Bile

This article is about the fluid produced by the liver. For other uses, see Bile (disambiguation).

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

Bile or gall is a dark green to yellowish brown fluid, produced by the liver of most vertebrates, that aids the digestion of lipids in the small intestine. In humans, bile is produced continuously by the liver (liver bile), and stored and concentrated in the gallbladder (gallbladder bile). After eating, this stored bile is discharged into the duodenum. The composition of gallbladder bile is 97% water, 0.7%^[1] bile salts, 0.2% bilirubin, 0.51% fats (cholesterol, fatty acids and lecithin),^[1] and 200 meq/l inorganic salts.^[2]

Bile was the yellow bile in the four humor system of medicine, the standard of medical practice in Europe from 500 B.C. to the early nineteenth century.

Function

Action of bile salts in digestion

Recycling of the bile

Bile acts to some extent as a surfactant, helping to emulsify the lipids in food. Bile salt anions are hydrophilic on one side and hydrophobic on the other side; consequently, they tend to aggregate around droplets of lipids (triglycerides and phospholipids) to form micelles, with the hydrophobic sides towards the fat and hydrophilic sides facing outwards. The hydrophilic sides are negatively charged, 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 around 14–33 μm.

The dispersion of food fat into micelles thus provides a greatly 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, the fatty acids reform into triglycerides, before being absorbed into the lymphatic system through lacteals. Without bile salts, most of the lipids in food would be excreted 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 A, D, E, and K.

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.

Bile tends to be alkali on average. The pH of common duct bile (7.50 to 8.05) is higher than that of the corresponding gallbladder bile (6.80 to 7.65). It becomes more acidic in the gallbladder the longer you go without eating, though resting slows this fall in pH.^[3] As an alkali, it also has the function of neutralizing any excess stomach acid before it enters the duodenum, the first section of the small intestine. Bile salts also act as bactericides, destroying many of the microbes that may be present in the food.

Clinical significance

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.^[4] 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.

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 may be bile or normal digestive juices originating in the stomach. The color of bile is often likened to "fresh-cut grass", unlike components in the stomach that look greenish yellow or dark yellow. Bile may be forced into the stomach secondary to a weakened valve (pylorus), the presence of certain drugs including alcohol, or powerful muscular contractions and duodenal spasms.

Biliary obstruction

Biliary obstruction can be caused by a variety of dietary factors. Most biliary obstructions are caused by the high consumption of sugar, fat and processed foods. These foods can cause gallstones. Primarily, biliary obstruction is caused by blockage in the bile ducts. Bile ducts carry bile from the liver and gallbladder through the pancreas. A huge amount of the bile is then released into the small intestine duodenum. The remaining bile is stored in the gallbladder. After food consumption the bile in the gallbladder is released to help with digestion and fat absorption.

Society and culture

In medical theories prevalent in the West from Classical Antiquity to the Middle Ages, the body's health depended on the equilibrium of four "humors", or vital fluids, two of which related to bile: blood, phlegm, "yellow bile" (choler), and "black bile". These "humors" are believed to have its roots in the appearance of a blood sedimentation test made in open air, which exhibits a dark clot at the bottom ("black bile"), a layer of unclotted erythrocytes ("blood"), a layer of white blood cells ("phlegm") and a layer of clear yellow serum ("yellow bile").^[5]

Excesses of black bile and yellow bile were thought to produce depression and aggression, respectively, and the Greek names for them gave rise to the English words cholera (from Greek kholé) and melancholia. Those same theories explain the derivation of the English word bilious from bile, the meaning of gall in English as "exasperation" or "impudence", and the Latin word cholera, derived from the Greek kholé, which was passed upon several Romance languages in words meaning "anger" such as colère (French) and cólera (Spanish).

Bile soap

Bile from deceased mammals can be mixed with soap. This mixture, called bile soap,^[6] 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.

Bile in food

"Pinapaitan" is a dish in Philippine cuisine that uses bile as flavoring.

Animal abuse

In regions like Asia where bile products are created, there are several cases of abuse of bears.

Principal bile acids

References

1 2 Barrett, Kim E. (2012). Ganong's review of medical physiology. (24th ed.). New York: McGraw-Hill Medical. p. 512. ISBN 978-0-07-178003-2.
↑ Guyton and Hall (2011). Textbook of Medical Physiology. U.S.: Saunders Elsevier. p. 784. ISBN 978-1-4160-4574-8.
↑ Sutor, D. June (1976). "Diurnal Variations in the pH of Pathological Gallbladder Bile" (PDF). Gut (17): 971–974. doi:10.1136/gut.17.12.971.
↑ 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.
↑ Johansson, Ingvar; Lynøe, Niels (2008). Medicine & Philosophy: A Twenty-First Century Introduction. Walter de Gruyter. p. 27. ISBN 9783110321364. Retrieved 23 April 2015.
↑ 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.