_Cholesterol gallstones are primarily made up of cholesterol. Cholesterol is one of the substances that liver cells secrete into bile. Secretion of cholesterol into bile is an important mechanism by which the liver eliminates excess cholesterol from the body. In order for bile to carry cholesterol, the cholesterol must be dissolved in the bile. Cholesterol is a fat and bile is an aqueous or watery solution; fats do not dissolve in watery solutions. In order to make the cholesterol dissolve in bile, the liver also secretes two detergent-like substances (bile acids and lecithin) into the bile. These detergents, just like dish-washing detergents, dissolve the fatty cholesterol so that it can be carried by bile through the ducts.
There are several types of gallstones and each type has a different cause.
Cholesterol gallstones are the most common type of gallstone, comprising 80% of gallstones in individuals in Europe and the Americas. If the liver secretes too much cholesterol for the amount of bile acids and lecithin it secretes, some of the cholesterol does not dissolve. Similarly, if the liver does not secrete enough bile acids and lecithin, some of the cholesterol also does not dissolve. In either case, the undissolved cholesterol sticks together and forms particles of cholesterol that grow in size and eventually form gallstones. In either case, the undissolved cholesterol sticks together and forms particles of cholesterol that grow in size and eventually form gallstones.
There are two other processes that promote the formation of cholesterol gallstones though neither process is able to form cholesterol gallstones by itself. The first is an abnormally rapid formation and growth of cholesterol particles into gallstones. Thus, with the same concentrations of cholesterol, bile acids and lecithin in their bile, patients with gallstones form particles of cholesterol more rapidly than individuals without gallstones. The second process that promotes the formation and growth of gallstones is reduced contraction and emptying of the gallbladder that allows bile to stay in the gallbladder longer than normal so that there is more time for cholesterol particles to form and grow.
Pigment gallstones are the second most common type of gallstone. Pigment is a waste product formed from hemoglobin, the oxygen-carrying chemical in red blood cells. The hemoglobin from old red blood cells that are being destroyed is changed into a chemical called bilirubin and released into the blood. Bilirubin is removed from the blood by the liver. The liver modifies the bilirubin and secretes the modified bilirubin into bile. There are two types of pigment gallstones 1) black pigment gallstones, and 2) brown pigment gallstones.
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_Black pigment gallstones:
If there is too much bilirubin in bile, the bilirubin combines with
other constituents in bile, for example, calcium, to form pigment
(so-called because it is dark brown in color). Pigment dissolves
poorly in bile and, like cholesterol, it sticks together and forms
particles that grow in size and eventually form gallstones. The pigment
gallstones that form in this manner are called black pigment
gallstones because they are black and hard. Black pigment gallstones form whenever an increased load of bilirubin reaches the liver. Black pigment gallstones sometimes occur with cirrhosis, sickle cell disease and thalassemia.
Brown pigment gallstones: If there is reduced contraction of the gallbladder or obstruction to the flow of bile through the ducts, bacteria may ascend from the duodenum into the bile ducts and gallbladder. The bacteria alter the bilirubin in the ducts and gallbladder, and the altered bilirubin then combines with calcium to form pigment. The pigment then combines with fats in bile (cholesterol and fatty acids from lecithin) to form particles that grow into gallstones. This type of gallstone is called a brown pigment gallstone because it is more brown than black. It also is softer than black pigment gallstones. Brown pigment gallstones form when there is stasis of bile (decreased flow), for example, when there are narrow or obstructed bile ducts.