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. 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.