Bile

Bile or gall is a bitter yellow or green alkaline fluid secreted by hepatocytes from the liver of most vertebrates. In many species, bile is stored in the gallbladder between meals and upon eating is discharged into the duodenum where the bile aids the process of digestion of lipids.

General information (summary)

Bile has various components, some of which are produced by hepatocytes (liver cells). Its constituents include:

• Water
• Cholesterol
• Bile pigments
• Bile salts

The bile acids cholate and chenodeoxycholate are typically conjugated with taurine or glycine and are produced by the liver from cholesterol. They are secreted in bile by hepatocytes along the bile canaliculi, which then join the bile duct, and thence into the gall bladder. Ordinarily the concentration of bile salts in bile is 0.8%, however the gall bladder removes water from the bile, concentrating it between meals. It concentrates it up to 5 times (increasing concentration to 4%), before contracting the walls and releasing it into the duodenum once chyme has entered the small intestine.

Components- in more detail

The components of bile:

• Cholesterol
• Phospholipids (mainly Lecithin)
• Bile pigments (bilirubin diglucoronoide)
• Bile salts (sodium glycocholate & sodium taurocholate)
• Bicarbonate ions and other ions
• Metabolized red blood cells
  

Production

(Digestive system diagram showing the bile duct)

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 gallbladder, 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 salts are composed of a hydrophilic side, and a hydrophobic side. This means that they are more likely to aggregate to form micelles, with the hydrophobic sides towards the centre and hydrophilic towards the outside. In the centre of these micelles are triglycerides, which are separated from a larger globule of lipid, as shown in the diagram (Diagram missing).

Pancreatic lipase is able to get to the molecules of triglyceride through gaps between the bile salts, providing a largely increased surface area for digestion. Ordinarily, the micelles in the duodenum have a diameter of around 14-33μm. However, it is possible for these to be much smaller, as small as 160nm when using artificial means.

Should bile not be present in the duodenum, not all of the lipid is able to be digested during digestion, and a lot of it is passed out in feces. As a result the time taken for the lipid to be broken down would be greatly increased if there was no bile present in the duodenum. This is how the body is able to efficiently digest and absorb lipids for metabolism.

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 breakdown of erythrocytes, which are conjugated by glucuronidation in the liver ; 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 soaps

Bile from slaughtered animals can be mixed with soap. This mixture, called bile soap, can be applied to textiles a few hours before washing and 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.
• On an empty stomach, for example, after repeated vomiting, a person's vomit may be green or dark yellow, and very bitter. The bitter and greenish component is bile.(The color of bile is often likened to “fresh-cut grass,” but in a vomit it may be mixed with other components in the stomach to look greenish yellow or dark yellow.)
• In the absence of bile, fats become indigestible and are instead excreted in feces, a condition called steatorrhea. Feces lack their characteristic brown colour and instead are white or grey, and greasy. Steatorrhea can lead to deficiencies in essential fatty acids and fat-soluble vitamins. In addition, past the small intestine (which is normally responsible for absorbing fat from food) the gastrointestinal tract and gut flora are not adapted to processing fats, leading to problems in the distal parts of the intestine.

Four humors

Yellow bile (sometimes called ichor) and black bile were two of the four vital fluids or humors of ancient and medieval Greco-Roman alternative medicine (the other two were phlegm and blood). The Greek 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." Depression and other mental illnesses (melancholia) were ascribed to a bodily surplus of black bile. This is the origin of the word "melancholy."