Here's how gallstones form, broken down step-by-step:

Gallstones are solid, pebble-like deposits that form in the gallbladder, a small organ located under the liver. They can range in size from as small as a grain of sand to as large as a golf ball.[1] The formation of gallstones is a complex process, but it generally involves an imbalance in the composition of bile, the digestive fluid produced by the liver and stored in the gallbladder.

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The primary factors contributing to gallstone formation are:

• Excess Cholesterol in Bile: Bile is composed of cholesterol, bile salts, and bilirubin. When the liver secretes bile with too much cholesterol, the cholesterol can crystallize and form stones. This can happen if the liver secretes more cholesterol than the bile can dissolve, or if the gallbladder doesn't empty properly, allowing cholesterol to accumulate.[2]
• Elevated Bilirubin: Bilirubin is a pigment produced during the breakdown of red blood cells. Certain conditions, such as liver cirrhosis, biliary tract infections, and some blood disorders, can lead to an increase in bilirubin levels in bile. Excess bilirubin can also contribute to the formation of gallstones, particularly pigment stones.[3]
• Incomplete Emptying of the Gallbladder (Bile Stasis): If the gallbladder doesn't empty completely or frequently enough, the bile can become overly concentrated. This stasis allows the components of bile, including cholesterol and bilirubin, to crystallize and form stones. Factors that can contribute to bile stasis include pregnancy, rapid weight loss, and certain medications.[4]

The interplay of these factors leads to the development of different types of gallstones, with cholesterol stones being the most common.