The gallbladder is an organ found in one form or another in most, but not all, vertabrate species. There are several species of mammals (like horses, deer, and rats), birds and fish, however, that lack gallbladders.


ANATOMY OF THE HUMAN GALL BLADDER
In humans, the gallbladder is a small pouch-like organ located just below the liver. It connects to the small intestine via a single 'bile duct.' In other vertebrates, the form and the arrangement of the bile ducts vary. In many cases there is no single common bile duct, rather there may be several separate ducts which all run between the gallbladder and the small intestine.

In adult humans, when fully distended the gallbladder is approximately 3 inches long and 1.6 inches in diameter. It is divided into three sections - fundus, body and neck.


FUNCTION OF THE GALL BLADDER
Between meals, the gallbladder fills with bile that is originally produced in the liver. While in the gallbladder this bile becomes very concentrated, increasing its potency.

When fatty foods are eaten, a substance known as CCK, or cholecystokinin,is produced. In the presence of CCK, the gallbladder releases the bile via the bile duct into the duodenum. This concentrated bile then serves to emulsify the fats remaining in the partially digested food.


GALLSTONES
Gallstones are small, hard masses - like pebbles - generally composed of cholesterol, calcium salts, and bile pigments. The exact composition of the stones varies based on age, diet and ethnicity.

Who gets gallstones? Anyone can develop gallstones, but those at greatest risk are overweight Caucasian pre-menopausal females at or above the age of 40.

These 'stones', ranging in size from very small like a grain of rice to very large (think golf ball!), may develop in the gallbladder or elsewhere in the biliary tract. The gallbladder may be filled with many small gallstones or one big gallstone.

Gallstones are divided in the following types, based on their composition:

Cholesterol Stones
To be a cholesterol stone, the stone musch be at least 80% cholesterol by weight. The color ranges from light yell to brown. Small stones, they are general 2 - 3 cm long ovals.

Pigment Stones
These small, dark stones are composed of bilirubin and calcium salts, with less than 20% of their composition from cholesterol.

Mixed Stones
This type of gallstone contain betwen 20% - 80% cholesterol, and other substances including calcium carbonate, palmitate phosphate, and bilirubin. These stones are the tye most often spotted in x-rays because of their calcium content.


GALL BLADDER REMOVAL
The creation and presence of gallstones often leads to an inflammatory condition known as 'cholecystitis'. In these cases, the surgical removal of the gallbladder, a 'cholecystectomy, may become necessary. Cholecsystectomies are 99% succesful in halting the creation of gallstones.

While the gallbladder's function is important, humans can live perfectly normal lives without one. However, approximately 10 - 15% of cholecystectomy patients develop 'postcholectystomy syndrome', a condition that causes increased gastronintestinal distress, chronic diarrhea, and persistent pain in the upper-right abdomen.