101 Anatomy

The average adult liver weighs approximately 1,200 to artery. Blood is drained from the hepatic sinusoids to the 1,600 grams. The liver is located in the right upper quad- hepatic veins and then back to the systemic system rant beneath the rib cage. The inferior border of the liver through the IVC. is the costal margin, and the superior portion lies just The portal vein forms from the connection of the supe- beneath the diaphragm. The liver is located at the level rior mesenteric vein and the splenic vein. The portal vein of the fifth rib on the right and the sixth rib on the left. runs in a superior direction behind the duodenum to Most of the liver is encapsulated except for the gallblad- enter the posterior portion of the hepatoduodenal liga- der bed, the porta hepatis, and the posterior surface adja- ment. The portal vein branches into a right lobar and left cent to the inferior vena cava (IVC). are lobar branch at the porta hepatis. The right lobar branch formed from peritoneal reflections. The major ligaments runs through the liver tissue and then makes anterior and supporting the liver are the coronary ligaments (attach posterior divisions. These vessels divide further into liver to diaphragm), the triangular ligaments (attach liver anterosuperior, anteroinferior, posterosuperior, and pos- to diaphragm), the falciform (connects liver to teroinferior segments. The left lobar branches run diaphragm and anterior ), the ligamentum through the liver tissue for some distance before dividing teres (contains the left umbilical vein), the gastrohepatic into superior and anteroinferior branches. The anteroin- ligament, the (contains the ferior branch is the larger of the two. It continues to portal vein, common bile duct, and hepatic artery), the divide into a lateral inferior branch and makes medial hepatocolic ligament, and the hepatorenal ligament. superior and medial inferior divisions. The functional unit of the liver is the hepatic lobule. In most cases, the common hepatic artery originates Lobules are composed of a central vein surrounded by from the celiac axis off of the abdominal aorta. At the four to six terminal portal triads (portal vein, hepatic superior edge of the duodenum, the common hepatic artery, and bile duct) to form a polygon-shaped unit. artery gives off the right gastric artery as well as the gas- Hepatocytes are placed single file, radiating outward troduodenal artery. From here, the common hepatic from the central vein. Endothelial-lined sinusoids run artery continues on as the proper hepatic artery within between each row of hepatocytes. The lateral walls of the anterior medial portion of the hepatoduodenal liga- the hepatocytes form bile canaliculi, which flow toward ment. The proper hepatic artery divides into the left and the portal triads. The hepatocytes are divided into three right hepatic arteries at the porta hepatis. The right zones traveling from the perimeter of the lobule to hepatic artery gives rise to the cystic artery before enter- the center. Zone 1 (periportal zone) is located at the ing the hepatic parenchyma. The hepatic arteries tend to perimeter closest to the portal triads. This zone is rich follow the same course as the portal tributaries within in oxygen and nutrients and is the least susceptible to the liver itself. The medial hepatic artery arises from the hypoxic injury. Zone 2 (intermediate zone) is located left hepatic artery where it enters the quadrate lobe. in the middle. Zone 3 (perivenular zone) is the most Common variations in the arterial anatomy include the distant from the portal triads and, thus, has the least origin of the right hepatic artery from the superior amount of oxygen and nutrients. It is this zone that is pri- mesenteric artery (17%) and the origination of the left marily affected during hypoxic insults (i.e., centrilobular hepatic artery from the left gastric artery (23%). necrosis). The central hepatic veins of the hepatic lobule inter- Blood is delivered to the liver from the portal venous connect to form sublobular veins. The sublobular veins system and from the systemic system via the hepatic coalesce to form collecting veins. The collecting veins

235 236 Part XII. Gastrointestinal Disorders unite to form three major hepatic vein conduits (i.e., the descriptions organize the liver anatomy according to the left, right, and middle veins).The right hepatic vein drains arrangement of hepatic vasculature and bile ducts the right posterolateral segments and superior portion (Couinaud’s system). Couinaud’s system provides a of the anteromedial segments. The right hepatic vein better anatomic road map for hepatic surgery. In this empties separately into the IVC. The left hepatic vein system, the liver is divided into eight segments, each with drains the left superolateral and inferolateral segments of its own pedicle (portal vein, hepatic artery, and bile duct). the liver and then empties directly into the IVC. The The segments are organized further into four sectors middle hepatic vein drains the inferior portion of the determined by the hepatic veins. The sectors are sepa- right anteromedial segments as well as the left inferome- rated by three scissurae that run in the course of the dial portions of the liver. The middle vein unites with the hepatic veins. The four sectors can be grouped into com- left hepatic vein in 60% of cases. ponents of the right and left liver. The middle scissura The division of the intrahepatic biliary system follows (also called Cantlie’s line) runs anterior to posterior the course of the portal vein divisions. On the right side, between the gallbladder fossa to the left of the IVC. It the right anterosuperior and anteroinferior ducts unite to divides the liver into the left and right hemiliver. The form the right anterior bile duct. Likewise their posterior middle scissura contains the middle hepatic vein. The counterparts combine to form the right posterior bile right scissura contains the right hepatic vein and divides duct.The right hepatic duct is formed when the two ante- the right liver into anterior and posterior segments. The rior and posterior segments unite before the porta anterior portion contains liver segments V and VIII. The hepatis. The left hepatic duct is formed from the union of posterior portion contains segments VI and VII. Likewise the lateral duct and the medial duct. The lateral duct is the left liver is divided into anterior and posterior seg- composed of the superior lateral and inferior lateral ments by the left scissura. The left scissura contains the ducts.The medial duct arises from the union of the medial left hepatic vein and is located posterior to the ligamen- superior and inferior ducts. The right and left hepatic tum teres. The anterior portion is composed of segments ducts join outside of the liver at the porta hepatis to form III and IV, and the posterior segment is made up of only the common hepatic duct. The common hepatic duct segment II. Segment I (the caudate lobe) is located on travels within the hepatoduodenal ligament and then the under surface of the liver anterior to the IVC. The joins with the cystic duct to form the common bile duct. caudate lobe receives vascular inflow and biliary drainage Original descriptions of liver anatomy were based on from both the left and right systems. The caudate lobe lobar divisions. The lobes were defined by ligamentous drains venous blood directly into the IVC via multiple attachments as well as fissures and grooves. Modern small venules.