Bile

Bile (or gall) is a thick, greenish-yellow alkaline (pH > 7) fluid that assists in digestion by breaking down fats, mostly triglycerides, into monoglycerides and fatty acids. Fats are the primary form of lipid in animals and plants, and generally the main source of fat calories in the Western diet. Bile is also valuable in the absorption of fat-soluble vitamins and the elimination of waste products, which are secreted into the bile and eliminated in feces. A major pathway for eliminating cholesterol is via secretion into bile.

In most vertebrates, bile is made in the liver and stored in the gallbladder between meals. When fats are present in the digestive tract after the consumption of a meal, a signal from cholecystokinin, or CCK (a hormone released from the small intestine), stimulates the gallbladder to contract and release bile. The bile is discharged into the duodenum (first part of the small intestine, where most of digestion occurs), where it consequently aids the process of fat digestion.

The components of bile are:

• Water
• Cholesterol
• Lecithin (a phospholipid)
• Bile pigments (bilirubin & biliverdin)
• Bile salts and bile acids (sodium glycocholate & sodium taurocholate)
• Small amounts of copper and other excreted metals

Physiology

Bile salts are bile acids conjugated with amino acids. Bile acids are steroid compounds (deoxycholic and cholic acid), often combined with the amino acids glycine and taurine. The most important compounds are the salts of taurocholic acid and deoxycholic acid. Bile acids act as detergents, helping to emulsify fats by increasing their surface area in order to help enzyme action; thus bile acids and salts aid in the absorption of fats in the small intestine.

Bile acids have also been reported to produce relaxation of smooth muscle both in vitro and in vivo. The cellular mechanisms underlying bile acid–induced relaxation are largely unknown; however, it is known that natural bile acids and synthetic analogues reversibly increase BKCa channel activity in smooth muscle cells. The magnitude of bile acid–induced increase in BKCa channel activity is inversely related to the number of hydroxyl groups in the bile acid molecule, only if it is a naturally occurring bile acid (Dopico 2002).

Bile salts function by combining with phospholipids to break down large fat globules in a process known as emulsification. Bile salts associate their hydrophobic side with lipids and their hydrophilic side with water. These emulsified droplets are then organized into many micelles, or small droplets of phospholipid arranged so that the interior is filled with hydrophobic fatty acid tails, which increases overall absorption by helping make large fat globules into smaller particles.

Aside from its digestive function as an emulsant, bile serves as the route for excretion of the hemoglobin breakdown product bilirubin, which gives bile its yellowish color, and for cholesterol as well. Bile, which contains cholesterol (not particularly water- soluble), occasionally aggregates into lumps in the gallbladder, resulting in cholelithiasis, or the formation and/or presence of gallstones. Bile acid sequestrants are medications which can aid in the removal of cholesterol from the blood in order to prevent cholelithiasis.

In species with a gallbladder (humans and most domestic animals, except horses and rats), further modification of bile occurs in the organ. The gallbladder stores and concentrates bile during the fasting state (between meals). Typically, bile is concentrated five-fold in the gallbladder 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. Ninety five percent of secreted bile salts are reabsorbed in the ileum (terminal portion of the small intestine) and re-used. Since bile increases the absorption of fats, it is also an important part of the absorption of the fat-soluble vitamins: A, D, E, and K.

Commercial Uses

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.

In Vietnam and China, bile is sold as traditional Chinese medicine (TCM). Bear bile contains ursodeoxycholic acid (UDCA), which is believed to reduce fever, protect the liver, improve eyesight, break down gallstones, and act as an anti-inflammatory. Bile, or battery, bears, from whom the bile is extracted, are kept in captivity until they stop producing bile, when they are then left to starve to death or are killed for their paws and gallbladders, both of which are considered delicacies.

Four humors

Yellow bile, sometimes called ichor, along with black bile were two of the four vital fluids, or humors, of ancient and medieval medicine. The relative proportions of the humors in the body were thought to determine a person's disposition and general health. For example, melancholia, a mental disorder characterized by severe depression, guilt, withdrawal and hopelessness, was believed to be caused by a bodily surplus of black bile. The other two vital fluids were phlegm and blood.

See also

• Bile acid sequestrant

References

ISBN links support NWE through referral fees

Dopico, A., J. Walsh, Jr., J. Singer. 2002. Natural Bile Acids and Synthetic Analogues Modulate Large Conductance Ca2+-activated K+ (BKCa) Channel Activity in Smooth Muscle Cells. Journal of General Physiology, Vol. 119, No. 3.

Silverthorn, D. 2004. Human Physiology, An Integrated Approach (3rd Edition).

San Francisco: Benjamin Cummings. ISBN 013102153

The Trade in Bear Bile. World Society for the Protection of Animals. 2000.

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