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The gallbladder is a blind-ended sac which hangs on the under surface of the liver and it is attached to the main bile duct by a small tube called the cystic duct. There are many variations in the anatomy of the area where the cystic duct joins the bile duct.
The function of the gallbladder is to store bile and to concentrate it as well. Food entering the duodenum, fats in particular, leads to the release of the hormone cholecystokinin. This stimulates the gallbladder to contract thus releasing bile into the duodenum. Concentrated bile helps to emulsify fats in the process of digestion.
Bile is a complex solution of cholesterol, bile-pigments, bile salts, calcium and water. Under certain situations the lining of the gallbladder becomes diseased and the solution becomes unstable leading to crystal formation. Eventually crystals provide a nidus for nucleation and stones form.