102 Hepatic Anatomy and Physiology

Hepatic Anatomy into right and left sides. The right is divided into an anterior (segments V and VIII) and a posterior (segments The liver is the largest solid organ in the body, weighing VI and VII) sector by the right scissura containing the about 1.5kg in an adult. It lies in the right upper quad- right hepatic vein. The left liver is split into an anterior rant of the completely protected by the thoracic (segments III and IV) and posterior (segment II—the rib cage. It extends from the nipple line at the 4th inter- only sector composed of a single segment) sector by the costal space down to the costal margin in the midclavic- left scissura, which runs posterior to the ligamentum teres ular line. It is completely surrounded by a peritoneal and contains the left hepatic vein. Segment I, the caudate membrane (Glisson’s capsule) enveloping the portal lobe, is located on the posterior aspect of the liver not triad structures as it enters the liver. The cephalad aspect visible on frontal view and receives its blood supply from is in contact with both hemidiaphragms, and the caudal both the left and right portal pedicles; bile ducts from surface is in contact with the stomach, duodenum, and segment I also drain into the right and left hepatic ducts. colon. The posterior aspect is in contact with the right The portal vein is a valveless structure formed by kidney and adrenal gland. The gross anatomic landmarks the confluence of the superior mesenteric vein and the include the falciform and the ligamentum teres splenic vein.The portal vein provides approximately 75% hepaticus (round ligament of the liver) separating the left of the total liver blood supply by volume. In the hepato- lateral segment of the liver (segments II and III) from the duodenal ligament, the portal vein is found most com- remaining liver. The round ligament, a remnant of the monly posterior to the and hepatic artery. The , is an external marker for the intrahepatic normal pressure in the portal vein is between 3 and 5mm portion of the left portal vein. The ligamentum venosum, Hg. Because the portal vein and its tributaries are without remnant of the , runs from the intrahep- valves, increases in venous pressure are distributed atic portal vein to the vena cava. It marks the border throughout the splanchnic circulation. In the setting of between the caudate lobe (segment I) and the left lateral portal venous hypertension, portosystemic collaterals sector. The lies on the caudal surface of the develop secondary to the increased pressure. The most liver. The left and right triangular secure the clinically important portosystemic connections include liver to the retroperitoneum.The gastrohepatic omentum those fed through the coronary (left gastric) and short connects the caudal surface with the lesser curvature of gastric veins through the fundus of the stomach and distal the stomach. esophagus to the azygos vein, resulting in gastroe- The functional anatomy is composed of eight segments, sophageal varices. Recanalization of the round liga- each supplied by a single portal triad (pedicle) composed ment/umbilical vein leads to a caput medusa around the of a portal vein, hepatic artery, and a bile duct. The liver umbilicus. Portal hypertension through the inferior is divided into sectors by longitudinal planes by scissurae mesenteric veins and hemorrhoidal plexuses can lead to drawn through each hepatic vein to the vena cava and engorged external hemorrhoids. further divided into segments by a transverse plane at the Most of the venous drainage occurs through three level of the main portal vein bifurcation. Segments are hepatic veins. The right hepatic vein drains segments V, numbered clockwise on frontal view. The main scissura VI, VII, and VIII and enters directly into the vena cava. contains the middle hepatic vein that runs in an antero- The middle hepatic vein drains segments IVA, IVB, V, posterior direction from the gallbladder fossa to the left and VIII and enters into a common orifice with the left side of the vena cava (Cantlie’s line) and divides the liver hepatic vein that drains segments II and III. A scissural

237 238 Part XII. Gastrointestinal Disorders branch of the left hepatic vein may run underneath the adherent to biliary ductule structures and play an impor- . A number of small, short hepatic tant role in bile homeostasis. Hepatic arterioles ulti- veins enter directly into the vena cava from the under- mately feed into the sinusoids and contribute to the surface of the liver in segment I. oxygen gradient across zones one, two, and three. The The hepatic arterial anatomy is part of the portal triad liver is also the largest repository of the reticuloen- and follows the segmental anatomy. The extrahepatic dothelial system. Kupffer cells (tissue-based macro- arterial anatomy can be highly variable. In 50% of the phages) line the sinusoidal spaces and are exposed to population, the common hepatic artery arises from the portal venous blood. celiac trunk, giving off the gastroduodenal artery fol- are highly metabolically active, polarized lowed by a right gastric artery. The proper hepatic artery cells found in platelike orientations in the acinus. They gives rise to the right and left hepatic arteries. There is are covered with microvilli and are in close proximity great variation in hepatic artery anatomy that is impor- with each other, allowing their common membranes to tant to understand and recognize during cholecystec- generate vital canaliculi. They also are in close proximity tomies, portal dissections, and liver resections. Replaced through the to sinusoidal endothe- hepatic arteries are lobar vessels that arise from either lial cells and blood. Sinusoidal endothelial cells are highly the superior mesenteric artery (replaced right hepatic permeable, allowing free flow of both large- and small- artery) or left gastric artery (replaced left hepatic artery). molecular-weight substances into the . The left hepatic artery, regardless of its origin, enters the liver at the base of the round ligament. A replaced or accessory left hepatic artery will run in the lesser Hepatic Physiology omentum anterior to the caudate lobe and is typically very easily identified. Accessory right hepatic arteries The liver is the center of metabolic homeostasis. It serves often supply the posterior sector of the right lobe as the regulatory site for energy metabolism by coordi- (segment VI and VII). An accessory left hepatic artery nating the uptake, processing, and distribution of nutri- typically supplies the left lateral segment. The cystic ents and their subsequent energy products. The liver is artery most commonly arises from the right hepatic crucial to the production and release of a variety of cir- artery but has a variety of common anomalies as well. culating factors critical to the coagulation cascade. The The spaces of Disse and clefts of Mall produce lymph most sensitive tests of liver function are measures of fluid at the cellular level that is collected through sub- coagulation function: international normalized ratio, Glissonian and periportal lymphatics draining into larger factor VII level, and factor V level. The liver also synthe- lymphatics and emptying through the into sizes a wide variety of plasma proteins; most important, the cisterna chyli. The anatomy and physiology of lym- albumin constitutes one seventh of total protein synthe- phatic drainage is important in the development of sis. The liver also makes a variety of acute-phase proteins ascites and in the process of tumor metastasis. Evaluation and cytokines that have important interactions with a of portal lymph nodes must be included during surgical variety of inflammatory, infectious, and regulatory operations for hepatic malignancies to exclude the pres- processes. The liver also synthesizes a large number of ence of extrahepatic nodal disease. proteins, enzymes, and vitamins that participate in a Parasympathetic fibers from the hepatic branches of tremendously broad range of bodily functions. the vagus nerve and both parasympathetic and sympa- The liver is the critical intermediary between dietary thetic fibers derived from the celiac plexus innervate sources of energy and the extrahepatic tissues. The liver the liver and gallbladder, the latter traveling along the accounts for about 4% of total body weight, yet it con- hepatic arteries. Irritation or stretching of the Glisson’s sumes approximately 28% of the total body blood flow capsule or the gallbladder causes referred pain to the and 20% of the total oxygen intake. The liver also right shoulder through the third and forth cervical nerves. expends about 20% of the total kilocalories used by the The microscopic anatomy of the liver is characterized whole body. The liver receives dietary byproducts by the acinar unit involving an afferent portal venule, through the portal circulation, sorts them, metabolizes hepatic arteriole, and a bile ductule flowing antegrade them, and distributes them to the systemic circulation. It along plates of hepatocytes. Portal venous blood flows also has a major role in regulating systemic sources of along sinusoids and contacting hepatocytes through a energy, such as fatty acids and glycerol from adipose perisinusoidal space of Disse; blood in these sinusoids tissues and lactate, pyruvate, and amino acids from skele- flows toward a hepatic venule. Concentration gradients tal muscle.The liver releases two major sources of energy of oxygen and solutes occur along the sinusoidal spaces; into the extrahepatic circulation: glucose and acetoac- three zones have been described, with zone one being etate. Glucose is derived from glycogenolysis of stored closest to the portal triad and zone three being closest to glycogen and from gluconeogenesis from lactate, pyru- the terminal hepatic vein. Hepatic arterioles are closely vate, glycerol, propionate, and alanine. Acetoacetate is 102. Hepatic Anatomy and Physiology 239 derived from the oxidation of fatty acids. Storage lipids, lesterol is synthesized in the liver and is used most impor- such as triacylglycerols and phospholipids, are synthe- tantly for bile salt synthesis. sized and stored as lipoproteins by the liver.These can be The excretion of bilirubin, a product of heme metabo- transported in the systemic circulation to the peripheral lism from erythrocytes, occurs in bile. Bilirubin circulates tissues. These complex and essential functions are regu- bound to albumin in the blood. It is actively taken up lated by hormones, by the overall nutritional state of the by hepatocytes, where it is glucuronidated and actively organism, and by the needs of obligate glucose-requiring secreted into bile. Benign disorders of bilirubin metabo- tissues. The liver also is a critical storage site of glycogen lism include Dubin-Johnson and Rotor’s syndromes, and is essential to the maintenance of systemic glucose which produce conjugated (direct) hyperbilirubinemia. homeostasis through a complex process involving broad Unconjugated hyperbilirubinemia is seen in Crigler- interactions with lipid metabolism. The liver also metab- Najjar type II and Gilbert’s syndromes. Crigler-Najjar olizes lactate, and the Cori cycle is important in main- type I syndrome causes neonatal kernicterus and invari- taining peripheral glucose availability in the setting of ably is fatal. The liver has an immense capacity to metab- anaerobic metabolism. It also is an important modulator olize bilirubin, such that correction of jaundice in hilar of lipid metabolism, performing a critical role in the syn- biliary obstruction requires drainage of only one liver thesis of lipoproteins, triglycerides, gluconeogenesis from sector. Even with complete lobar biliary obstruction, the fatty acids, and cholesterol metabolism. In humans, cho- serum bilirubin level may be normal.