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Bile or gall is a bitter-tasting, 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), stored and concentrated in the gallbladder (gallbladder bile) and when the organism eats, is discharged into the duodenum. The composition of gallbladder bile is 92% water, 6% bile salts, 0.3% bilirubin, 0.9-2.4% fats (Cholesterol, fatty acids and lecithin), and 200 mEq/L inorganic salts. 
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: blood, phlegm, "yellow bile" (choler), and "black bile". Excesses of the last two humors were thought 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". These theories derived from the four-element theory. Underlying this is the idea that the organs of the body are connected to the soul, specifically the astral body, and reflect the emotional state of the soul. Thus excess anger, for example, gave rise to liver derangement and imbalances in the humors. This is similar to beliefs in traditional Chinese medicine. In the course of history, medical science helped people to understand diseases better, and the humorism theory fell from use.
Bile acts to some extent as a surfactant, helping to emulsify the fats in food. Bile salt anions are hydrophilic on one side and hydrophobic on the other side; consequently, they tend to aggregate around droplets of fat (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 of 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.
Bile is alkaline and 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.
Bile from deceased mammals 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 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, the presence of certain drugs including alcohol, or powerful muscular contractions and duodenal spasms.
- 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
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