David Ferber, age 13, of Baltimore, Md., for his question:

WHERE IS YOUR GALLBLADDER

Your gallbladder is a small pouch that stores bile. It is pear shaped and it rests on the underside of the right portion of the liver. The gallbladder can hold about an ounce and a half of bile at one time.

During digestion, bile flows from the liver through the hepatic duct into the common bile duct and empties into the duodenum, which is the first section of the small intestine. Between meals, the bile is not needed but it continues to flow from the liver into the common bile duct. It is kept out of the duodenum by a small, ringlike muscle called the sphincter of Oddi, which tightens around the opening and will not allow the bile to pass.

The fluid is then forced to flow into the gallbladder, where it remains until it is needed for digestion.

The gallbladder is made to contract by action of the hormone called chlesystokinin. This hormone is formed in the upper part of the small intestine.

Sometimes the bile stagnates in the gallbladder. The bile becomes excessively concentrated and gallstones form. Sometimes doctors must remove the gallstones by operating.