Gallstones are a concretion formed from bile. Bile is a thick, greenish-yellow fluid that assists in digestion, and in most vertebrates is made in the liver and stored in the gallbladder between meals. The human liver produces about a quart (or liter) of bile per day. In species with a gallbladder (including humans and most domestic animals, but not horses or rats), further modification of bile occurs in this organ. The gallbladder stores and concentrates bile during the fasting state (between meals). Typically, bile is concentrated five-fold in the gallbladder by absorption of water and small electrolytes. Virtually all of the organic molecules are retained. When fats are present in the digestive tract after the consumption of a meal, a hormonal signal causes the gallbladder to contract and release bile in to the small intestine.
The biliary tree or biliary system refers to the pathway in most members of the mammal family whereby bile is secreted by the liver on its way to the duodenum, or small intestine—a path that includes the liver, gallbladder, pancreas, and various ducts, such as the common bile duct leading to the small intestine. Gallstones may occur in any part of the biliary system.
Among the components of bile are water, cholesterol, lecithin, bile pigments (bilirubin and biliverdin), and bile salts and bile acids. Gallstones typically are composed of cholesterol. Bile pigments and calcium salts may also be involved. Cholesterol stones make up about eighty percent of all gallstones (Balentine 2007); pigments stones are less common and form when bilirubin precipitates in bile (Breslow 2002; Balentine 2007).
Gallstones often cause no symptoms. However if they are large, or many, or move into areas where they cause blockage of a duct, whether those connecting the liver, gallbladder, or pancreas with the intestine, then there can be serious complications, including inflammation, severe pain, organ damage, and death (Balentine 2007).
There are several disorders associated with gallstones. Cholelithiasis is the presence of stones in the gallbladder. (Chole- means “bile,” lithia means “stone,” and -sis means “process.”) Obstruction of the common bile duct is known as choledocholithiasis. Cholecystitis is inflammation of the gallbladder and is usually caused by a gallstone passing from the gallbladder to the cystic duct, but in five to ten percent of the cases, a gallstone is not the cause (Haggerty 2002). Gallstone ileus results from the blockage of the entrance to the large intestine caused by a gallstone. Obstruction of the biliary tree can cause jaundice; obstruction of the outlet of the pancreatic exocrine system can cause pancreatitis.