The gallbladder is a 3-inch long pear-shaped sac situated beneath the liver. Connected to the bile ducts of the liver through the cystic duct, the gallbladder holds bile produced by the liver for storage until it is needed for the digesting fatty foods in the duodenum of the intestine. Like the small intestine, the inner surface of the gallbladder wall is lined with mucous-membrane tissue, which forms hundreds of microvilli for increasing the area of fluid absorption and allowing the lining to concentrate the dilute bile. In order to propel bile out of the gallbladder, smooth muscle tissue is contained in the deeper layer of the gallbladder wall. When the fats or proteins in the duodenum are detected, the gallbladder will release the stored concentrated bile into the small intestine for digesting. The gallbladder is commonly subject to a lot of disorders, especially gallstones developed by the deposition of insoluble bile salts, pigments, and cholesterol. A variety of target molecules on the gallbladder tissue is used in the detection of associated diseases.