The force of the contraction propels the bile back through the common bile duct and then into the small intestine, where it emulsifies fatty molecules so that fat and the fat-absorbable vitamins A, D, E, and K can enter the blood stream through the intestinal lining. About three-quarters of the gallstones found in the US population are formed from cholesterol. If the liver secretes too much cholesterol into the bile, if the bile becomes stagnant and forms sludge because of a defect in the mechanisms that cause the gallbladder to empty, or if other factors are present, supersaturation can occur. Cholesterol may then precipitate out of the bile solution to form gallstones, a condition known as cholelithiasis. The process is very slow and most often painless.
Gallstones can range from a few millimeters to several centimeters in diameter.
They are composed of calcium bilirubinate, or calcified bilirubin, the substance formed by the breakdown of hemoglobin in the blood. Pigment stones can be black or brown and often form in the gallbladders of people with hemolytic anemia or cirrhosis. Patients may also have a mixture of pigment and cholesterol gallstones. On average, symptoms take about eight years to develop. The reason for the decline in incidence after ten years is not known, although some physicians suggest that younger, smaller stones may be more likely to cause symptoms than larger ones.