Bile

# Overview
Bile (or gall) is a bitter, yellow or green 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 excretes waste and aids the process of digestion of lipids.

# Components
The components of bile:
- Water
- Cholesterol
- Lecithin (a phospholipid)
- Bile pigments (bilirubin & biliverdin)
- Bile salts (sodium glycocholate & sodium taurocholate)
- Bicarbonate ions

# Production
Bile is produced by hepatocytes in the liver, draining through the many bile ducts that penetrate the liver.  During this process, the epithelial cells add a watery solution that is rich in bicarbonates that dilutes and increases alkalinity of the solution.   Bile then flows into the common hepatic duct, which joins with the cystic duct from the gallbladder to form the common bile duct.  The common bile duct in turn joins with the pancreatic duct to empty into the duodenum.  If the sphincter of Oddi is closed, bile is prevented from draining into the intestine and instead flows into the gall bladder, where it is stored and concentrated to up to five times its original potency between meals.  This concentration occurs through the absorption of water and small electrolytes, while retaining all the original organic molecules.  Cholesterol is also released with the bile, dissolved in the acids and fats found in the concentrated solution.  When food is released by the stomach into the duodenum in the form of chyme, the gallbladder releases the concentrated bile to complete digestion.
The human liver can produce close to one litre of bile per day (depending on body size). 95% of the salts secreted in bile are reabsorbed in the terminal ileum and re-used. Blood from the ileum flows directly to the hepatic portal vein and returns to the liver where the hepatocytes resorb the salts and return them to the bile ducts to be re-used, sometimes two to three times with each meal.

# Physiological functions
Bile acts to some extent as a detergent, helping to emulsify fats (increasing surface area to help enzyme action), and thus aids 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. 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.  Besides its digestive function, bile serves as the route of excretion for the hemoglobin breakdown product (bilirubin) created by the spleen which gives bile its colour; it also neutralises any excess stomach acid before it enters the ileum, the final section of the small intestine. Bile salts are also bacteriocidal to the invading microbes that enter with food.
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.

# Abnormal conditions associated with bile
- The cholesterol contained in bile will occasionally accrete into lumps in the gall bladder, forming gallstones.
- After excessive consumption of alcohol, a person's vomit may be green. The green component is bile.
- In the absence of bile, fats become indigestible and are instead excreted in feces.  In this case, the feces lacks its characteristic brown colour and instead are white or grey, and greasy.  This causes significant problems in the distal parts of the intestine as normally all fats are absorbed earlier in the gastrointestinal tract.  Past the small intestine the organs and gut flora are not adapted to processing fats.

# Four humours
Yellow bile (sometimes called ichor) 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). Excessive bile was supposed to produce an aggressive temperament, known as "choleric". This is the origin of the word "bilious." Depressive and other mental illnesses (melancholia) were ascribed to a bodily surplus of black bile. This is the origin of the word "melancholy."