles William McLaughlin, Susan Johnson, Maryanna Quon Warner, David LaHart, Jill D. Wright (1993). Human Biology and Health. Englewood Cliffs, New Jersey, USA: Prentice Hall. ISBN 0-13-981176-1. 

External links

• Photo of bear bile sample in an animal welfare blog by Paul Kraaijer.