The Penn State Milton S. Hershey Medical describes the gallbladder is a hollow organ located beneath the liver that secretes a digestive fluid called bile into the intestines. Gallstones form when substances precipitate out of the bile and form hard, small, pebble-like stones. Small gallstones often do not cause any symptoms. However, large gallstones can get stuck in the common bile duct, which is the passageway between the gallbladder and the small intestine. There are three main types of gallstones, which differ based on their composition.
Cholesterol
The University of Maryland Medical Center reports that cholesterol is a normal component of bile. However, cholesterol is not a very soluble molecule, and cholesterol may precipitate out of bile that has too much cholesterol in it. Initially, the cholesterol causes the bile to thicken and turn into sludge. Then the cholesterol may form very tiny crystals. At even higher concentrations of cholesterol, the crystals get larger and larger and eventually form yellowish-green stones. These stones can also form if the gallbladder does not squeeze out the bile properly, because cholesterol is more likely to precipitate out of stagnant bile. Additionally, gallbladder cells normally absorb fat and cholesterol from the bile, and if these cells are defective, the cholesterol concentration may abnormally rise. Interestingly, MayoClinic.com notes that cholesterol levels in the bile are not related to the amount of cholesterol in the blood.
Bilirubin
Like cholesterol, bilirubin is normally present in the bile, according to MayoClinic.com. Bilirubin is a substance that is produced when the body breaks down red blood cells. If the concentration of bilirubin gets too high in the bile, then bilirubin stones may form. These stones are also sometimes called pigment stones because these stones are often black due to the dark color of bilirubin.
There are some conditions that are associated with the development of bilirubin stones, including: liver cirrhosis, in which the liver develops scar tissue and reduces its function; infections of the biliary tract, the passageway through which bile travels from the liver to the small intestine; and some disorders of the blood.
Mixed
If stones are not made of cholesterol or bilirubin individually, they are commonly mixed stones, meaning they are made of both cholesterol and bilirubin. This occurs when the bile has elevated levels of both bilirubin and cholesterol, and the crystals that precipitate out of the bile combine and formed mixed stones.
