Gallbladder and Biliary Tract
Bile flows out of the liver through the right and left hepatic ducts (see Section 10, Chapter 133), 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.
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 these actions:
- Bile salts increase the solubility of cholesterol, fats, and fat-soluble vitamins to aid 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 bacteria break them down into various constituents. Some constituents are reabsorbed; the rest are excreted with the stool.
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