Anatomy of the liver

General

The liver is the largest organ in the body, weighing 1.5 kg in the average 70-kg male. Its position under the right hemi diaphragm allows it to he protected from trauma by the costal margin. The liver parenchyma is entirely covered by a thin capsule and by visceral peritoneum in all but the posterior surface of the liver, termed the ‘bare area’. At first appearance the liver is divided into two main lobes: a large right lobe which comprises three-quarters of the liver parenchyma and a smaller left lobe the remaining quarter. Surgical resection of these lobes would be termed a right or left lobectomy.

Ligaments and peritoneal reflections

The liver is fixed in the right upper quadrant by peritoneal reflections which form ligaments. On the superior surface of the left lobe is the left triangular ligament. Dividing the anterior and posterior folds of this ligament allows the left lobe to mobilised from the diaphragm and the left lateral wall of the inferior vena cava (IVC) to be exposed. Large veins drain venous blood from the diaphragm to the hepatic veins and IVC at this level. The right triangular ligament fixes the entire right lobe of the liver to the undersurface of the right hemidiaphragm. Division of this ligament allows the liver to be rnobilised from under the diaphragm and rotated to the left. Another major supporting structure is the falciforrn ligament, which runs from the umbilicus to the liver between the right and left lobes, passing into the interlobar fissure. Embryologically this ligament contained the umbilical vein which carried maternal nutrients to the liver of the foetus. From the fissure it passes anteriorly on the surface of the liver attaching it to the posterior aspect of the anterior abdominal wall. On the superior aspect of the liver the falciform liga­ment divides into two leaves, between which is loose areolar tissue and some small vessels. Division of this layer allows exposure of the suprahepatic IVC lying within a thin sheath of fibrous tissue. The final peritoneal reflection is between the stomach and the liver. This lesser omentum is often thin and fragile but contains the hilar structures in its free edge.

Liver blood supply

The blood supply to the liver is unique, being derived 80 per cent from the portal vein and 20 per cent from the hepatic artery. The arterial blood supply in most individuals is derived from the coeliac trunk of the aorta where the hepatic artery arises along with the splenic artery. After branching to form the gastroduodenal artery it branches at a very variable level to produce the right and left hepatic arteries. The right artery supplies the majority of liver parenchyma and is therefore the larger of the two arteries. There are many anatomical variations which are essential knowledge for safe surgery on the liver. The blood supply to the right lobe of the liver may be partly or completely supplied by a right hepatic artery arising from the superior mesenteric artery. This vessel passes posterior to the uncinate process and head of pancreas, and runs to the liver on the posterior wall of the bile duct. Similarly, the arterial blood supply to the left lobe of liver may be derived from the coeliac trunk via its left gastric branch. This vessel runs between the lesser curve of the stomach and the left lobe of the liver in the lesser omentum (Fig. 52.1).

Structures in the hilum of the liver

The hepatic artery, portal vein and bile duct are present within the free edge of the lesser omentum or the ‘hepato­duodenal ligament’. To expose these structures requires the peritoneum overlying the hilar triad to be divided followed by division of small vessels and an extensive lymphatic plexus. The standard relationship of these three structures is for the bile duct to be within the free edge, the hepatic artery to be above and medial, and for the portal vein to lie posteriorly. Within this ligament the common hepatic duct is joined by the cystic duct at varying levels to form the com­mon bile duct. The common hepatic artery branches at a variable level within the ligament to form two or often three main arterial branches to the liver. The right hepatic artery often crosses the bile duct anteriorly or posteriorly before giving rise to the cystic artery. Multiple small hepatic arterial branches provide blood to the bile duct, principally from the right hepatic artery. The portal vein arises from the joining of the splenic vein with the superior mesenteric vein behind the neck of pancreas. It rarely branches until close to the liver parenchyma but has some important tributaries, including the left gastric vein joining just above the pancreas and a tributary anteriorly from the head of pancreas.

Division of structures at the hilum

At the hilum the major structures are divided into right and left branches. The right and left hepatic ducts arise from the hepatic parenchyma and join to form the common hepatic duct. The left duct has a longer extra hepatic course of approximately 2 cm. Once within the liver parenchyma the duct accompanies the branches of the hepatic artery and portal vein within a fibrous sheath. The portal vein often gives off two large branches to the right lobe which are acces­sible outside the liver for a short length before giving a left portal vein branch which runs behind the left hepatic duct.

Venous drainage of the liver

The venous drainage of the liver is via the hepatic veins into the IVC. The vena cava lies within a groove in the posterior wall of the liver. Above the liver it immediately penetrates the diaphragm to join the right atrium, whereas below the liver parenchyma there is a short length of vessel before the insertion of the renal veins. The inferior hepatic veins are short vessels which pass directly between the liver parenchyma and the anterior wall of the IVC. The major venous drainage is through three large veins which join the IVC immediately below the diaphragm. Outside the liver these vessels are sur­rounded by a thin fibrous layer. The right hepatic vein can be exposed fully outside the liver, but the middle and left veins usually join within the liver parenchyma. Immediately adja­cent to the retro hepatic IVC lies the right kidney and adrenal gland. The right adrenal vein drains into the IVC at this level, usually via one main branch. The IVC can be mobilised fully from the retroperitoneal tissues and in the healthy state there are no significant vessels in this tissue plane. The right inferior phrenic vein often drains to the IVC via the right hepatic vein.

The internal anatomy of the liver

Safe liver surgery has been enormously facilitated by a better understanding of the internal anatomy of the liver which has been extensively investigated by the French anatomist Couinaud. He described the liver as being divided into eight segments (Fig. 52.2). Each of these segments can be consider­ed as functional units, each with a branch of hepatic artery, portal vein and bile duct, and drained by a branch of the hepatic vein. The overall anatomy of the liver is divided into a functional right and left along the line between the gallblad­der fossa and the middle hepatic vein. Liver segments (v—viii) to the right of this line are supplied by the right hepatic artery and the right branch of portal vein, and drain bile via the right hepatic duct. The liver parenchyma to the left of this line (segments i—iv) is functionally the left liver and is supplied by the left branch of the hepatic artery and the left portal vein branch, and drains bile via the left hepatic duct.

The hepatic lobules

The functional units within the liver segments are the liver lobules. These comprise plates of liver cells separated by the hepatic sinusoids, large thin-walled venous channels which carry blood to the central vein, a tributory of the hepatic vein from the portal tracts which contains branches of the hepatic artery and portal vein. During passage through the sinusoids the many functions of the liver take place, including bile formation which is channeled in an opposite direction to the blood flow to drain via the bile duct tributaries within the portal tracts.

Embryology

The liver is a foregut structure and forms as a small endodermal bud early in gestation. The cell population within the bud grows rapidly and forms two cell populations which differentiate into the liver and extra hepatic biliary tree and gallbladder. The liver cells are bipotential and may develop into hepatocytes or intrahepatic ductal cells. The liver endothelium is derived from the vitelline and umbilical veins which merge with the endodermal bud to form the liver sinusoids. The supporting connective tissue, haemopoetic cells, which are important during intrauterine life, and the Kupffer cells are derived from the mesoderm of the septum transversum.