Bile, a digestive fluid produced and secreted by the liver, is transported by a series of branching bile ducts known collectively as the biliary tree. At the cellular level, several narrow tubular channels called canaliculi collect the bile generated by each hepatocyte. These canaliculi drain into an intralobular bile duct which collects all the bile from each lobule, the functional unit of the liver. Intralobular ducts then drain into the interlobular ducts which are located between lobules. The interlobular ducts merge to form the two main bile ducts of the liver: the right hepatic duct and the left hepatic duct. Extrahepatically, the right and left hepatic ducts coalesce to form the common hepatic duct which travels within the hepatoduodenal ligament until coming into contact with the cystic duct, the bile duct which connects to the gallbladder. The common hepatic and cystic duct merge to form the common bile duct. The hepatopancreatic ampulla, also called the hepatopancreatic duct or ampulla of Vater, is a spherical structure located at the site of the confluence of the common bile duct and pancreatic duct, marking the entry point of bile into the second portion of the duodenum. This is controlled by the smooth muscle fibers of the sphincter of Oddi which open at the duodenal papilla, allowing bile to flow into the small intestine. Alternatively, bile can travel into the gallbladder for storage via the cystic duct.
The caudate lobe of the liver (segment I) is drained by small ducts from both the right and left lobes.
The major function of the biliary tract is to control the transport and release of bile. The gallbladder stores and concentrates bile. Bile assists with the absorption and digestion of fats and fat-soluble vitamins in addition to the removal of lipid-soluble waste products. The main components of bile are cholesterol, bile salts, and bilirubin. Bilirubin, responsible for the signature yellow-green color of bile, is also excreted in bile and is responsible for the pigmentation of feces.
The biliary ducts originate embryologically from the foregut. Early in the 4th week of development, the hepatic diverticulum arises from the endodermal epithelium of the ventral foregut. The hepatic diverticulum serves as the anlage for the liver, extrahepatic ducts, gallbladder, and ventral pancreas. The hepatic diverticulum is divided into two buds, the cranial bud, and the caudal bud. The intrahepatic bile ducts develop from the cranial bud, whereas the extrahepatic biliary tree develops from the caudal bud. Notably, the hepatic diverticulum must recanalize to form the ducts. Hepatoblasts are stimulated to differentiate into the cholangiocytes that line the intrahepatic bile ducts.
Congenital anomalies associated with the biliary tract include biliary atresia, choledochal cysts, and congenital biliary ectasia. Biliary atresia is due to failure of recanalization during bile duct formation and is associated with significant morbidity and mortality, often progressing to end-stage liver failure. These patients present with severe obstructive jaundice and pale stools shortly after birth. The diagnosis is both confirmed and treated surgically with the Kasai portoenterostomy procedure.
The blood supply of the biliary system originates from the celiac trunk which branches off of the abdominal aorta at T12, giving rise to the splenic artery, common hepatic artery, and left gastric artery. The majority of the blood supply to the biliary tree comes from branches off of the common hepatic artery, including the proper hepatic, cystic, gastroduodenal, and superior pancreaticoduodenal arteries. The cystic artery is also responsible for the blood supply of the gallbladder.
The venous drainage of the biliary ducts themselves is not well described. Many small veins serve the liver and gallbladder. The main vein of the gallbladder, the cystic vein, drains into the portal vein.
Similarly, the lymphatic drainage of the gallbladder is more commonly described than that of the biliary ducts. From the gallbladder, lymph flows through the cystic nodes, hepatic nodes, and celiac nodes.
The biliary tree is innervated by both sympathetic and parasympathetic fibers from the celiac plexus. The splanchnic nerves carry sympathetic fibers, while the vagus nerve carries parasympathetic fibers. Stimulation of splanchnic nerve visceral efferent fibers causes gallbladder relaxation, allowing bile to flow into the gallbladder through the cystic duct for storage.
Conversely, gallbladder contraction with bile secretion is primarily mediated by the hormone cholecystokinin (CCK) which the duodenum secretes in response to the arrival of food in the small intestine. Parasympathetic fibers from the vagus nerve assist CCK in this process.
Approximately two-thirds of patients will have variant anatomical configurations that differ from the classic description of the biliary tree and its blood supply.
Cholecystectomy, surgical removal of the gallbladder, is usually performed laparoscopically. Indications for cholecystectomy include symptomatic cholelithiasis, cholecystitis, porcelain gallbladder, and gallbladder polyps greater than 0.5 cm. Cholecystectomy is recommended for patients with asymptomatic cholelithiasis if there is an increased risk of gallbladder carcinoma or other complications.
Cholelithiasis, the presence of gallstones within the gallbladder, is very common. While the majority of people with gallstones are asymptomatic, biliary pathology is often associated with gallstones. For example, acute cholecystitis is most commonly caused by gallstone obstruction of the cystic duct. This obstruction leads to inflammation of the gallbladder. Classically, the patient will present with right upper quadrant abdominal pain, fever, nausea, vomiting, and an elevated white blood cell count. If the subdiaphragmatic parietal peritoneum is irritated by the inflamed gallbladder, pain may also be felt in the right shoulder. This referred pain is due to stimulation of the phrenic nerve which supplies the diaphragm and originates in the neck (C3, C4, and C5). The pain is referred to the shoulder because the supraclavicular nerves supplying the skin over the shoulder also originate from C3 and C4.
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