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101 Liver Anatomy The average adult liver weighs approximately 1,200 to 1,600 grams. The liver is located in the right upper quadrant beneath the rib cage. The inferior border of the liver is the costal margin, and the superior portion lies just beneath the diaphragm. The liver is located at the level of the fifth rib on the right and the sixth rib on the left. Most of the liver is encapsulated except for the gallbladder bed, the porta hepatis, and the posterior surface adjacent to the inferior vena cava (IVC). Ligaments are formed from peritoneal reflections. The major ligaments supporting the liver are the coronary ligaments (attach liver to diaphragm), the triangular ligaments (attach liver to diaphragm), the falciform ligament (connects liver to diaphragm and anterior abdominal wall), the ligamentum teres (contains the left umbilical vein), the gastrohepatic ligament, the hepatoduodenal ligament (contains the portal vein, common bile duct, and hepatic artery), the hepatocolic ligament, and the hepatorenal ligament. The functional unit of the liver is the hepatic lobule. Lobules are composed of a central vein surrounded by four to six terminal portal triads (portal vein, hepatic artery, and bile duct) to form a polygon-shaped unit. Hepatocytes are placed single file, radiating outward from the central vein. Endothelial-lined sinusoids run between each row of hepatocytes. The lateral walls of the hepatocytes form bile canaliculi, which flow toward the portal triads. The hepatocytes are divided into three zones traveling from the perimeter of the lobule to the center. Zone 1 (periportal zone) is located at the perimeter closest to the portal triads. This zone is rich in oxygen and nutrients and is the least susceptible to hypoxic injury. Zone 2 (intermediate zone) is located in the middle. Zone 3 (perivenular zone) is the most distant from the portal triads and, thus, has the least amount of oxygen and nutrients. It is this zone that is primarily affected during hypoxic insults (i.e., centrilobular necrosis). Blood is delivered to the liver from the portal venous system and from the systemic system via the hepatic artery. Blood is drained from the hepatic sinusoids to the hepatic veins and then back to the systemic system through the IVC. The portal vein forms from the connection of the superior mesenteric vein and the splenic vein. The portal vein runs in a superior direction behind the duodenum to enter the posterior portion of the hepatoduodenal ligament. The portal vein branches into a right lobar and left lobar branch at the porta hepatis. The right lobar branch runs through the liver tissue and then makes anterior and posterior divisions. These vessels divide further into anterosuperior, anteroinferior, posterosuperior, and posteroinferior segments. The left lobar branches run through the liver tissue for some distance before dividing into superior and anteroinferior branches. The anteroinferior branch is the larger of the two. It continues to divide into a lateral inferior branch and makes medial superior and medial inferior divisions. In most cases, the common hepatic artery originates from the celiac axis off of the abdominal aorta. At the superior edge of the duodenum, the common hepatic artery gives off the right gastric artery as well as the gastroduodenal artery. From here, the common hepatic artery continues on as the proper hepatic artery within the anterior medial portion of the hepatoduodenal ligament. The proper hepatic artery divides into the left and right hepatic arteries at the porta hepatis. The right hepatic artery gives rise to the cystic artery before entering the hepatic parenchyma. The hepatic arteries tend to follow the same course as the portal tributaries within the liver itself. The medial hepatic artery arises from the left hepatic artery where it enters the quadrate lobe. Common variations in the arterial anatomy include the origin of the right hepatic artery from the superior mesenteric artery (17%) and the origination of the left hepatic artery from the left gastric artery (23%). The central hepatic veins of the hepatic lobule interconnect to form sublobular veins. The sublobular veins coalesce to form collecting veins. The collecting veins 235 236 unite to form three major hepatic vein conduits (i.e., the left, right, and middle veins). The right hepatic vein drains the right posterolateral segments and superior portion of the anteromedial segments. The right hepatic vein empties separately into the IVC. The left hepatic vein drains the left superolateral and inferolateral segments of the liver and then empties directly into the IVC. The middle hepatic vein drains the inferior portion of the right anteromedial segments as well as the left inferomedial portions of the liver. The middle vein unites with the left hepatic vein in 60% of cases. The division of the intrahepatic biliary system follows the course of the portal vein divisions. On the right side, the right anterosuperior and anteroinferior ducts unite to form the right anterior bile duct. Likewise their posterior counterparts combine to form the right posterior bile duct. The right hepatic duct is formed when the two anterior and posterior segments unite before the porta hepatis. The left hepatic duct is formed from the union of the lateral duct and the medial duct. The lateral duct is composed of the superior lateral and inferior lateral ducts.The medial duct arises from the union of the medial superior and inferior ducts. The right and left hepatic ducts join outside of the liver at the porta hepatis to form the common hepatic duct. The common hepatic duct travels within the hepatoduodenal ligament and then joins with the cystic duct to form the common bile duct. Original descriptions of liver anatomy were based on lobar divisions. The lobes were defined by ligamentous attachments as well as fissures and grooves. Modern Part XII. Gastrointestinal Disorders descriptions organize the liver anatomy according to the arrangement of hepatic vasculature and bile ducts (Couinaud’s system). Couinaud’s system provides a better anatomic road map for hepatic surgery. In this system, the liver is divided into eight segments, each with its own pedicle (portal vein, hepatic artery, and bile duct). The segments are organized further into four sectors determined by the hepatic veins. The sectors are separated by three scissurae that run in the course of the hepatic veins. The four sectors can be grouped into components of the right and left liver. The middle scissura (also called Cantlie’s line) runs anterior to posterior between the gallbladder fossa to the left of the IVC. It divides the liver into the left and right hemiliver. The middle scissura contains the middle hepatic vein. The right scissura contains the right hepatic vein and divides the right liver into anterior and posterior segments. The anterior portion contains liver segments V and VIII. The posterior portion contains segments VI and VII. Likewise the left liver is divided into anterior and posterior segments by the left scissura. The left scissura contains the left hepatic vein and is located posterior to the ligamentum teres. The anterior portion is composed of segments III and IV, and the posterior segment is made up of only segment II. Segment I (the caudate lobe) is located on the under surface of the liver anterior to the IVC. The caudate lobe receives vascular inflow and biliary drainage from both the left and right systems. The caudate lobe drains venous blood directly into the IVC via multiple small venules.