Bile

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 between meals and upon eating is discharged into the duodenum.

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 ichor) 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", meaning of "gall" in English as "exasperation" or "impudence".

Production

Bile is produced by hepatocytes in the liver, draining through the many bile ducts that penetrate 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, which joins with the cystic duct from the gallbladder to form the common bile duct. The common bile duct in turn joins with the pancreatic duct to empty into the 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 five times its original potency between meals. This concentration occurs through the absorption of water and small electrolytes, while retaining all the original organic molecules. Cholesterol is also released with the bile, dissolved in the acids and fats found in the concentrated solution. When food is released by the stomach into the duodenum in the form of chyme, the duodenum releases cholecystokinin, which causes the gallbladder to release the concentrated bile to complete digestion.

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.

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 triglicerides, 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 hemoglobin 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,^[1] 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.

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 bile.^{[citation needed]} (The color of bile is often likened to "fresh-cut grass", but in a vomit it may be mixed with other components in the stomach to 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 colour and instead are white or grey, and greasy.^[2] 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.

Footnotes

^ NEWTON, W. (1837). "The invention of certain improvements in the manufacture of soap, which will be particularly applicable to the felting of woollen cloths". The London Journal Of Arts And Sciences; And Repertory Of Patent Inventions. IX: 289. Retrieved 2007-02-08. {{cite journal}}: Cite has empty unknown parameter: |coauthors= (help)
^ Barabote RD, Tamang DG, Abeywardena SN; et al. (2006). "Extra domains in secondary transport carriers and channel proteins". Biochim. Biophys. Acta. 1758 (10): 1557–79. doi:10.1016/j.bbamem.2006.06.018. PMID 16905115. {{cite journal}}: Explicit use of et al. in: |author= (help)CS1 maint: multiple names: authors list (link)

References

Krejčí, Z (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. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
Bowen, R. (2001-11-23). "Secretion of Bile and the Role of Bile Acids In Digestion". Colorado State Hypertextbook article on Bile. Retrieved 2007-07-17.
Maton, Anthea (1993). Human Biology and Health. Englewood Cliffs, New Jersey, USA: Prentice Hall. ISBN 0-13-981176-1. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

External links

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

[1] NEWTON, W. (1837). "The invention of certain improvements in the manufacture of soap, which will be particularly applicable to the felting of woollen cloths". The London Journal Of Arts And Sciences; And Repertory Of Patent Inventions. IX: 289. Retrieved 2007-02-08. {{cite journal}}: Cite has empty unknown parameter: |coauthors= (help)

[2] Barabote RD, Tamang DG, Abeywardena SN; et al. (2006). "Extra domains in secondary transport carriers and channel proteins". Biochim. Biophys. Acta. 1758 (10): 1557–79. doi:10.1016/j.bbamem.2006.06.018. PMID 16905115. {{cite journal}}: Explicit use of et al. in: |author= (help)CS1 maint: multiple names: authors list (link)

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