Bile (or gall) is a bitter, greenish-yellow alkaline fluid secreted by hepatocytes from 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.
The components of bile:
- Lecithin (a phospholipid)
- Bile pigments (bilirubin & biliverdin)
- Bile salts (sodium glycocholate & sodium taurocholate)
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. The most important compounds are the salts of taurocholic acid and deoxycholic acid.
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. 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; it also neutralises any excess stomach acid before it enters the ilium. Bile also contains cholesterol, which occasionally accretes into lumps in the gall bladder, forming gallstones.
In species with a gall bladder (humans 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 organic molecules are retained.
The human liver produces about a quart (or liter) 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.
After excessive consumption of alcohol on the previous night your vomit may be green. The green component is bile. See Hangover.
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.
Bile salts are also bacteriocidal to the invading microbes that enter with food.
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); for example, melancholia was believed to be caused by a bodily surplus of black bile.
Yellow bile is sometimes called ichor.