Physiology and structure
The gall bladder is a small sac-like organ that is located under the right side of the liver, from which it receives bile. It can hold about 50 mililitres of bile, which which are expelled through the cystic duct as a reaction to the hormone cholecystokinin.

Why have a gall bladder?
Although not essential in the same way that a heart is, a large proportion of mammals have a gall bladder. The gall bladder stores bile, which is used to emulsify fats, and allow them to be digested. The liver produces bile, but it cannot do so quickly enough to produce a suitable amount when a meal is eaten. Bile is therefore stored in the gall bladder until needed.

The gall bladder both stores bile and concentrates it by removing water. Despite this, it is not an exocrine gland, as the bile is not originally produced within the gall bladder.

Cholecystokinin is secreted by cells in the mucosa of the duodenum and ileum in response to food. It is also secreted by the hypothalamus. As well as several other effects important for digestion it stimulates the gall bladder, causing it to contract. This increases the pressure in the gall bladder, causing bile to flow out, first into the cytic duct, then through the common bile duct into the duodenum.

Disorders of the gall bladder
A disturbance to the gall bladder can cause any of all of these symptoms:

The most common disoder of the gall bladder is gallstone formation (cholelithiasis), in which hard stones form in the gall bladder from compounds present in bile.

Other disorders include:
All of which are much more common in people with gallstones.

Severe problems with the gall bladder may lead to its removal: a process known as a cholecystectomy.