Biliary tract

Biliary tract's main purpose is to transport bile from the liver to the duodenum. Bile is a fluid of yellowish-brown colour which consists of bile salts, bile pigments (bilirubin, biliverdin), fatty acids, lipids, cholesterol etc. It partakes in emulsification of fats. Around 0,5-0,7 l of bile is produced per day.

Biliary tract can be divided by the localisation into intrahepatic (inside of the liver) and extrahepatic (outside of the liver) bile ducts.

Intrahepatic bile ducts
Intrahepatic bile ducts start in the liver as bile capillaries (bile canaliculi) between hepatocytes. They are thin and they are lined just by hepatocytes. Their diameter is approx. 1-2 µm. Bile flows from the centre of the liver lobule to the periphery where intrahepatic bile ductules (canals of Hering) are located. Canals of Hering are lined by cuboidal epithelium. They connect to interlobular bile ducts which are located in the portobiliary space as a part of the portal triad (as well as portal vein and interlobular artery). Interlobular bile ducts are lined by cuboidal epithelium supported by basal lamina. They connect into segmental bile ducts which then form right and left hepatic ducts. Right and left hepatic ducts exit liver at the porta hepatis (together with v. portae and a. hepatica propria).



Extrahepatic bile ducts
Extrahepatic bile ducts connect to intrahepatic bile ducts at the porta hepatis. They start as right hepatic duct (made from the anterior and posterior ramus) and left hepatic duct (made from medial and lateral ramus). They are lined by simple columnar epithelium. Right and left bile ducts then merge together to creat the common bile duct which is about 3-5 cm long and which enters the hepatoduodenal ligament (together with hepatic proper artery on the left and portal vein in the back). Hepatoduodenal ligament is a part of lesser omentum and it makes the anterior margin of omental bursa - omental foramen. The common bile duct connects with the cystic duct (2-3 cm long) from gallblader in the hepatoduodenal ligament and together they create the common bile duct. The lenght of the common bile duct is approx. 6-9 cm.

Common bile duct can be divided into 4 parts according to its position:


 * pars supraduodenalis - located above duodenum;
 * pars retroduodenalis - crossing behind the superior duodenal flexure;''
 * pars pancreatica - located behind the head of the pacreas;''
 * pars intramuralis - part inside the wall of duodenum; the common bile duct and the pancreatic duct come together here and open into the duodenum as major duodenal papilla

In 77 % cases both the bile duct and the pancreatic duct lead into the major duodenal papilla. In the rest of the cases both ducts have their own termination. The bile duct a forms the longitudinal fold of duodenum which is located in the dorsal medial part of the descending part of duodenum. The major duodenal papilla is located In the middle of the longitudinal fold of duodenum. The major duodenal papilla leads into the hepatopancreatic ampulla (ampulla of Vater) where the common bile duct and the pancreatic duct come together. In 50 % cases the ampulla is absent. It is closed by the m. hepatopancreatic sphincter (sphincter of Oddi). There are also other smooth muscle sphincters around both common bile duct and pancreatic duct. Sphincters are contracted when duodenum is empty. When the sphincter around the common bile duct is relaxed and bile flows into the hepatopancreatic ampulla, the sphincter of the pancreatic duct is closed in order to protect pancreas from bile which would activate pancreatic enzymes inside the pancreas and by that it would cause acute pancreatic necrosis.

Around 2 centimeters in cranial direction the major duodenal papilla the accessory pancreatic duct (duct of Santorini) opens into the duodenum as the minor duodenal papilla

Extrahepatic bile ducts are lined by simple columnar epithelium with microvilli, nuclei located in the base of the cell and mucin granules in the apical part of the cell. Cells' lamina propria mucosae is made from loose connective tissue. Tunica muscluaris consists of smooth muscle oriented circularly which forms sphincters along the end of the common bile duct, pancreatic duct and hepatopancreatic ampulla.

Gallbladder
Gallbladder is located beneath the liver in fossa for gallbladder. Surface of the gallbladder that is not in touch with the liver is covered by peritoneum which comes from the visceral surface of the liver. The tunica mucosa of the gallbladder is lined by simple columnar epithelium with lamina propria mucosae made of loose connective tissue. The tunica mucosa forms folds distributed evenly over the lumen of the gallbladder. The tunica muscularis is thin and its smooth muscle cells are oriented circularly. On top of the muscular layer there is the tunica serosa.

The main function of the gallbladder is to store bile produced in the liver and to concentrate it before it is released into the duodenum.

Related articled

 * Liver
 * Portal:Atlas of histology/Digestive tract
 * Diagnostic imaging methods of the gallbladder and biliary tract
 * Operations of the gallbladder and biliary tract
 * Inflammatory diseases of the gallbladder and biliary tract
 * Cholelithiasis
 * Summery of gastrointestinal hormones