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A hollow muscular organ, present in humans and most vertebrates, which receives dilute bile from the liver, concentrates it, and discharges it into the duodenum. It also participates in the entero-hepatic (re)circulation of bile, and in secretion and removal of conjugated xenobiotics, including radiopaque substances taken orally or intravenously for diagnostic purposes. Although not a vital organ, it stores bile, regulates biliary tract pressures, and, when diseased, enhances precipitation of various constituents of the bile as gallstones.
The system of bile ducts lying outside the liver is known as the extrahepatic biliary tract. In humans (Illus.) right and left hepatic ducts empty into the common hepatic duct, which continues to the duodenum as the common bile duct, or ductus choledochus. The gallbladder and cystic duct thus appear to be accessory organs and therefore are removable. However, they are converted into main-line structures by the presence of a sphincter (sphincter of Oddi) at the choledochoduodenal junction. Tonic contraction of this sphincter between meals forces the bile to back up into the gallbladder.
In most other vertebrates essentially similar relations exist except when the gallbladder is absent, but there is considerable variation in proportion and arrangement of ducts, including the pancreatic ducts. See Liver, Pancreas
In humans, evacuation of the gallbladder is accomplished by a trigger mechanism which is set off by the presence of fatty foods, meat, and hydragogue cathartics in the duodenum and upper jejunum. Absorption of these substances by the mucous membrane results in the release of cholecystokinin (CCK), a hormone which rapidly circulates in the bloodstream and simultaneously produces contraction of the gallbladder and relaxation of the sphincter of Oddi. The most effective food is egg yolk, which contains certain l-amino acids. Resorption of bile salts by the intestine stimulates secretion of bile for hours after a meal. See Digestive system
the organ of vertebrate animals and man in which bile accumulates. The degree of gallbladder development is determined by the nutritional method of the organism. In predatory animals, whose diet contains much fat, it is better developed than in herbivores. The gallbladder is usually located in the liver itself or is close to it and has an effluent duct, which, merging with the effluent duct of the liver, forms the bile duct, which opens into the duodenum. Some animals have two hepatic ducts: one of them, the hepatocystic duct, connects directly with the gallbladder; the other either opens into the cystic duct (usually the case) or directly into the duodenum and is then called the hepatoenteric duct. A gallbladder is present in hagfish and the larvae of lampreys, the majority offish, all amphibians and reptiles, and the majority of birds. (It is especially large in predators.) Among mammals, the gallbladder is absent in many rodents (mice, rats, and hamsters), whales, elephants, and perissodactyls (horses, tapirs, and rhinoceroses), and some artiodactyls (deer and camels).
In man the gallbladder is a thin-walled, pear-shaped pouch 10-14 centimeters (cm) long and 3.5-4 cm wide, with a capacity of 30-70 cm3. It is located on the lower surface of the liver, with which it is connected by porous tissue. The wall of the gallbladder consists of mucous, muscular, and serous coats. Gradually narrowing, the gallbladder becomes the cystic duct, which merges with the hepatic duct to form the common bile duct; this, together with the pancreatic duct, empties into the duodenum. Bile produced in the liver is carried through the hepatic duct into the gallbladder. Contractions of the gallbladder musculature, induced by the entrance of bile into the duodenum, begin as a result of the effect of stimuli connected with food intake and are repeated periodically during digestion. [9-474.2]