bile - Physiology, Four humours

A golden-yellow fluid produced by the liver, and stored and concentrated in the gall bladder, until released into the duodenum in response to certain dietary substances (eg fats) in the duodenal cavity. In humans it contains sodium and potassium salts of certain organic acids (which facilitate the digestion and absorption of fats, and the absorption of fat-soluble vitamins), excretory products (bile pigments, cholesterol), and other alkaline dissolvable substances.

The components of bile:

Water Cholesterol Lecithin (a phospholipid) Bile pigments (bilirubin & sodium taurocholate)

Physiology

Bile acts 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 by associating its hydrophobic side with lipids and the hydrophilic side with water. Besides its digestive function, bile serves as the route of excretion for the hemoglobin breakdown product (bilirubin) which gives bile its colour;

In species with a gall bladder (humans and most domestic animals, excluding 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 organic molecules are retained.

The human liver produces about 1 L (1.1 quart) of bile per day. Since bile increases the absorption of fats, it is an important part of the absorption of the fat-soluble vitamins D, E, K and A. In the absence of bile, fats become undigestible and are instead excreted in feces.

Bile from slaughtered animals can be mixed with soap.

Bile salts are also bacteriocidal to the invading microbes that enter with food.

Four humours

Yellow bile and black bile were two of the four vital fluids or humours of ancient and medieval medicine (the other two were phlegm and blood). The Latin names for the terms gave rise to the words "choler" (bile) and "melancholia" (black bile). Depressive and other mental illnesses (melancholia) were ascribed to a bodily surplus of black bile.

Yellow bile is sometimes called ichor.