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Anatomy - Right hemihepatectomy - general and visceral surgery
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Functional liver anatomy
In terms of surface relations, on the superior parietal surface the insertion of the ligamentum teres and the falciform ligament, and the sagittal fissure on the inferior aspect divide the liver into a large right and small left lobe (about 80:20 by volume), this gross anatomical division does not correspond with the functional anatomy of the liver. The functional structure of the liver is governed by the branches of the structures in the porta hepatis: Portal vein, hepatic artery and hepatic duct. Not only do these three anatomical structures branch, mostly in matching fashion, in the porta hepatis but also in the hepatic parenchyma. Because the blood supply and bile drainage of each hepatic segment are completely independent of all other segments, each segment may be resected without compromising the function of the remaining liver.
Therefore, the concept of “functional anatomy” refers to the hepatic substructure based on circumscribed, hemodynamically independent areas of parenchyma, the knowledge of which is essential for surgical strategy in liver resections.
Portal vein and hepatic veins
The functional structure of the liver rests on the portal vein ramifying into individual, completely independent subunits, the liver segments.
In the porta hepatis, the portal vein usually bifurcates into a left and right lobar branch. The outer margin of these areas is defined by Cantlie’s line, i.e., the vertical plane extending from the inferior vena cava posteriorly to the middle of the gallbladder fossa anteriorly. The right lobar branch divides into an anteromedial and posterolateral branch supplying the liver segments V/VIII and VI/VII respectively. The left lobar branch of the portal vein courses transversely to the left, continuing anteriorly as umbilical branch and terminating at the insertion of the ligamentum teres in the recessus of Rex. The left lobar branch of the portal vein gives off branches to both left lateral segments II and III, and to the median segments IVa and IVb. The small caudate lobe is special because it may receive sizable tributaries from the left and also the right lobar branch of the portal vein.
Couinaud identified eight portovenous liver segments which are numbered clockwise starting with the caudate lobe:
Segment I………………………..caudate lobe
Segments I/II/III………………….left lateral hepatic lobe
Segment IV………………………left paramedian sector (quadrate lobe)
Segments I/II/III/IV………………left lobe of liver
Segments V/VIII…………………right paramedian sector
Segments VI/VII…………………right lateral sector
Segments V/VI/VII/VIII…………..right lobe of liver
Three major venous trunks traverse the liver caudocephalad; these are the right, median and left hepatic veins which divide the liver into a total of four hepatic sectors. The left hepatic vein almost exclusively drains the left lateral hepatic lobe, and just before its junction with the inferior vena cava it usually merges with the middle hepatic vein which courses along the cava-gallbladder line. The right hepatic vein travels between the posterolateral and anteromedial segments. The caudate lobe has its own venous drainage, the Spieghelian veins, comprising multiple small posterior veins draining directly into the inferior vena cava.
In reality, the regularity of the venous branching described by Couinaud is seen only in few cases, and there are numerous branch variants supplying areas of varying size.
The portal hila of liver segments II, III and IV are extrahepatic and can be easily exposed in the anterior section of the left umbilical fissure. The portal hila of the right liver segments are intrahepatic; while there are exceptions at times, these mostly pertain to segment VI. The anatomy of the hepatic veins is even more variable than that of the portal vein.
While normally the common hepatic artery arises from the celiac axis, in rare cases its origin is directly with the aorta or superior mesenteric artery. After giving off the gastroduodenal artery, the proper hepatic artery bifurcates in the porta hepatis into the left and right hepatic arteries. Quite often, there are even more extrahepatic branches, such as the artery for liver segment IV, which usually arises from the left hepatic artery just before the umbilical fissure. Variants of this normal anatomy of the hepatic arteries have been described in about 30% of cases.
The extrahepatic section of the left hepatic duct is about 3 cm to 5 cm long and arises in the umbilical fissure from the junction of the two bile ducts draining segments II and III. In most cases the bile duct from segment IV will join proximal to this junction. With about 25%, the most common anatomical variant is a common junction of the bile duct from segment IV with those bile ducts draining segments II and III. This variant has clinical ramifications in left hemihepatectomy, split liver and living-donor liver transplants because it may easily impair the bile drainage from segment IV.
Barely 1 cm long, the right hepatic duct is rather short and drains bile from segments V, VI, VII and VIII via an anterior and posterior branch. Sometimes the right hepatic duct may be missing altogether. The variant junctions of the posterior branch are clinically relevant because this branch may empty directly into the cystic duct, common hepatic duct or common bile duct.
Regional lymph nodes
The liver has two lymphatic systems:
1. Most of the liver lymph (90%) drains into the lymph nodes at the porta hepatis and from there via the celiac lymph nodes into the intestinal trunk.
2. The second system, a subserous network of lymphatics (10%), drains the superficial aspects of the phrenic surface and the area nuda. The lymph passes through the diaphragm into the superior phrenic lymph nodes and from there via mediastinal lymphatics into the right venous angle.