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 free 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:
- Lecithin (a phospholipid)
- Bile pigments (bilirubin & biliverdin)
- Bile salts and bile acids (sodium glycocholate & sodium taurocholate)
- Small amounts of copper and other excreted metals
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 salts function by combining with phospholipids to break down large fat globules in a process known as emulsification. Bile acids are facial amphipathic, meaning they contain both hydrophobic (lipid soluble) and hydrophilic (water soluble) components. 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. In essence, bile greatly increases the surface area of fat, allowing easier digestion by lipases, as well as transport of lipids by suspension in water.
Aside from its digestive function as an emulsifier, bile serves as the route for excretion of the hemoglobin breakdown product bilirubin, which gives bile its yellowish color, and for elimination of cholesterol as well. Free cholesterol is almost insoluble in water. The body converts free cholesterol to the bile acids cholic and chenodeoxycholic acids. In humans, about 500 mg of cholesterol are converted to these acids and eliminated each day. Cholesterol occasionally precipitates from solution and aggregates into lumps in the gallbladder, resulting in cholelithiasis, or the formation and/or presence of gallstones. Bile acid sequestrants are medications that can aid in the removal of cholesterol from the blood in order to prevent cholelithiasis.
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 is made in the liver. The human liver produces about a quart (or liter) of bile per day.
In species with a gallbladder (including humans and most domestic animals, but not horses or rats), further modification of bile occurs in this 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.
Although bile acids are secreted into the intestine, most is re-absorbed prior to excretion. In humans, approximately 95 percent of secreted bile salts are reabsorbed in the ileum (terminal portion of the small intestine) and re-used.
Bile in the animal world
The use of bile is widespread among vertebrates. It is used for digestion by fish, amphibians, reptiles, birds, and mammals.
The presence of gallbladders to store bile, while found in each of these classes of vertebrates, is not as widespread. Among mammals, humans and many domestic animals have gallbladders, but horses and rats do not. Pigeons also lack gallbladders. In general, omnivores that eat animal flesh have gallbladders.
Bile has been used commercially. It is used in traditional ointments, shampoos, wine, and eye drops. 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.
Bear bile is used in some traditional Chinese medicine (TCM). Bear bile contains ursodeoxycholic acid (UDCA), which is believed by practitioners to reduce fever, protect the liver, remedy kidney aliments, improve eyesight, break down gallstones, and act as an anti-inflammatory. It is more common in bear bile than in other animals. Although there is an international ban on trade in bear parts, bear farms exist in China and Vietnam for extraction of bile. These farms are reported to involve a great deal of cruelty for the animals, with bears being kept in small cages and having a tube or metal inserted to capture bile from the gallbladder, or by allowing it to seep freely from an open wound created by pushing a hollow steel stick though the bear's abdomen. When they stop producing, bears have been left to starve to death or killed for their paws and gallbladders, both of which are considered delicacies. There is a movement to ban such bear farms or to at least develop more humane procedures.
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.
- Dopico, A., J. Walsh, and 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 119(3).
- Johnson, T. 2006. China refuses to abolish bear farms despite international pressure. The Mercury News. January 12, 2006.
- Silverthorn, D. 2004. Human Physiology, An Integrated Approach (3rd Edition). San Francisco: Benjamin Cummings. ISBN 013102153
- World Society for the Protection of Animals. 2000. The Trade in Bear Bile. http://www.animalsvoice.com/PAGES/writes/editorial/features/misc/bearbile_wspa1.html
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