The biliary tree (biliary system or biliary tract) is a system of ducts that collect and transport bile from the liver to the duodenum. Bile is a yellowish-green liquid that helps with digesting fats in the small intestine. It is stored and concentrated in the gallbladder (Read more!).

The gallbladder is connected to the common bile duct via the cystic duct. The bile is then further released into the descending part of the duodenum to aid in the digestion of fats by breaking them into smaller fat droplets. This process happens in response to a hormone called cholecystokinin secreted by cells of the duodenum.

Overall, the biliary tree is composed of two parts - the ducts that are found inside the liver (intrahepatic biliary tree) and ducts located outside the liver (extrahepatic biliary tree).

Intrahepatic biliary tree

The intrahepatic biliary tree is formed by bile canaliculi, canals of Hering, bile ductules and various ducts surrounded by hepatic and connective tissue cells. The smallest unit of the intrahepatic biliary tree is the bile canaliculus (bile capillary).

The bile canaliculus is a thin bile tube surrounded by hepatocytes, and it receives and collects hepatocyte-secreted bile. The canal of Hering is a larger intrahepatic bile ductule surrounded by three types of cells - hepatocytes, cholangiocytes, and hepatic progenitor (stem) cells. In contrast, bile ductules are intrahepatic bile tubes that are lined only by the cholangiocytes.

Various bile ductules join together, forming interlobular bile ducts, and then these merge into area bile ducts that further form segmental bile ducts. The segmental ducts combine to form sectoral ducts. Finally, sectoral bile ducts fuse and create the right and left hepatic ducts.

Extrahepatic biliary tree

The extrahepatic biliary tree is formed by the extrahepatic segments of the right and left hepatic ducts, common hepatic duct, cystic duct, gallbladder, and the common bile duct. The segment of the right hepatic duct outside the liver is short, while the left hepatic duct is longer and situated more horizontally.

The right and left hepatic ducts unite and form the common hepatic duct that is about 1.18 to 1.57 inches (3 - 4 cm) long tube. It further descends and is joined by the cystic duct that emerges from the neck of the gallbladder. The union of both ducts forms the common bile duct.

The common bile duct starts near the porta hepatis and is about 2.36 to 3.15 inches (6 to 8 cm) long. It is divided into four segments according to their localization:

Supraduodenal segment - located within the hepatoduodenal ligament; it runs above the superior (first) part of the duodenum;
Retroduodenal segment - located behind the superior part of the duodenum to the right of the gastroduodenal artery;
Retropancreatic segment - goes down on the posterior surface of the pancreatic head and runs to the right side to reach the descending (second) part of the duodenum; it is also referred to as the intrapancreatic segment because, most often, tissues of the pancreas surround and cover this segment;
Intraduodenal segment - a short portion of the common bile duct that goes through the wall of the duodenum.

The common bile duct descends posterior to the duodenum within the hepatoduodenal ligament. As it approaches the main pancreatic duct, both ducts merge in a Y-shaped configuration. This configuration forms a short channel that has a dilation called the ampulla of Vater (hepatopancreatic ampulla).

The ampulla of Vater pierces the wall of the duodenum and terminates with an opening in the posteromedial wall of the descending part of the duodenum. Here, the duodenal wall contains a slight mucosal elevation called the major duodenal papilla (papilla of Vater). The mucosa in the terminal part of the duct contains circumferential folds that have an anti-reflux function.

Note: To read more about the biliary system, please visit our 3D Biliary system article!

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Biliary ducts