GALLBLADDER

a muscular membranous sac occurring in many vertebrates; its function is to store and concentrate bile, a fluid that is received from the liver and is important in digestion. Situated in humans on the underside of the liver, the gallbladder is pear-shaped and expandable, with a capacity of about 1.7 fluid ounces (50 ml). The inner surface of the gallbladder wall is lined with mucous membrane tissue similar to that of the small intestine. Cells of the mucous membrane perform the function of absorption; each cell's surface has hundreds of microscopic projections called microvilli, which increase the area of fluid absorption. The absorption of water and inorganic salts from the bile by the mucous-membrane cells leaves the stored bile about 5 timesand sometimes as much as 18 timesmore concentrated than when it was produced in the liver. Contraction of the muscle wall in the gallbladder is stimulated by the vagus nerve of the parasympathetic system and by the hormone cholecystokinin, which is produced in the upper portions of the intestine. The gallbladder's contraction results in the discharge of bile through the bile duct into the duodenum of the small intestine. The human bile duct is composed of three branches (hepatic duct, cystic duct, and common duct), which are arranged into the shape of the letter Y. The lower segment is the common bile duct; it terminates in the duodenal wall of the small intestine. A constriction at the end of the common duct, called the sphincter of Oddi (for Ruggero Oddi, a 19th-century Italian physician), regulates the flow of bile into the duodenum. The upper right branch is the hepatic duct, which leads to the liver, where bile is produced. The upper left branch, the cystic duct, passes to the gallbladder, where bile is stored. Bile flows from the two lobes of the liver into the hepatic and common ducts. If food is present in the small intestine, the bile will continue directly into the duodenum. If the small intestine is empty, the sphincter of Oddi is closed, and bile flowing down the common duct accumulates and is forced back up the tube until it reaches the open cystic duct. It flows into the cystic duct and gallbladder, where it is stored and concentrated until needed. When food enters the duodenum, the common duct's sphincter opens, the gallbladder contracts, and bile enters the duodenum to aid in the digestion of fats. In animals without a gallbladder, such as horses, rats, and pigeons, bile comes directly from the liver. Some animals have a gallbladder that does not concentrate fluids well; these animals usually have a continuous flow of bile into the intestine. The gallbladder is commonly subject to many disorders, particularly the formation of solid deposits called gallstones. Despite its activity, it can be surgically removed without serious effect. See also bile.