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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 many species, bile is stored in the gallbladder and, when the organism eats, is discharged into the duodenum. Bile is 85% water, 10% bile salts, 3% mucus and pigments, 1% fats, and 0.7% 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.