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bile, or GALL, golden-yellow secretion produced in the liver and passed to the gallbladder for concentration and either for storage or for transport into the first region of the small intestine, the duodenum. Its function is to aid in the digestion of fats. Bile is composed of bile acids and salts, pigments, water, and electrolyte chemicals that keep the total solution slightly acidic (with a pH of about 5-6). Bile continually secreted from the cells of the liver into the common bile duct and gallbladder (q.v.); once in the gallbladder it is concentrated about 20 times that of the original secretion. The amount of bile secreted into the duodenum is controlled by the hormones secretin, gastrin, and cholecystokinin (qq.v.) and also by the vagus nerve. About 250 to 1,000 millilitres of bile (before concentration) are produced daily by the liver.
Bile salts are composed of the salts of four different kinds of free bile acids (cholic, deoxycholic, chenodeoxycholic, and lithocholic acids); each of these acids may in turn combine (conjugate) with glycine or taurine to form more complex acids and salts. The salts of both conjugated and unconjugated bile acids are found in bile. Bile salts and acids can be synthesized from cholesterol or extracted from the bloodstream by the liver. They pass from the liver into the intestine, where they emulsify fat and reduce the surface tension on fat droplets to prepare them for the action of pancreatic and intestinal fat-splitting enzymes.
The salts are large negatively charged ions that are not readily absorbed by the upper region of the small intestine; consequently, they remain in the intestine until most of the fat
digested. In the lower part of the intestine, the salts and acids are absorbed and passed back into the blood stream until they are once again extracted by the liver; this cycle, from liver to the intestine and blood, and then back to the liver, is called enterohepatic circulation. Some salts and acids are lost during this process; these are replaced in the liver by continual synthesis from cholesterol. The rate of synthesis is directly related to the amount of acids and salts Lost. Bile salts do not normally reach the colon; in the rare cases when they do, however, they inhibit the absorption of water and sodium, causing a watery diarrhea. Bile salts and acids are transported in a fluid that contains water, sodium, chloride, and bicarbonates. This fluid is produced in the liver, and it serves to neutralize hydrochloric acid passed from the stomach into the small intestine. The major bile pigment is bilirubin (q.v.); it has no known function other than that of
colouring agent. Traces of other substances can also be found in bile, but their significance here is not fully understood; a few of these substances are mucus, serum proteins, lecithin, neutral fats, fatty acids, and urea.
