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Updated: 03/13/08 01:44 PM |
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| HOME | HEAL | EDUCATE | RESEARCH | DIRECTORY | OUTREACH |
Authors: W. Volwiler, R.A. Willson, A.M. Larson, and J.D. Ostrow
Download Chapter ♦ Download Lecture Slides & Notes
Download Chapter ♦ Download Lecture Slides & Notes
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B. Embryology
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Falciform ligament |
The hepatobiliary system arises during the fourth week of embryogenesis from an endodermal outgrowth of the foregut called the hepatic diverticulum. This diverticulum gives rise to the hepatic parenchyma, gallbladder, cystic duct, common bile duct, and head of the pancreas. Occasional alterations in the normal embryological development give rise to clinical disorders of the liver and biliary system (i.e., liver cysts, congenital cysts of the intrahepatic and extrahepatic bile ducts). An important embryological remnant persists as an anatomical structure of the adult liver: the falciform ligament. It is important both anatomically and clinically because (a) it divides the liver into topographical right and left lobes, (b) it carries the partly obliterated left umbilical vein, which, after birth, offers access to the portal circulation for both angiographic and hemodynamic studies, and (c) it contains small paraumbilical veins which connect the portal venous system with the systemic venous system of the anterior wall. These venous channels may enlarge in patients with portal hypertension.
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C. Gross Anatomy
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Topographic vs. functional anatomy |
The liver is the largest organ in the human body, normally weighing about 1.5 kg (1200-1500 g) and comprises about 1/50 of the total adult body weight. The liver’s functional division differs from its topographic division (Figure 1 & Figure 2). On a visual basis (topographically), the liver is divided into a large right lobe and smaller left lobe by the falciform ligament, which connects the liver to the diaphragm and the anterior abdominal wall. The right lobe is about six times the size of the left. Functionally, however, the liver is divided according to the distribution of its main vascular and biliary channels. These are the “true” anatomic right and left lobes. This functional anatomy is important in surgical decision-making regarding resection of lobes or segments of the liver for pathologic reasons (i.e., tumors, cysts, traumatic rupture), or to use as donor "split grafts" for liver transplantation.
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Figure 1 Distribution of bile ducts (light) and hepatic artery (dark) 
Figure 2 Venous Drainage of the Liver |
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| Hepatic artery supplies most of oxygen despite lower flow than portal vein |
The liver has a dual blood supply (Figs. 1 & 2). The hepatic artery (a branch of the celiac axis) carries well-oxygenated arterial blood to the liver. The second source is the portal vein, which carries nutrient-rich/oxygen-poor venous blood to the liver from the intestines and spleen. From 50 to 80% of the liver’s oxygen supply is furnished by the hepatic artery, the remainder comes from the portal vein. Approximately 20% to 30% of hepatic blood flow is normally derived from the hepatic artery, with the portal vein contributing 70% to 80%. Nearly one-third of the cardiac output passes through the liver.
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Bile ducts perfused only by arteries
Portal triads Opposite flow of blood in sinusoids and bile in canaliculi Venous drainage |
The portal vein, hepatic artery, and the common hepatic bile duct (accompanied by nerves and lymphatics) ascend to the hilum of the liver (Porta Hepatis) where each penetrates the liver parenchyma and bifurcates into right and left branches going to the anatomic right and left lobes, respectively. These vascular and biliary channels divide into progressively smaller branches, until they reach the “portal triads.” The arterial branches first supply the bile ducts, both delivering nutrients and oxygen to the ducts and carrying metabolites from the ductal epithelium to the hepatocytes in the liver cords (the "chole-hepatic shunt"). The bile duct epithelium is susceptible to ischemia because its blood supply is entirely arterial. Each portal triad contains an artery, a portal vein, and one or two bile ducts (as well as lymph vessels and nerves). From the portal triad, blood from the artery and vein empty into the hepatic sinusoids (Fig. 3) which are endothelial-lined channels that run between the cords of hepatocytes, arranged radially from the central vein. In the sinusoids, the blood flows centrally towards the hepatic veins, whereas bile, secreted by the hepatocytes, flows in the canaliculi in the opposite direction, emptying via the Duct of Hering into the interlobular bile duct in the portal triad. The central hepatic veins merge to form progressively larger channels, that ultimately empty into the main hepatic veins that emerge at the posterior, dorsal surface of the liver and promptly enter the inferior vena cava near its point of entry into the right atrium.
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Figure 3 Anatomy of a Traditional Hepatic Lobule Cords (plates) of hepatocytes, 1-2 cells thick, with intervening sinusoids are arranged radially around a central hepatic venule. At the periphery of the lobule are multiple portal triads, each containing a portal venule, an hepatic arteriole, and 1-2 intralobular bile ducts. Bile is secreted by the hepatocytes into an anastomosing network of bile canaliculi, then drains peripherally into ductules at the margins of the portal triads, and from there empties into the interlobular bile ducts. ©American Gastroenterological Assn., Undergraduate Teaching Project, Unit 21, Slide 2. Milner-Fenwick Timonium, MD, 1989. Modified from Levy CM, Fig. 2c in Evaluation of Liver Function in Clinical Practice, Eli Lilly & Co., Indianapolis, IN 1965. |
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| Hepatic lymphatics |
Lymph drains from the liver via superficial and deep channels. The main lymph drainage of the liver is via the deep channels. These exit the liver at the hilum and follow through nodes and trunks along the common bile duct. They merge with lymphatics from the gallbladder, stomach, and duodenum en route to the cisterna chyli and the left thoracic duct, which empties into the left subclavian vein. Channels from the upper one-third (dome) of the liver drain via the right thoracic duct into the right subclavian vein. Lymphatic channels become important in the spread of tumor as well as contributing to the formation of ascites.
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| Hepatic Nerve supply |
The nerve supply of the liver is derived predominantly from the celiac plexus (sympathetic fibers), plus branches from the anterior and posterior vagal trunks (parasympathetic fibers). These nerves help regulate hepatic blood flow (such as vasoconstriction) and visceral pain. Sensory nerves extend from the diaphragm onto the superior portion of the liver. These nerves are part of the phrenic nerves (originating in dermatome segments C3 C5, which also serve cutaneous sensation on the top of the shoulders). This explains the perception of shoulder pain (referred pain) arising from certain diseases of the liver and biliary system (i.e., abscess, tumor) or following liver biopsy.
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| Subdiaphragmatic abscess |
Because of the liver's position under the diaphragm, several potential subphrenic (supra- and infra-hepatic) spaces exist. Localized abscesses may develop in these spaces following intraabdominal sepsis. Knowledge of their location is important for diagnostic and therapeutic drainage procedures.
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Next Section (D): Microscopic Anatomy » |
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