The gallbladder is a small, pear-shaped, muscular storage sac that holds bile and is interconnected to the liver by ducts known as the biliary tract. (See also Overview of the Liver and Gallbladder.)
Bile is a greenish yellow, thick, sticky fluid. It consists of bile salts, electrolytes (dissolved charged particles, such as sodium and bicarbonate), bile pigments, cholesterol, and other fats (lipids). Bile has two main functions:
Bile salts aid in digestion by making cholesterol, fats, and fat-soluble vitamins easier to absorb from the intestine.
Bilirubin is the main pigment in bile. Bilirubin is a waste product that is formed from hemoglobin (the protein that carries oxygen in the blood) and is excreted in bile. Hemoglobin is released when old or damaged red blood cells are destroyed.
Bile flows out of the liver through the left and right hepatic ducts, which come together to form the common hepatic duct. This duct then joins with a duct connected to the gallbladder, called the cystic duct, to form the common bile duct. The common bile duct enters the small intestine at the sphincter of Oddi (a ring-shaped muscle), located a few inches below the stomach.
About half the bile secreted between meals flows directly through the common bile duct into the small intestine. The rest of the bile is diverted through the cystic duct into the gallbladder to be stored. In the gallbladder, up to 90% of the water in bile is absorbed into the bloodstream, making the remaining bile very concentrated. When food enters the small intestine, a series of hormonal and nerve signals triggers the gallbladder to contract and the sphincter of Oddi to relax and open. Bile then flows from the gallbladder into the small intestine to mix with food contents and perform its digestive functions.
After bile enters and passes down the small intestine, about 90% of bile salts are reabsorbed into the bloodstream through the wall of the lower small intestine. The liver extracts these bile salts from the blood and resecretes them back into the bile. Bile salts go through this cycle about 10 to 12 times a day. Each time, small amounts of bile salts escape absorption and reach the large intestine, where they are broken down by bacteria. Some bile salts are reabsorbed in the large intestine. The rest are excreted in the stool.
The gallbladder, although useful, is not necessary. If the gallbladder is removed (for example, in a person with cholecystitis), bile can move directly from the liver to the small intestine.
Gallstones are hard masses consisting mainly of cholesterol. Gallstones may form in the gallbladder or bile ducts. Gallstones usually cause no symptoms. However, gallstones may block the flow of bile from the gallbladder, causing pain (biliary colic) or inflammation. They may also migrate from the gallbladder to the bile duct, where they can block the normal flow of bile to the intestine, causing jaundice (a yellowish discoloration of the skin and whites of the eyes) in addition to pain and inflammation. The flow of bile can also be blocked by tumors. Other causes of blocked flow are less common.