Gallstones are solid deposits that form in the gallbladder or bile ducts when substances in bile, such as cholesterol and bilirubin, crystalize.

The gallbladder is a pear-shaped organ located in the upper, right side of the abdomen, just beneath the liver. Small tubes called bile ducts connect the gallbladder to the liver and the small intestine.  The gallbladder stores and releases bile, a greenish-brown fluid produced in the liver and used to help the body digest fats.

A series of ducts move bile from the liver to the upper portion of the small intestine, or duodenum. Bile leaves the liver through the hepatic duct and travels to and from the gallbladder through the cystic duct. The junction of the cystic and hepatic ducts forms the common bile duct, which transports bile to the duodenum. The common bile duct joins the duodenum alongside or with the pancreatic duct, which originates in the pancreas.

Bile contains water, cholesterol, bile salts and other fatty substances called lipids. It also contains waste products, such as bilirubin. When the body needs to digest fat, the gallbladder contracts and pushes bile into the ducts that carry it to the small intestine. There, bile dissolves excess cholesterol and bile salts help to break down fat.

Gallstones may form when the balance of components in bile changes. When there is too much cholesterol, the other components may be unable to dissolve the cholesterol and some of the bile hardens into stones. Diminished protein levels in the bile can also cause the cholesterol to crystalize. In addition, when the gallbladder does not contract regularly and empty itself of bile, the concentration of cholesterol in bile can increase to form stones.

Patients can develop one gallstone or up to several hundred. They may be tiny or as large as a golf ball. Most gallstones are smaller than 1 inch (2.54 centimeters). In some cases, patients have tiny, sandlike gallstones called biliary sludge.

When gallstones form in the gallbladder, the condition is called cholelithiasis. When they form in the bile ducts, the condition is called choledocholithiasis.

Gallstones can block the flow of bile from the liver to the small intestine if they become lodged in ducts. If any of the ducts remain blocked for a significant period of time, severe and possibly even fatal complications can occur in the gallbladder, liver or pancreas. Complications of gallstones may include:

• Common bile duct blockage. Gallstones can block the ducts that lead from the gallbladder, liver or pancreas to the small intestine. This can result in an inflamed gallbladder (cholecystitis) or infection of the bile duct (cholangitis).
  
  
• Pancreatitis (inflammation of the pancreas). This condition typically causes intense pain in the upper abdomen.
  
  
• Gallbladder cancer. People with gallstones face a higher risk of developing gallbladder cancer.