Gallstones and Gallbladder Disease - Introduction

Cholesterol Stones. Although cholesterol makes up only five percent of bile, about three-quarters of the gallstones found in the US population are formed from this substance. Cholesterol gallstones typically form in the following way:

Cholesterol is not very soluble, so in order to remain suspended in fluid it must be transported within clusters of bile salts called micelles. If there is an imbalance between these bile salts and cholesterol, then the bile fluid turns to sludge. This thickened fluid consists of a mucus gel containing cholesterol and calcium bilirubinate.
If the imbalance worsens, cholesterol crystals form (called supersaturation), which can eventually form gallstones.
This process of gallstone formation is referred to as cholelithiasis. It is very slow and most often painless.

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Supersaturation and cholelithiasis can occur as a result of various abnormalities, although the cause is not entirely clear. There are many events that may promote cholelithiasis.

The liver secretes too much cholesterol into the bile.
The gallbladder may not be able to empty normally, so that the bile becomes stagnant.
The cells lining the gallbladder may not be able to absorb cholesterol and fat from bile efficiently.
High levels of bilirubin have been observed in patients with gallstones. Bilirubin is a substance normally formed by the breakdown of hemoglobin in the blood and it is excreted in bile. Some experts believe it may play an important role in cholesterol gallstones.

Pigment Stones. Pigment stones are composed of calcium bilirubinate, or calcified bilirubin. Pigment stones can be black or brown.

Black stones form in the gallbladder and are the more common type. In fact, they represent 20% of all gallstones in the US. They are more likely to develop in people with hemolytic anemia (a relatively rare anemia where red blood cells are destroyed) or cirrhosis (a scarred liver).
Brown pigment stones are more common in Asian populations. They contain more cholesterol and calcium than black pigment stones and are more likely to occur in the bile ducts. Infection plays a role in the development of these stones. One report suggested that bacteria or other microorganisms may trigger oxidation--a damaging chemical process in the body--which, in this case, can cause changes that lead to pigment stone formation.