Bile that is produced in the liver is released in the small bile ducts. These small bile ducts merge with other bile ducts and eventually end in the left or right hepatic duct and exit the liver (Fig. 9.1).

The right and left hepatic duct merges at the level of the liver hilum and together form the common hepatic duct. Further distally the cystic duct branches off and runs to the gallbladder. The hepatic duct is called common bile duct from that point on. In the last few centimeters, the common bile duct runs through the pancreas and merges with the pancreatic duct. The point where the common bile duct and pancreatic duct merge and open in the duodenum is called papilla of Vater or major duodenal papilla (Fig. 9.2).

The sphincter of Oddi surrounds the most distal part of the common bile duct and pancreatic duct and controls the outflow of bile and pancreatic juices. The sphincter of Oddi consists of circular and longitudinal muscle layers, and the largest part of the sphincter is located within the duodenal wall.

The liver produces approximately 1 l bile every day. In fasting conditions the bile flows to the gallbladder, where it is stored until the next meal. When nutrients and fat in particular reach the duodenum, the hormone cholecystokinin (CCK) is released. This hormone stimulates hepatic bile production and promotes contractions of the gallbladder, which results in emptying of the gallbladder and flow of bile through the bile ducts into the duodenum. Bile is a mixture of bile salts, cholesterol, phospholipids, bile pigments, and water. Bile salts together with fat and fat-soluble contents from the foods form micelles that make the fat and fat-soluble contents soluble in water and allow absorption in the intestine. In case bile does not reach the bowel, for example, because of an obstruction of the bile ducts by a stone or tumor, much less fat absorption will take place. An absolute shortage of bile can occur after prolonged percutaneous drainage of the bile ducts.

The vast majority of bile produced in the bowel is absorbed again in the terminal ileum, and this is called the enterohepatic cycle. A patient can also develop a shortage of bile when a large part of ileum is missing, for example, after a large resection and reuptake of bile is not possible anymore.

The main function of the gallbladder is the storage of bile. Because the gallbladder also absorbs water from the bile, concentration of the bile takes place. Sometimes the gallbladder has been surgically removed, usually because of problems related to bile stones. This situation however does not result in problems in fat digestion and absorption.

The contractions of the gallbladder are related to its function. The muscles in the gallbladder wall contract, causing the bile to get squeezed out. The duration and force of the contractions are dependent on the concentration of fat in the ingested meal. High-fat meals require a lot of bile and result in the strongest gallbladder wall contractions.

In fasting conditions the migrating motor complex (MMC) controls motility of the small bowel but also the gallbladder. A 90-min cycle exists of motor quiescence (phase I), increasing motor activity (phase II) and a short period of strong contractions (phase III). In phase I the muscles of the gallbladder wall are relaxed and the pressure in the gallbladder is low. Most bile that is produced in the liver flows into the gallbladder, and only a small amount flows through the bile ducts into the duodenum. When the bowels enter phase III, strong contractions of the gallbladder wall start causing bile to flow out of the gallbladder. At the end of phase III, the gallbladder relaxes again. Bile thus contributes chemical cleansing to the mechanical cleaning provided by the MMC.