Liver

Fissures of the liver

Knowledge of the fissures of the liver is essential for understanding liver surgery. Three major fissures, not visible on the surface, run through the liver parenchyma and harbour the three main hepatic veins (main, left and right portal fissures). Three minor fissures are visible as physical clefts of the liver surface (umbilical, venous and fissure of Gans).

Sectors and segments of the liver

Sectors

The sectors of the liver are made up of between one and three segments: right lateral sector = segments VI and VII; right medial sector = segments V and VIII; left medial sector = segments III and IV (and part of I); left lateral sector = segment II (Fig. 68.5). Segments are numbered in an ante-clockwise spiral centered on the portal vein with the liver viewed from beneath, starting with segment I up to segment VI, and then back clockwise for the most cranial two segments VII and VIII (Fig. 68.6).

Fig. 68.5 Segments of the liver (after Couinaud). A, superior view; B, posterior view; C, anterior view; D, inferior view. The segments are sometimes referred to by number (name) – I (caudate) (sometimes subdivided into left and right parts called segment IX); II (left lateral superior); III (left medial inferior); IV (left medial superior) (sometimes subdivided into superior and inferior parts); V (right medial inferior); VI (right lateral inferior); VII (right lateral superior); VIII (right medial superior).

Fig. 68.6 Segments of the liver seen on axial CT scan. A, Contrast enhanced CT shows the left (L), middle (M), and right (R) hepatic veins at the superior aspect of the liver marking the left main and right portal fissures. B, Inferior to this the caudate lobe (segment I) lies between the inferior vena cava (IVC) and the main portal vein (PV). The left portal vein (LPV) separates segment II superiorly from segment III inferiorly. C, The right portal vein (RPV) divides segments V and VI inferiorly (C) from segments VII and VIII superiorly (B).

Segment I

Segment I corresponds to the anatomical caudate lobe and lies posterior (dorsal) to segment IV with its left half directly posterior to segments II and II and its medial half surrounded by major vascular branches. The Glissonian sheaths to segment I arise from both right and left main sheaths: the segment therefore receives vessels independently from the left and right portal veins and hepatic arteries, and it drains independently into the inferior vena cava by multiple small branches (referred to as the lower group). They nearly always arise in the lower and occasionally from the middle third, but never from the upper third of the segment. The bile ducts draining the segment are closely related to the confluence of the right and left hepatic ducts such that excision of central bile duct tumours usually requires removal of segment IV.

Segment II

Segment II is the only segment in the left lateral sector of the liver and lies postero-lateral to the left fissure. It often has only one Glissonian sheath and drains into the left hepatic vein. Rarely, a separate vein drains directly into the inferior vena cava.

Segment III

Segment III lies between the umbilical fissure and the left fissure and is often supplied by one to three Glissonian sheaths: it drains into the left hepatic vein. The vein of the falciform ligament can provide an alternative drainage route for segment III.

Segment IV

Segment IV lies between the umbilical fissure and the main fissure, anterior to the dorsal fissure and segment I. Segment IV is supplied by three to five Glissonian sheaths, of which the majority arise in the umbilical fissure; their origins are often close to those that supply to segments II and III. Occasionally segment IV is supplied by branches from the main left pedicle. The main venous drainage segment is into the middle hepatic vein; the segment can also drain into the left hepatic vein through the vein of the falciform ligament.

Segment V

Segment V is the inferior segment of the right medial sector and lies between the middle and the right hepatic veins. Its size is variable, as are the number of Glissonian sheaths that supply it. Venous drainage is into the right and middle hepatic veins.

Segment VI

Segment VI forms the inferior part of the right lateral sector posterior to the right portal fissure. It is often supplied by two to three branches from the right posterior Glisson’s sheath, but occasionally the Glisson’s sheath to segment VI can arise directly from the right pedicle. Venous drainage is normally into the right hepatic vein, but may be via the right inferior hepatic vein directly into the inferior vena cava.

Segment VII

Segment VII forms the superior part of the posterior sector and lies behind the right hepatic vein. The sheaths to segment VII are often single. The venous drainage is into the right hepatic vein; occasionally the segment can drain through the right middle hepatic vein directly into the inferior vena cava.

Segment VIII

Segment VIII is the superior part of the right anterior sector. The right anterior sectoral sheaths end in segment VIII and supply it after giving off branches to segment V. The venous drainage is to the right and middle hepatic veins.

Segment IX

Segment IX is a recent subdivision of segment I, and describes that part of the segment that lies posterior to segment VIII.

Supports of the liver

The liver is stabilized and maintained in its position in the right upper quadrant of the abdomen by both static and dynamic factors. Richelme & Bourgeon (1973) suggested a three-tier classification of the anatomical factors: the suspensory attachments at the posterior abdominal wall to the inferior vena cava, hepatic veins, coronary and triangular ligaments (primary factors); the support provided by the right kidney, right colonic angle and duodenopancreatic complex (secondary factors); the attachment to the anterior abdominal wall and diaphragm by the falciform ligament (tertiary factors).

The surgical implications of these different factors are important for traumatologists in terms of achieving a better understanding of the pathophysiology of liver trauma. The inferior vena cava and the supra-hepatic veins, especially the right hepatic vein, appear to be the most important anatomical structures that support the bulk of the liver. Other factors which are responsible for the maintenance of the position of the liver within the abdominal cavity include positive intra-abdominal pressure and the movement of the diaphragm during respiration.

Peritoneal attachments and ligaments of the liver

The liver is attached to the anterior abdominal wall, diaphragm and other viscera by several ligaments which are formed from condensations of the peritoneum.

Porta hepatis, hepatoduodenal ligament and hilar plate

The porta hepatis is a deep fissure on the inferior surface of the liver. It is situated between the quadrate lobe in front and the caudate process behind, and contains the portal vein, hepatic artery and hepatic nervous plexuses as they ascend into the parenchyma of the liver, and the right and left bile ducts and some lymph vessels as they emerge from the liver. The hepatic ducts lie anterior to the portal vein and its branches, and the hepatic artery with its branches lies between the two (Fig. 68.7). All these structures are enveloped in the perivascular fibrous capsule, the hepatobiliary capsule of Glisson, a sheath of loose connective tissue which surrounds the vessels as they course through the portal canals in the liver and is also continuous with the fibrous hepatic capsule. The dense aggregation of vessels, supporting connective tissue, and liver parenchyma just above the porta hepatis is often referred to as the ‘hilar plate’ of the liver (Fig. 68.8). Understanding of the concept of hilar plate is important in surgical approaches to the hilar structures. Division or lowering of the hilar plate are essential for surgical access to the left hepatic duct. The hepatic artery, bile duct and portal vein extend from the porta hepatis towards the duodenum in the free edge of the hepatoduodenal ligament, which forms the anterior boundary of the epiploic foramen. Rapid control of the vessels entering the porta hepatis can be obtained by dividing the lesser omentum to the left of these structures and passing a tape around them from left to right through the epiploic foramen (a ‘Pringle’ manoeuvre).

Fig. 68.7 Cross-section of the structures at the porta hepatis.

Fig. 68.8 Axial CT of the porta hepatis. The hepatic ducts lie anteriorly, the portal vein posteriorly, and hepatic artery between the two.

The left hepatic duct remains extrahepatic as it runs down to the bifurcation along the base of segment IV (the quadrate lobe). This extrahepatic length of duct is particularly useful when performing high biliary duct reconstructions where a length of jejunum is anastomosed to form a biliary-enteric bypass, to treat strictures of the common hepatic duct.

Hepatic artery

In adults the hepatic artery is intermediate in size between the left gastric and splenic arteries. In fetal and early postnatal life it is the largest branch of the coeliac axis. The hepatic artery gives off right gastric, gastroduodenal and cystic branches as well as direct branches to the bile duct from the right hepatic and sometimes the supraduodenal artery (Fig. 68.9A). After its origin from the coeliac axis, the hepatic artery passes anteriorly and laterally below the epiploic foramen to the upper aspect of the first part of the duodenum. It may be subdivided into the common hepatic artery, from the coeliac trunk to the origin of the gastroduodenal artery, and the hepatic artery ‘proper’, from that point to its bifurcation. It passes anterior to the portal vein and ascends anterior to the epiploic foramen between the layers of the lesser omentum. Within the free border of the lesser omentum the hepatic artery is medial to the common bile duct and anterior to the portal vein. At the porta hepatis it divides into right and left branches before these run into the parenchyma of the liver. The right hepatic artery usually crosses posterior (occasionally anterior) to the common hepatic duct (Fig. 68.9B). This close proximity often means that the right hepatic artery is involved in bile duct cancer earlier than the left hepatic artery. Occasionally the right hepatic artery crosses in front of the common bile duct and may be injured in surgery of the common bile duct. It almost always divides into an anterior branch supplying segments V and VIII, and a posterior branch supplying segments VI and VII. The anterior division often supplies a branch to segment I and the gallbladder. The segmental arteries are macroscopically end-arteries although some collateral circulation occurs between segments via fine terminal branches.

Fig. 68.9 The hepatic artery. A, Branches. B, Relations of the hepatic artery, bile duct and portal vein to each other in the lesser omentum: anterior aspect, portion of the liver removed.

A small number of normal variants are important to demonstrate angiographically because they may influence surgical and interventional radiological procedures. A vessel that supplies a lobe in addition to its normal vessel is defined as an accessory artery. A replaced hepatic artery is a vessel that does not originate from an orthodox position and provides the sole supply to that lobe. Rarely a replaced common hepatic artery arises from the superior mesenteric artery (Fig. 68.10) and is identified at surgery by a relatively superficial portal vein (reflecting the absence of a common hepatic artery that would normally cross in front of the vein). More commonly a replaced right hepatic artery or an accessory right hepatic artery arises from the superior mesenteric artery (Fig. 68.11). In this case they run behind the portal vein and bile duct in the lesser omentum and can be identified at surgery by pulsation behind the portal vein. The accessory right hepatic artery may be injured during resections of the pancreatic head because the artery lies in close proximity to the portal vein. Occasionally, a replaced left hepatic artery or an accessory branch arises from the left gastric artery: these vessels provides a source of collateral arterial circulation in cases of occlusion of the vessels in the porta hepatis but may also be injured during mobilization of the stomach as it lies in the upper portion of the lesser omentum. Rarely, accessory left or right hepatic arteries may arise from the gastroduodenal artery or aorta. The presence of replaced arteries can be lifesaving in patients with bile duct cancer: because they are further away from the bile duct they tend to be spared from the cancer, making excision of the tumour feasible. Knowledge of these variations is also important in planning whole and split liver transplantation.

Fig. 68.10 Most common variants of the main hepatic artery.

Fig. 68.11 Hepatic arteriogram. A, A selective hepatic arteriogram shows normal left hepatic artery branches and small right hepatic artery branches. B, The right hepatic artery is arising from the origin of the superior mesenteric artery.

Variations in the intrahepatic arteries are common and may be surgically important. For example, the segment VI artery most commonly arises from the left hepatic artery, but in about 10–20% of cases it arises from the right hepatic artery or the main hepatic artery. Failure to recognize this variation may compromise the blood supply to segment IV following right hepatectomy, and is especially important following right lobe donation for live donor liver transplantation.

Veins

The liver has two venous systems. The portal system conveys venous blood from the majority of the gastrointestinal tract and its associated organs to the liver (see Ch. 60). The hepatic venous system drains blood from the liver parenchyma into the inferior vena cava.

Portal vein

The portal vein begins at the level of the second lumbar vertebra and is formed from the convergence of the superior mesenteric and splenic veins (Fig. 68.12; see also Fig. 71.4 and Fig. 60.8. It is approximately 8 cm long and lies anterior to the inferior vena cava and posterior to the neck of the pancreas. It lies obliquely to the right and ascends behind the first part of the duodenum, the common bile duct and gastroduodenal artery. At this point it is directly anterior to the inferior vena cava. It enters the right border of the lesser omentum, ascends anterior to the epiploic foramen to reach the right end of the porta hepatis and then divides into right and left main branches which accompany the corresponding branches of the hepatic artery into the liver. In the lesser omentum the portal vein lies posterior to both the common bile duct and hepatic artery. It is surrounded by the hepatic nerve plexus and accompanied by many lymph vessels and some lymph nodes.

Fig. 68.12 Coronal CT of the portal vein and superior mesenteric vein.

The main extrahepatic tributaries of the portal vein are the coronary or the left gastric vein, which ends in the left margin of the portal vein, and the posterior superior pancreatoduodenal vein nearer to the head of the pancreas. The portal vein divides into right and left branches at the hilum (Fig. 68.13). The left portal vein has a longer extraparenchymal course (4–5 cm) and tends to lie slightly more horizontally than the right portal vein, but is often of smaller calibre. It has horizontal and vertical portions. The horizontal portion runs along the base of segment VI and often gives branches to segment I and sometimes to segment VI in this part of its course. The branch to segment II continues laterally but the main left portal vein takes a more anterior and vertical course in the umbilical fissure (the vein of the umbilical fissure) where it gives branches to segments III and IV and receives the obliterated left umbilical vein (ligamentum teres). The majority of the supply to segment IV comes from the left portal vein, and only occasionally from the right via proximal branches of the main vein or branches from veins to segments V or VIII. The right portal vein is only 2–3 cm in length and usually divides into a right medial (anterior) sectoral division supplying segments V and VIII, and a right lateral (posterior) sectoral division supplying segments VI and VII. The medial division may give a branch to segment I.

Fig. 68.13 The main portal vein and its intraheptic branches. (Right lateral = right posterior; right medial = right anterior.)

Variations usually involve the right portal vein: absence of a right portal vein with the resulting portal trifurcation in the form of left portal vein, right medial and right lateral portal veins, is present in 10–15% of livers. Occasionally the right medial vein arises from the left portal vein, a variant which is important to remember during left sided liver resection. The portal trifurcation has implications for split liver and live donor liver transplantation, where its presence might be considered as a relative contra-indication. On rare occasions, the portal bifurcation is absent, in which case the main portal vein enters the liver giving off the right segmental branches and then turns left to supply the left lobe of the liver (a contraindication to major liver surgery). Occasionally one or more of the segmental branches of the right lobe (especially segment IV) arises proximally.

Porto-systemic shunts

Increased pressure within the portal venous system may result in dilatation of the portal venous tributaries: a reversal of flow may occur where these veins form anastomoses with veins which drain into the systemic venous circulation. The common sites where porto-systemic shunts may occur, and the associated clinical implications, are listed in Table 68.1.

Table 68.1 Common sites of occurrence of porto-systemic shunts, and associated clinical implications.

Portal veins	Systemic veins	Clinical presentations
Left gastric and lower oesophageal veins	Lower branches of oesophageal veins that drain into azygos and accessory hemiazygos veins	Oesophageal or gastric varices
Superior rectal veins	Middle and inferior rectal veins that drain into internal iliac and pudendal veins	Rectal varices
Persistent tributaries of left branch of portal vein, running in ligamentum teres	Peri-umbilical branches of superior and inferior epigastric veins	‘Caput medusae’
Intraparenchymal branches of right branch of portal vein, lying in liver tissue exposed in ‘bare area’	Retroperitoneal veins that drain into lumbar, azygos and hemiazygos veins	Retroperitoneal dilated veins are at risk during surgery or interventional procedures
Omental and colonic veins in the region of the hepatic and splenic flexure	Retroperitoneal veins in the region of the hepatic and splenic flexure
Patent ductus venosus connected to the left branch of the portal vein	Inferior vena cava	Is rare

Hepatic veins

The liver is drained by three major hepatic veins into the suprahepatic part of the inferior vena cava and a multitude of minor hepatic veins that drain into the intrahepatic inferior vena cava. The three major veins are located between the four major sectors of the liver (Figs 68.6A, 68.14 and 68.15).

Fig. 68.14 Sagittal ultrasound of the middle hepatic vein. Middle hepatic vein draining into the inferior vena cava.

Fig. 68.15 Arrangement of the hepatic venous territories. Multiple lower group veins may be present. Individual segments may drain into more than one hepatic venous territory.

Right hepatic vein

This is the longest and the largest vein and also the most variable. It is usually single, but occasionally it remains as two trunks until it terminates by draining into the inferior vena cava. The right hepatic vein runs into the intersectoral plane between the right medial and right lateral sectors. It drains the whole of segments VI and VII and parts of segments V and VIII. The extent of its contribution to the drainage of segments V and VIII is variable, and depends upon the size of the veins that drain these segments into the middle hepatic vein. The right hepatic vein is formed near the anterior and inferior edge of the liver and lies in a coronal plane through most of its course. It drains into the inferior vena cava near the upper border of the caudate lobe. Of all the three major veins the right hepatic vein is the one which is the most variable in its size not only due to the differential contribution to segments V and VIII drainage along with the middle hepatic vein but also due to the occasional presence of an accessory right middle and right inferior veins.

Middle hepatic vein

The middle hepatic vein lies along the plane between the right medial and the left medial sectors of the liver. It usually joins with the left hepatic vein and terminates into the inferior vena cava as a common trunk. It drains the central part of the liver receiving constant branches from segments IV, V and VIII. The sizes of the branches draining segments V and VIII are variable and are of surgical importance in terms of right lobe living donation.

Left hepatic vein

The left hepatic vein lies between the left medial and left lateral sectors of the liver. It drains segments II, III and occasionally IV. Small veins draining segment II and occasionally the superior part of segment IV may drain directly into the inferior vena cava in the minority of livers. Usually a major tributary of the left hepatic vein, the umbilical fissure vein runs in the intersegmental plane between segments III and IV and contributes to their drainage. Occasionally the vein draining segment III ends separately in the confluence of the left and middle hepatic veins. These variations in the venous drainage are of significance in terms of split liver transplantation and live donor liver transplantation.

Minor veins

Segment I veins drain directly into the inferior vena cava and vary in number from one to five. Since it has an independent drainage from the rest of the liver in patients with Budd–Chiari syndrome, where all the major hepatic veins are blocked, segment I often continues to drain effectively and undergoes compensatory hypertrophy. Rarely there is an accessory right middle or inferior hepatic vein. When they are present they are of surgical importance especially if they are of more than 5 mm in diameter; they drain segments V and VI independently of the three major veins and therefore any tumour involving the three major veins can be safely resected as long as venous drainage from the accessory veins is ensured. In live donor and split liver transplantation these veins must be individually anastomosed to the recipient inferior vena cava to ensure adequate venous drainage.

Transjugular intraparenchymal porto-systemic shunt (TIPS) procedure for portal hypertension

In extreme cases of chronic portal hypertension, large calibre anastomoses between portal and systemic circulations may be formed within the liver parenchyma using a balloon catheter, introduced via the internal jugular vein and under radiological control, to puncture across and rupture through a thinned strip of liver parenchyma between hepatic veins and dilated portal branches.

Lymphatics

Lymph from the liver has abundant protein content. Lymphatic drainage from the liver is wide and may pass to nodes above and below the diaphragm. Obstruction of the hepatic venous drainage increases the flow of lymph in the thoracic duct. Hepatic collecting vessels are divided into superficial and deep systems.

Lobulation of the liver

The structural unit of the liver is the lobule: a roughly hexagonal arrangement of plates of hepatocytes, separated by intervening sinusoids which radiate outward from a central vein, with portal triads at the vertices of each hexagon (Fig. 68.16). The central vein is a tributary of the hepatic vein that drains the tissue. In some species, the classic lobular units are delimited microscopically by distinct connective tissue septa. However, the lobular organization of the human liver is not immediately evident in histological sections: the lobules do not have distinct boundaries, and connective tissue is sparse. The plates do not pass straight to the periphery of a lobule like the spokes of a wheel but run irregularly as they anastomose and branch.

Detailed studies of human liver, using three-dimensional reconstruction and morphometric analysis combined with histopathological observations, have revealed a highly ordered arrangement of functional units, the hepatic (portal) acini. Each acinus is an approximately oval mass of tissue, oriented around the afferent vascular system, i.e. a terminal branch of a hepatic arteriole and portal venule, and with its long axis defined by the territory between two adjacent central veins. It includes the hepatic tissue served by these afferent vessels, and is bounded by the territories of other acini.

The acinar definition of hepatic micro-organization has clarified important problems of liver histopathology, especially the development of zones of anoxic damage, glycogen deposition and removal, and of toxic trauma, which are all related to the direction of blood flow and thus tend to follow the acinar pattern. There are also real metabolic differences between hepatocytes within the acini, and they have been divided into three zones: zone 1 (periportal) is nearest to the terminal branches of afferent vessels; zone 2 is the intermediate zone; and zone 3 is the area closest to the central venous drainage.

Blood supply

Preterminal hepatic arterioles in the portal canals branch to convey arterial blood to the sinusoids by several routes. The main pattern is via a fine capillary plexus which drains to branches of the portal veins. Some arterial blood passes directly to the hepatic sinusoids, bypassing these capillary plexuses, but this represents only a small part of the total flow. Sinusoids thus contain mixed venous and arterial blood. Central veins from adjacent lobules form interlobular veins which unite as hepatic veins and drain to the inferior vena cava. Hepatic veins draining the tissue run quite separately with respect to the portal triad system, freely crossing the boundaries of triad territories.

Hepatic plates (cords)

The endothelial linings of the sinusoids are separated from hepatocytes forming the hepatic plates by a narrow gap, the perisinusoidal space of Disse, which is normally about 0.2–0.5 μm wide, but which distends in anoxic conditions. The space contains fine collagen fibres (mostly type III, with some types I and IV), the microvilli of adjacent hepatocytes, and occasional non-myelinated nerve terminals. There is no basal lamina within the space of Disse.

Minute bile canaliculi form nets with polygonal meshes in the hepatic plates. Each polygonal hepatocyte is surrounded by canaliculi (see below) except on the surfaces, at least two per cell, that face sinusoids. Hepatic plates thus enclose a network of canaliculi that pass to the lobular periphery, where they join to form narrow intralobular ductules (terminal ductules or the canals of Hering) lined by squamous or cuboidal epithelium. The intralobular ductules enter bile ductules, lined by cuboidal or columnar cells, in the portal canals. The flow of bile is thus towards the periphery of lobules, in the opposite direction to the blood flow, which is centripetal.

Cells of the liver

Cells of the liver include hepatocytes, hepatic stellate cells (also known as perisinusoidal lipocytes, or Ito cells), sinusoidal endothelial cells, macrophages (Kupffer cells), the cells of the biliary tree (cuboidal to columnar epithelium) and connective tissue cells of the capsule and portal tracts (Figs 68.16B and 68.17).

Fig. 68.17 Hepatic microstructure. Sinusoidal endothelial cells are not shown.