Gallbladder and Biliary Tract
(See also Overview of the Digestive System.)
Bile is a greenish yellow, thick, sticky fluid. It consists of bile salts, electrolytes, bile pigments, cholesterol, and other fats. The gallbladder is the storage sac that holds bile.
Bile flows out of the liver through the right and left hepatic ducts, which come together to form the common hepatic duct. This duct then joins with a duct coming from the gallbladder, called the cystic duct, to form the common bile duct. The pancreatic duct joins the common bile duct just where it empties into the duodenum through the sphincter of Oddi (see Figure: The Digestive System).
Between meals, bile salts are stored in the gallbladder, and only a small amount of bile flows into the intestine. Food that enters the duodenum triggers a series of hormonal and nerve signals that cause the gallbladder to contract. As a result, bile flows into the duodenum and mixes with food contents.
Bile has two important functions: It assists in the digestion and absorption of fats, and it is responsible for the elimination of certain waste products from the body, particularly hemoglobin from destroyed red blood cells and excess cholesterol. Specifically, bile is responsible for the following actions:
Bile salts make cholesterol, fats, and fat-soluble vitamins more soluble (more dissolved), which aids in their absorption.
Bile salts stimulate the secretion of water by the large intestine to help move the contents along.
Bilirubin (the main pigment in bile) is excreted in bile as a waste product of destroyed red blood cells, giving stool a green-brown color.
Drugs and other waste products are excreted in bile and later eliminated from the body.
Various proteins that play important roles in bile's absorptive function are secreted in bile.
Bile salts are reabsorbed by the last portion of the small intestine, extracted by the liver, and resecreted into bile. This recirculation of bile salts is known as the enterohepatic circulation. All the bile salts in the body circulate about 10 to 12 times a day. During each pass, small amounts of bile salts reach the large intestine, where some are reabsorbed and the rest are excreted in the stool.