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 (glycine and taurine) act to some extent as a detergent, helping to emulsify fats, and thus aid in their absorption. Besides its digestive function, bile serves as the route of excretion for hemoglobin breakdown products (bilirubin) which give bile its colour. Bile also contains cholesterol, which occasionally accretes into lumps in the gall bladder, forming gallstones.

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.

The human liver produces about a quart (or roughly a litre) of bile per day. Since bile increases the absorption of fats, it can help the body absorb fat-soluble vitamins: A, D, E, and K.

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.