bitter, neutral, or slightly alkaline fluid secreted by the liver and passed through a duct into the gallbladder, where it is stored and, as necessary, released into the duodenum. As formed in the liver, bile is a thin, watery fluid to which the gallbladder adds a mucous secretion, forming a complex thickened and stringy substance consisting of salts and bile salts, proteins, cholesterol, hormones, and enzymes. The gallbladder then returns water that contains salts and other materials to the circulation and concentrates the complex further by a tenfold reduction of the bile salts, which the liver synthesizes from cholesterol. Such foods as fats, egg yolk, and foods rich in cholesterol cause concentrated bile, together with secretions from the pancreas, to be discharged into the duodenum to promote digestion, to stimulate peristalsis and absorption, and to carry off excess cholesterol and the disintegration products of overage red blood cells. The hemoglobin of such disintegrating cells degrades rapidly into bilirubin, which is a reddish-yellow pigment that is predominant in the bile of carnivorous and omnivorous animals, and into biliverdin, a green pigment that appears in the bile of herbivores. Under normal conditions, the liver efficiently clears these pigments.

Certain conditions create an inability to excrete bile, and this may create serious disabilities, such as JAUNDICE (q.v.). In obese and immobilized persons, in pregnant women, and in cases of obstruction of flow of bile, gallstones may be formed by precipitation of bilirubin in combination with calcium and cholesterol. Stasis frequently coexists with inflammation and infection of the gallbladder; this may alter the concentration of bile constituents and create debris around which bile and its components may precipitate to form gallstones, which may then block the common bile duct to reduce or stop the flow of bile. Inflammation and infection, with the consequent regurgitation of bile into the liver, may damage that organ, sometimes causing cirrhosis.