The biliary tree is composed of two parts - the intrahepatic biliary tree and the extrahepatic biliary tree. The ducts that are found inside the liver are called the intrahepatic ducts, while the ducts located outside the liver are known as the extrahepatic ducts.

Extrahepatic biliary tree

The extrahepatic biliary tree is formed by the extrahepatic segments of the right and left hepatic ducts, the common hepatic duct, the cystic duct, and the common bile duct.

The extrahepatic segment of the right hepatic duct is short, while the left hepatic duct is longer and situated more horizontally.

Both hepatic ducts unite and form the common hepatic duct, up to 1.57 inches (4 cm) long tube. The common hepatic duct travels together with the proper hepatic artery and main hepatic portal vein. It is joined by the cystic duct from the gallbladder. Both these ducts form the common bile duct.

The cystic duct is an outflow duct of the gallbladder. It is up to 0.28 inches (7 mm) in diameter and 0.79 - 1.57 inches (2 - 4 cm) long. It emerges from the neck of the gallbladder, runs posteroinferiorly, and to the left side, where it unites with the common hepatic duct.

The mucosa of the proximal portion of the cystic duct consists of 2 to 10 crescent-shaped folds, which create spiral-shaped valves (valves of Heister). These mucosal folds of the cystic duct join the folds in the neck of the gallbladder.

The common bile duct arises near the porta hepatis and is 2.36 to 3.15 inches (6 to 8 cm) long and 0.24 inches (6 mm) wide. Overall, it is divided into four segments according to their localization:

Supraduodenal segment - segment that runs above the superior (first) part of the duodenum; it is found within the hepatoduodenal ligament going together with the proper hepatic artery and main hepatic portal vein; this segment is the easiest to access during surgery;
Retroduodenal segment - segment that is located behind the superior part of the duodenum to the right of the gastroduodenal artery; anteriorly, this segment is crossed by the posterior superior pancreaticoduodenal artery, which is a branch of the gastroduodenal artery;
Retropancreatic segment - segment that goes down behind the head of the pancreas and runs to the right side to reach the descending (second) part of the duodenum; this segment sometimes forms a groove on the pancreatic head; it is also referred to as the intrapancreatic segment because pancreatic tissues usually surround and cover this segment;
Intraduodenal segment - short segment that goes through the wall of the descending part of the duodenum.

As the common bile duct approaches the main pancreatic duct, both ducts merge in a Y-shaped configuration. This configuration forms a short common channel that has a dilation called the ampulla of Vater (hepatopancreatic ampulla). It terminates with an opening in the posteromedial wall of the descending part of the duodenum. The duodenal wall contains a slight mucosal elevation called the major duodenal papilla (papilla of Vater).

The walls of the distal parts of the common bile duct, main pancreatic duct and hepatopancreatic ampulla contain smooth muscle cells. The circumferential organization of these muscle cells forms three sphincters:

Sphincter of the common bile duct (of Boyden)
Sphincter of the main pancreatic duct
Sphincter of the hepatopancreatic ampulla (sphincter of Oddi)

These sphincters regulate the flow of bile and pancreatic secretions and contribute to the anti-reflux system of the duodenal contents.

Note: Besides the main pancreatic duct, the pancreas usually has one more called the accessory pancreatic duct. It drains into the minor duodenal papilla found above the major duodenal papilla. The accessory duct joins the main pancreatic duct if the papilla is absent.

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Extrahepatic biliary tree