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 (RHD) and the left hepatic duct (LHD). Extrahepatically, the RHD and LHD coalesce to form the common hepatic duct (CHD) which travels within the hepatoduodenal ligament until coming into contact with the cystic duct, the bile duct which connects to the gallbladder. The CHD and cystic duct merge to form the common bile duct (CBD). 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 opens 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 primary function of the biliary tract is to transport and release the 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 fourth 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 differentiate into the cholangiocytes that line the intrahepatic bile ducts.
The blood supply of the biliary system originates from the celiac trunk, which branches off of the abdominal aorta at T12 level, giving rise to the splenic artery, common hepatic artery, and left gastric artery. Arterial supply to the biliary tract is well documented due to its clinical relevance in the liver transplant procedure. In general, the common hepatic artery plays a crucial role in providing oxygenated blood to the liver. Supplies to the biliary tree rely on an intricate network of plexuses from subsidiary branches of larger vessels, including hepatic proper, cystic, gastroduodenal, and posterior superior pancreaticoduodenal arteries. Paracholedocal plexus, as the name implies, supplies the medial and lateral side of the biliary duct, while epicholedocal plexus trails on the top of the duct. However, not all portions of the biliary tree receive equal blood supply from the nearby arteries. For example, the supraduodenal CBD (immediately inferior to the confluence between the cystic duct and the CHD) receives the least amount of blood supply, thus predisposing it to ischemic attack; the implication is especially strong in liver transplantation where the arterial supply is often disrupted.
The venous network that drains the biliary tree is parallel to that of its arterial counterpart. The main difference being that epicholedocal venous plexus drains into epicholedocal plexus. Epicholedocal plexus has a superior and an inferior outlet; the former joins the portal vein near the hilar of the liver, whereas the latter drains into the superior mesenteric vein before merging with the portal vein.
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 finally, the celiac chains.
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.
Different biliary tree configurations have been identified through the study of cadavers, MRCP, and intraoperative cholangiogram. Most commonly, the unity of a horizontally traversed right posterior duct (RPD) and a vertically traversed right anterior duct (RAD) forms the RHD. The RHD then joins with the LHD to create a CHD. The second variant describes a trifurcated configuration where the RPD, RAD, and the LHD join at a single confluence to form a CHD. Rarely, one can see the presence of an accessory duct (Duct of Luschka) that extends from the liver and enters the gallbladder. Configurations such as abnormal emptying of the RPD or direct drainage from the RHD into the cystic duct exist but are less frequently seen.
The cystic duct typically originated from the gallbladder, joining with the CHD to form the CBD. The confluence is found in the middle third of the extrahepatic duct in the majority of the population. However, anatomical variants exist, and one must recognize them during biliary procedures. For example, a low fusion of a long cystic duct and the CHD is seen in approximately 8.6% of study subjects. In contrast, 2.1% of cystic duct fuses with proximal CHD near the liver edge. Other variants, such as the absence of cystic duct or duplicated ducts, have all been reported. 
Bile duct injury is a potential complication of cholecystectomy. Misidentification of critical anatomical structures is a common cause, mainly due to the proximity of the cystic duct, the CHD, and the CBD. The incidence is higher in a laparoscopic than an open procedure. During the operation, it is crucial to identify Calot's Triangle - an anatomical space defined superiorly by the inferior border of the liver, medially by the common hepatic duct, and laterally by the cystic duct. Calot's triangle contains structures such as the cystic artery and Mascagni's node (Lund's node). Rarely, an accessory hepatic duct or right hepatic artery can also be found traversing through this space. To avoid arterial or unwanted biliary injury, surgeons need to establish a "critical view of safety." The critical view of safety comprises of three components: identifying Calot's Triangle, exposing the inferior portion (infundibulum) of the gallbladder, and revealing only the cystic duct and cystic artery for ligation.
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.
The biliary tract provides an outlet to transport bile into the gastrointestinal tract, which facilitates the absorption of fat-soluble nutrients. In general, acute obstruction of the biliary tract can lead to steatorrhea, while chronic obstruction can cause fat-soluble vitamin deficiency (A, D, E, and K). Proximal to the site of obstruction also harbors an opportunity for bacterial infection, as in the case of ascending cholangitis. If left untreated, a chronic biliary obstruction can cause portal hypertension and its associated complications.
Congenital anomalies associated with the biliary tract include biliary atresia, choledochal cysts, and congenital biliary ectasia. Biliary atresia is the most common cause of conjugated hyperbilirubinemia in young infants.  The etiology of the disease is not well understood; however, both genetic and environmental factors have been implicated. Several mechanisms have been proposed, including failure to recanalize during the bile duct formation, and repeated structural insult leading to extensive luminal fibrosis. Patients usually have normal growth but gradually develop jaundice and Catholic stools within two months after birth. The initial evaluation with ultrasound is aimed at excluding other causes of cholestasis, such as a choledochal cyst. Hepatobiliary scintigraphy can delineate the patency of the biliary tree. The liver function test often confers to cholestasis pattern. If surgical intervention is warranted, a definitive diagnosis can be confirmed by an intraoperative cholangiogram. Early surgical intervention with Kasai portoenterostomy procedure improves the disease prognosis. Ultimately, patients will require a liver transplant.
Cholelithiasis, commonly known as gallstones, is a common disease in western populations. 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, patients may also experience the pain in the right shoulder - a phenomenon called "referred pain." Rarely, an obstruction in the cystic duct is severe that the enlarged duct compresses on the nearby common hepatic duct, as seen in Mirrizi syndrome. Patients with Mirrizi syndrome show signs of jaundice in addition to the classic symptoms of cholecystitis.
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