Bile

Bile (or gall) is a thick, greenish-yellow alkaline (pH > 7) fluid. It assists in digestion by breaking down fats, mostly triglycerides (primary form of lipid in animals and plants as well as the main source of fat calories in the Western diet), into monoglycerides and two free fatty acids. In most vertebraes, 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 (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 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 (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, bile serves as the route for excretion of the hemoglobin breakdown product bilirubin (gives bile its yellowish color). Bile, which also contains cholesterol (not particularly water- soluble), occasionally aggregates into lumps in the gallbladder, resulting in cholelithiasis, or the formation and/or presence of gallstones.

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. 95% 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.

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.

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

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

San Francisco: Benjamin Cummings. ISBN 013102153

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