Adapted from http://www.mnsu.edu/emuseum/biology/humananatomy/digestive/digestivemainframe.htm.

The gall bladder is a sac-like about 7 to 10 cm (3 to 4 in) long. It is located in a depression on the underside of the liver. This organ stores and concentrates bile, which drains to it via the bile ducts, hepatic duct and cystic duct. The gall bladder receives blood from the cystic artery which branches from the right hepatic artery. The wall of the gall bladder consists of an inner layer of epithelium (skin) called the mucosal layer. The mucosal layer secretes mucous to protect the gall bladder from the basic (pH 7.6 -8.6) bile it stores. The mucosal layer is arranged into rugae (folds or ridges) similar to the stomach. Beneath the mucosal layer, there is a layer of smooth muscle.

A sphincter valve and the neck of the gall bladder allows a storage capacity of about 35 to 50 mL. When the gall bladder fills with bile, it expands to the size and shape of a small pear. Bile is continuously produced by the liver and drains to the duodenum (duo- DE- num) by way of the hepatic and common bile ducts. When the duodenum is empty, the bile is forced back up the cystic duct to the gall bladder for storage. After a meal, various stimuli cause contraction of the gall bladder and bile is released back into the common bile duct. Bile is partially an excretory product and partially a digestive secretion. It is a yellowish-green fliud composed of bile salts, bilirubin, cholesterol, and other compounds. Bile salts are used to assist in the breakdown of fat globules. Gallstones result when there are not enough salts in the bile.

See images:
http://curezone.com/ig/igallery.asp?d=\liver_gallbladder\gallbladder_removed\