Anatomy, Embryology, Anomalies, and Physiology of the Biliary Tract

Intrahepatic Ducts

The anatomy of the biliary tract can be divided into various segments, including the intrahepatic ducts, extrahepatic ducts, gallbladder and CD, and sphincter of Oddi. The anatomy of the intrahepatic ducts is intimately associated with the anatomy of the liver. The lobar and segmental anatomy of the liver is determined by the sequential branching of the portal vein, hepatic artery, and biliary tree as they enter the parenchyma at the hilum. All three of these structures follow approximately parallel courses and bifurcate just before entering the liver. This major bifurcation divides the liver into left and right lobes. According to the Couinaud classification, the caudate lobe is segment I; segments II to IV are on the left; and segments V to VIII are on the right ( Fig. 106.2 ).

The biliary drainage of the right and left liver is into the right and left hepatic ducts, respectively. The left hepatic duct is formed within the umbilical fissure from the union of the three segmental ducts draining the left side of the liver (segments II through IV). The left hepatic duct crosses the base of segment IV (medial segment of the left lobe) in a horizontal direction to join the right hepatic duct and form the common hepatic duct (CHD). The right hepatic duct drains segments V through VIII and is formed from the union of the right posterior and right anterior segmental ducts. The right posterior segmental duct is formed by the confluence of ducts draining segments VI and VII. The posterior segmental duct initially courses in a nearly horizontal direction before descending in a more vertical direction to join the anterior segmental duct. The right anterior segmental duct is formed by the union of the ducts draining segments V and VIII. In approximately 15% to 20% of cases, the right posterior duct drains into the left hepatic duct. The posterior right duct usually passes superior to the right anterior portal vein (80%). The ducts from segments II and III join to the left of the intrahepatic left portal vein, and the segment IV duct joins to the right of the umbilical fissure and forms the main left duct. The main left (horizontal) and right hepatic ducts (vertical) join at the hilum to form the CHD in 56% of the cases. The biliary drainage of the caudate lobe (segment I) is variable. In approximately 80% of the individuals, the caudate lobe drains into both the right and left hepatic ducts. In 15% of cases the caudate lobe drains only into the left hepatic duct, and in the remaining 5% of cases the caudate is drained exclusively by the right hepatic duct.

Extrahepatic Ducts

Most patients have a bifurcation where the right and left hepatic ducts join to form the CHD. This junction may occur as a wide or an acute angle, or the two hepatic ducts may run parallel to each other before joining. In some patients, three hepatic ducts join to form the CHD. Usually, the hepatic ducts meet just outside of the liver parenchyma, with the CD entering 2 to 3 cm distally. Occasionally, the two hepatic ducts do not unite until after the CD has joined the right hepatic duct. The CHD extends for a variable length from the junction of the right and left hepatic ducts to the entrance of the CD into the gallbladder ( Fig. 106.3 ).

The CBD is formed by the union of the cystic and CHDs. The CBD is approximately 8 cm in length, but, like the hepatic duct, it varies in length according to the point of union of the CD and the CHD. The normal diameter of the CBD ranges from 4 to 9 mm. The CBD is considered enlarged if the duct diameter exceeds 10 mm. The upper third, or supraduodenal portion, of the CBD courses downward in the free edge of the lesser omentum, anterior to the portal vein and to the right of the proper hepatic artery. The middle third, or retroduodenal portion, of the CBD passes behind the first portion of the duodenum, lateral to the portal vein and anterior to the inferior vena cava. The lower third, or intrapancreatic portion, of the CBD traverses the posterior aspect of the pancreas in a tunnel or groove to enter the second portion of the duodenum, where it is usually joined by the pancreatic duct. The intramural or intraduodenal portion of the CBD passes obliquely through the duodenal wall to enter the duodenum at the papilla of Vater.

The relationship between the lower CBD and pancreatic duct is variable: (1) the two structures may rarely unite outside the duodenal wall to form a long common channel; (2) the bile duct and pancreatic duct usually join within the duodenal wall to form a short common channel; or (3) the two structures may rarely enter the duodenum independently through separate orifices. The lower portion of the CBD and the terminal portion of the pancreatic duct are enveloped and regulated by a complex sphincter, the sphincter of Oddi. In 5% to 10% of patients who have pancreas divisum, the dorsal pancreatic duct enters the duodenum through an accessory sphincter, whereas the ventral pancreatic duct joins the CBD at the sphincter of Oddi.

The extrahepatic bile ducts contain a columnar mucosa surrounded by a connective tissue layer. The surface is relatively flat, with basal nuclei and an absent or small nucleolus. The lamina propria consists of collagen, elastic fibers, and vessels. Occasional lymphocytes are found, and pancreatic acini and ducts may be seen in the wall of the intrapancreatic portion of the distal CBD. Muscle fibers in the bile duct are sparse and discontinuous. The muscle fibers that are present are usually longitudinal, although occasional circular fibers are observed. The distal CBD begins to develop a more substantial muscle layer in the intrapancreatic portion of the duct, which becomes prominent at the sphincter of Oddi, where distinct bundles of longitudinal and circular fibers are clearly identified.

Gallbladder and Cystic Duct

The gallbladder is a pear-shaped organ that lies on the inferior surface of the liver at the junction of the left and right hepatic lobes between Couinaud segments IV and V ( Fig. 106.4 ). The gallbladder varies from 7 to 10 cm in length and from 2.5 to 3.5 cm in width. The gallbladder's volume varies considerably, being large during fasting states and small after eating. A moderately distended gallbladder has a capacity of 50 to 60 mL of bile but may become much larger with certain pathologic states and can get markedly distended containing up to 300 mL. The gallbladder has been divided into four areas: the fundus, body, infundibulum, and neck. The gallbladder fundus is commonly located at the level of the ninth costal cartilage and the external border of the right rectus muscle. It is covered by peritoneum because it projects beyond the inferior border of the liver. The body of the gallbladder occupies the gallbladder fossa of the liver and has intimate contact with the first and second portions of the duodenum. The infundibulum is the portion of the body between the neck and the point of entrance of the cystic artery (CA); when this portion is dilated, it becomes an asymmetric bulge called the Hartmann pouch. The neck curves forming an S -shaped structure that ultimately becomes the CD. The CA is usually found in this region coursing parallel within the connective tissue that attaches the neck of the gallbladder to the liver.

The gallbladder wall consists of five layers. The innermost layer is the epithelium, and the other layers are the lamina propria, smooth muscle, perimuscular subserosal connective tissue, and serosa. The gallbladder has no muscularis mucosae or submucosa. Most cells in the mucosa are columnar cells, and their main function is absorption, but they also are capable of active secretion. These cells are aligned in a single row, with slightly eosinophilic cytoplasm, apical vacuoles, and basal or central nuclei.

The lamina propria contains nerve fibers, vessels, lymphatics, elastic fibers, loose connective tissue, and occasional mast cells and macrophages. The muscle layer is a loose arrangement of circular, longitudinal, and oblique fibers without well-developed layers. Ganglia are found between smooth muscle bundles. The subserosa is composed of a loose arrangement of fibroblasts, elastic and collagen fibers, vessels, nerves, lymphatics, and adipocytes.

Rokitansky-Aschoff sinuses are invaginations of epithelium into the lamina propria, muscle, and subserosal connective tissue. These sinuses are present in approximately 40% of normal gallbladders and are present in abundance in almost all inflamed gallbladders. The ducts of Luschka are tiny bile ducts found around the muscle layer on the hepatic side of the gallbladder. They are found in approximately 10% of normal gallbladders and have no relation to the Rokitansky-Aschoff sinuses or to cholecystitis.

The CD arises from the gallbladder and joins the CHD to form the CBD (see Fig. 106.3 ). The length of the CD is variable, averaging between 2 and 4 cm. The CD usually courses downward in the hepatoduodenal ligament to join the lateral aspect of the supraduodenal portion of the CHD at an acute angle. Occasionally the CD may join the right hepatic duct, or it may extend downward to join the retroduodenal duct. In addition, the CD may join the CHD at a right angle, may run parallel to the CHD, or may enter the CHD dorsally, on its left side, behind the duodenum, or, rarely, may enter the duodenum directly. The CD contains a variable number of mucosal folds, similar to those found in the neck of the gallbladder. Although referred to as valves of Heister, these spiral folds do not have a valvular function. Variations in the length and course of the CD and its point of union with the CHD are common.

In 1891 Calot described a triangular anatomic region formed by the CHD medially, the CD laterally, and the CA superiorly. Calot triangle is considered by most to comprise the triangular area with an upper boundary formed by the inferior margin of the right lobe of the liver, rather than the CA ( Fig. 106.5 ). A thorough appreciation of the anatomy of Calot triangle is essential during performance of a cholecystectomy because numerous important structures pass through this area. In most instances, the CA arises as a branch of the right hepatic artery within the hepatocystic triangle. A replaced or aberrant right hepatic artery arising from the superior mesenteric artery usually courses through the medial aspect of the triangle, posterior to the CD. Aberrant or accessory hepatic ducts also may pass through Calot triangle before joining the CD or CHD. During performance of a cholecystectomy, clear visualization of the hepatocystic triangle is essential with accurate identification of all structures within this triangle.

Sphincter of Oddi

The entire sphincteric system of the distal bile duct and the pancreatic duct is commonly referred to as the sphincter of Oddi. It regulates the bile and pancreatic juice flow towards the duodenum, preventing the regurgitation of duodenal content into the biliary tree and also diverts bile into the gallbladder leading to its distention. This term is imprecise because the sphincter is subdivided into several sections and contains both circular and longitudinal fibers. The sphincter mechanism functions independently from the surrounding duodenal musculature and has separate sphincters for the distal bile duct, the pancreatic duct, and the ampulla ( Fig. 106.6 ). In more than 90% of the population, the common channel, where the biliary and pancreatic ducts join, is less than 1.0 cm in length and lies within the ampulla. In the rare situation in which the common channel is longer than 1.0 cm or the biliary and pancreatic ducts open separately into the duodenum, pathologic biliary or pancreatic problems are likely to develop. The entire sphincter mechanism is actually composed of four sphincters containing both circular and longitudinal smooth muscle fibers ( Fig. 106.7 ). The four sphincters are the superior and inferior sphincter choledochus, the sphincter pancreaticus, and the sphincter of the ampulla.

Vascular

The hepatic artery represents the 25% of the blood supply to the liver; the rest is provided by the portal vein. The hepatic artery is derived from the celiac trunk in 55% of the cases. The common hepatic and the right or left hepatic arteries may arise from vessels other than the celiac trunk.

The blood supply to the right and left hepatic ducts and upper portion of the CHD is from the CA and the right and left hepatic arteries. The supraduodenal bile duct is supplied by arterial branches from the right hepatic, cystic, posterior superior pancreaticoduodenal, and retroduodenal arteries. The axial blood supply of the supraduodenal bile duct has been emphasized by Terblanche et al. ( Fig. 106.8 ). The most important arteries to the supraduodenal bile duct run parallel to the duct at the 3 and 9 o'clock positions. Approximately 60% of the blood supply to the supraduodenal bile duct originates inferiorly from the pancreaticoduodenal and retroduodenal arteries, whereas 38% of the blood supply originates superiorly from the right hepatic artery and CD artery. Injury to this important axial blood supply may result in the formation of an ischemic ductal stricture. This configuration dictates surgical management when the CBD is injured or is opened purposefully when attempting exploration. Most surgeons would suggest that if more than 50% of the diameter of the CBD is transected it will lead unequivocally to stricture if primarily closed, rendering jejunal limb reconstruction necessary in this situation. Only 2% of the arterial blood supply to the supraduodenal bile duct is segmental (nonaxial). These small segmental arterial branches arise directly from the proper hepatic artery as it ascends in the hepatoduodenal ligament, adjacent to the CBD. The blood supply to the retroduodenal and intrapancreatic bile duct is from the retroduodenal and pancreaticoduodenal arteries.

The CA usually arises as a single branch from the right hepatic artery within Calot triangle ( Fig. 106.9 ). Infrequently, the CA may arise from the left hepatic, common hepatic, gastroduodenal, or superior mesenteric artery. When the CA arises from the right hepatic artery, it usually courses parallel, adjacent, and medial to the CD. However, this relation is far from constant; if the artery arises from the proximal right hepatic artery or from the common hepatic artery, it may lie close to the hepatic duct, which may be injured when the artery is ligated.

As it crosses Calot triangle, the CA often supplies the CD with one or more small arterial branches. Near the gallbladder, the CA usually divides into a superficial branch and a deep branch. The superficial branch of the CA courses along the anterior surface of the gallbladder, whereas the deep branch passes between the gallbladder and liver within the cystic fossa.

The right hepatic artery passes posterior to the CHD as it ascends to the liver in 85% of individuals and anterior to the CHD in the remaining 15%. In approximately 15% of individuals, a replaced or aberrant right hepatic artery originates from the superior mesenteric artery and courses through the medial aspect of Calot triangle, posterior to the CD.

The venous drainage from the hepatic ducts and hepatic surface of the gallbladder is through small vessels that empty into branches of the hepatic veins within the liver. A small venous trunk ascending parallel to the portal vein receives veins draining the gallbladder and bile duct before entering the liver, separate from the portal vein. Venous drainage of the lower portion of the bile duct is directly into the portal vein.

Lymphatic Drainage

Lymphatic vessels from the hepatic ducts and upper CBD drain into the hepatic lymph nodes, a chain of lymph nodes that follows the course of the hepatic artery to drain into the celiac lymph nodes. Lymph from the lower bile duct drains into the lower hepatic nodes and the upper pancreatic lymph nodes. Lymphatic vessels from the gallbladder and CD drain primarily into the hepatic nodes. Two main trunks descending along the lateral borders of the gallbladder join together by an oblique trunk, forming a large “N” on the surface. The trunks located to the left of the gallbladder drain into the cystic node, a constant lymph node located at the junction of the CD and CHD. The right trunks do not enter the node but continue down, joining the bile duct lymphatics. Lymphatic vessels from the hepatic surface of the gallbladder may also communicate with lymphatic vessels within the liver.

Neural Innervation

The gallbladder and biliary tree receive sympathetic and parasympathetic nerve fibers that are derived from the celiac plexus and course along the hepatic artery ( Fig. 106.10 ). The left (anterior) vagal trunk branches into hepatic and gastric components. The hepatic branch supplies fibers to the gallbladder, bile duct, and liver. Sympathetic fibers originating from the fifth to the ninth thoracic segments pass through the greater splanchnic nerves to the celiac ganglion. Postganglionic sympathetic fibers travel along the hepatic artery to innervate the gallbladder, bile duct, and liver. Visceral afferent nerve fibers from the liver, gallbladder, and bile duct travel with sympathetic afferent fibers through the greater splanchnic nerves to enter the dorsal roots of the fifth through ninth thoracic segments. Sensory fibers from the right phrenic nerve also innervate the gallbladder, presumably through the communications between the phrenic plexus and the celiac plexus. This innervation may explain the phenomenon of referred shoulder pain in patients with gallbladder disease.

Burnett et al. described three nerve plexuses within the gallbladder wall: mucosal, muscular, and subserous. There is a decrease in number of ganglion cells from subserous to mucosal plexus. The subserous plexus ganglia are larger and spaced farther apart, unlike the myenteric plexus of the gut.

Biliary Ducts

The anatomy of the extrahepatic biliary tree is highly variable. A thorough knowledge of this variable anatomy is important because failure to recognize the frequent anatomic variations may result in significant ductal injury. Anomalies of the extrahepatic biliary tree may involve the hepatic ducts, CBD, or CD.

Hepatic Ducts

In 57% to 68% of patients, the right anterior and right posterior intrahepatic ducts join and the right hepatic duct unites with the left hepatic duct to form the CHD ( Fig. 106.11 ). Three other common variations are recognized. In 12% to 18% of patients, the right anterior, right posterior, and left hepatic ducts unite to form the CHD. In 8% to 20% of patients, the right posterior and left hepatic ducts join to form the CHD and the right anterior duct joins below the union. In 4% to 7% of patients, the right posterior duct joins the CHD below the union of the right anterior and the left hepatic ducts. In 1.5% to 3% of patients, the CD joins at the union of all the ducts or with one of the right hepatic ducts.

Accessory hepatic ducts may emerge from the liver to join the right hepatic duct, CHD, CD, CBD, or gallbladder ( Fig. 106.12 ). These ducts are present in approximately 10% of individuals. Although accessory hepatic ducts may approach the size of a normal CD, they are often delicate, thin structures that may easily be overlooked. Accessory hepatic ducts often course through Calot triangle and may be injured during dissection in this area. Cholecystohepatic ducts are small biliary ducts that emerge from the liver to enter the hepatic surface of the gallbladder directly. If a cholecystohepatic duct is discovered during dissection of the gallbladder from the cystic fossa, it should be ligated to avoid a postoperative bile leak.

Gallbladder

Some apparent anomalies are acquired but most result from arrested or abnormal development at some stage of embryonic growth. These anomalies vary in their clinical significance: Some are only medical curiosities and require no attempt at correction, whereas others require surgical intervention. The gallbladder anomalies may be divided into three groups based on formation, number, and position ( Box 106.1 ).