Bile
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Bile (or gall) is a bitter, greenish-yellow alkaline fluid secreted by the liver of most vertebrates. In many species, it is stored in the gallbladder between meals and upon eating is discharged into the duodenum where it aids the process of digestion.
Physiology
Bile salts are steroid compounds conjugated with glycine and taurine, and act to some extent as a detergent, helping to emulsify fats (increasing surface area to help enzyme action), and thus aid in their absorption in the small intestine.
Bile salts combine with phospholipids to break down fat globules in the process of emulsification. Emulsified droplets then are organized into many micelles which increases absorption. Besides its digestive function, bile serves as the route of excretion for the hemoglobin breakdown product (bilirubin) which gives bile its colour. Bile also contains cholesterol, which occasionally accretes into lumps in the gall bladder, forming gallstones.
In species with a gall bladder (man and most domestic animals except horses and rats), further modification of bile occurs in that organ. The gall bladder stores and concentrates bile during the fasting state. Typically, bile is concentrated five-fold in the gall bladder by absorption of water and small electrolytes - virtually all of the the organic molecules are retained.
The human liver produces about a quart (or litre) of bile per day. 95% of secreted bile salts are reabsorbed in the terminal ileum and re-used. Since bile increases the absorption of fats, it is an important part of the absorption of the fat-soluble vitamins: A, D, E, and K.
Bile from slaughtered animals can be mixed with soap. This mixture, applied to textiles a few hours before washing, is a traditional and rather effective method for removing various kinds of tough stains.
Four humours
Yellow and black bile were two of the four vital fluids or humours of ancient and medieval medicine; for example, melancholia was believed to be caused by a bodily surplus of black bile.
Yellow bile is sometimes called ichor.