Figure 1.1  Schematic diagram of the segmental anatomy of the liver.

There are no real discrepancies between the ‘areas’ of Healey and Schroy and the ‘segments’ of Couinaud on the right; however, the division of the left hemiliver is more controversial. Thus, Couinaud described the left hepatic vein ‘scissura’ dividing obliquely segment 2 and segment 3, with the position of the umbilical vein and falciform ligament being irrelevant. This actually leaves a much larger Couinaud ‘sector’ between the middle and left hepatic veins than is currently appreciated, and is not the common line of surgical division [7].

1.1.3.1  CAUDATE (SPIEGEL’S) LOBE^*

This lobe (segment 1) engulfs the inferior vena cava with a prominent fissure separating it from the left lobe proper, in which lies the obliterated ductus venosus. The caudate lobe is predominantly to the left side of the cava and extends between the portal vein and vena cava to merge with the right lobe as the caudate process. Superiorly, there may be a retrocaval component or at least a dense fibrous ligament attaching it to the left crus of the diaphragm. Venous input derives from both the right (usually posterior) and left portal veins branching from the confluence. There is a similar bilateral arterial input and bile drainage. The venous drainage is, however, obviously different from the other segments, being via 3–10 veins draining directly into the cava. Typically, there is a dominant left-sided vein in about half, and several smaller ones arranged along the length of the cava. This independent venous drainage accounts for selective caudate hypertrophy and parenchymal preservation when there is chronic hepatic venous occlusion (Budd–Chiari syndrome) (see Chapter 23).

At one point, the existence of a further independent segment (IX or 9) was proposed by Couinaud, among others [8,9], as equivalent to the right paracaval part of the caudate lobe and caudate process. However, as it is difficult to define specific vessels or biliary drainage for this segment, the concept has not been widely accepted.

1.1.4  Porta hepatis

This is the gateway to the liver, containing its entire vascular inflow and biliary drainage. The region is bounded by the quadrate (segment 4) lobe anteriorly and the caudate (segment 1) lobe posteriorly. On the left, it merges into the recess of Rex and umbilical fissure, which may be open or closed, by an isthmus of liver tissue joining segments 3 and 4. On the right, it is somewhat obscured by the gallbladder and cystic duct, but if these are dissected free, there is usually a ventral gallbladder fossa with the anterior bifurcation of the right vascular pedicle (to segments 5 and 8) and the more dorsally placed sulcus of Rouviere ^* (incisura dextra) containing the posterior vascular elements (to segments 6 and 7).

All these structures are enveloped by a condensation of connective tissue. The term hilar plate is used for this on its anterior aspect with the umbilical and cystic plates on either side. ‘Lowering’ the hilar plate implies an anterior dissection typically to expose the left hepatic duct, usually the most superficial structure. The right and left pedicles, consisting of duct, artery and vein, are bound together by a continuation of the Glissonian^† liver capsule, making surgical ligation or stapling en masse easier.

1.1.5  Portal vein

The portal vein supplies about 75% of the total blood flow to the liver, but only about 50% of its oxygen requirements. It is formed by the union of the superior mesenteric vein (SMV) and the splenic vein behind the neck of the pancreas. The large left gastric (coronary) vein and inferior mesenteric vein (in about 60% of subjects) typically join the splenic vein, but may drain directly into the medial aspect of the portal vein. There are also pancreatic veins on both the medial and lateral aspects, but it is relatively free of tributaries anteriorly. It then ascends in the free posterior aspect of the lesser omentum to divide into right and left portal veins. Small venous branches from this bifurcation emerge from the posterosuperior aspect and supply the caudate lobe. The right portal vein is shorter and divides outside or just inside the liver substance into anterior (to segments 5 and 8) and posterior (to segments 6 and 7) sectional (formerly sectorial) branches (Figure 1.2). The former tends to divide in a vertical plane, while the latter’s division is more horizontal. The left portal vein is more horizontal and passes into the left side of the porta hepatis, where it actually becomes the obliterated umbilical vein. Within the umbilical fissure, branches from the right side supply segment 4, and from the left supply segments 2 and 3 and segment 1 (Figure 1.2). The vein to segment 2 is usually solitary, while there are up to three veins to segment 3.

Figure 1.2  Schematic diagram of the division of the portal vein. RiAn, right anterior portal vein; RiPo, right posterior portal vein. Note: Rex fossa may be open or closed; relative size of veins exaggerated for effect.

Variations of the normal pattern are also seen. These include a trifurcation at the confluence (6%–15%), the right anterior branch originating from the main left portal vein (~7%), the right posterior branch originating from the main portal vein and, finally, absence of the left portal vein entirely [10]. The venous supply in the latter variation is entirely from the right side, through the substance of the liver.

1.1.6  Hepatic artery

Hepatic arterial input is pulsatile, making up about 25% of the liver blood flow and providing about 50% of its oxygen requirement. Ligation of the hepatic artery can be tolerated in the otherwise intact human liver, although not in most other species. This is due to an extensive collateral network, and previous authors have described up to 26 different extrahepatic anatomical pathways [11–13].

The liver is supplied by the artery of the foregut – the coeliac trunk or axis. This is a short vessel given off from the front of the aorta, between the diaphragmatic crura at the level of T12 and surrounded by a dense collection of preaortic lymph nodes. Three named vessels arise almost immediately: the left gastric, the splenic and the common hepatic arteries. The left gastric artery curves upwards around the lesser sac to reach the lesser curve of the stomach and folds of the lesser omentum. The tortuous splenic arises at the upper border of pancreas and follows it towards the splenic hilum. It generally divides into four or five vessels to supply the spleen with some vessels (vasa brevia or short gastric arteries), continuing along the gastrosplenic ligament to supply the greater curve and fundus of the stomach and contributing to the gastroepiploic arterial arcade. It is also the principal blood supply to the body and tail of the pancreas via the arteria pancreatica magna.

The common hepatic artery passes to the right, along the upper border of the pancreas as far as the first part of the duodenum. It then gives off a right gastric artery, which runs within the leaves of the lesser omentum to complete the lesser curve arterial arcade with the left gastric artery. A larger gastroduodenal artery is given off to pass behind the duodenum and supply this and the head of the pancreas, while also contributing to the right gastroepiploic arcade. This leaves the common hepatic artery proper within the free edge of the lesser omentum. Although commonly found on the left side of the portal triad, it is very variable. It branches into a right and left hepatic artery, the former either passing behind (80%) or in front (20%) of the bile duct to access the portal pedicle to the right hemiliver. The left enters the left side of the porta hepatis, with a fairly consistent vessel given off to segment 4 as the middle hepatic artery. Alternatively, segment 4 receives a branch from the right hepatic artery, crossing the principal plane. As a general rule, each artery is an end artery without intraparenchymal anastomoses [5,12]. However, this supposition was based upon cadaveric injection studies rather than clinical practice, and Mays in 1974, for instance, first showed that in vivo translobar collaterals do open up with ligation of one or the other of the arteries [13].

The above textbook description occurs only in about 60% of subjects (Table 1.2), and variations in hepatic arterial anatomy are common and can be surgically important. These include either accessory arteries (i.e. additional to the standard named arteries) or replacement arteries. The largest series of arterial variants is that of Hiatt’s, first published in 1994 from Los Angeles, and is based on a study of 1000 donor liver dissections [14]. More recently, there have been several large series based on mesenteric angiography [15,16]. The most common, and therefore most important, variations include

1.  Right hepatic artery (usually a replacement) from the superior mesenteric artery (SMA) ascending the portal triad either on the right side or closely behind the common bile duct (~15%)

2.  Left hepatic artery (usually an accessory) from the left gastric artery ascending in the lesser omentum (~10%)

Exceptional anomalies include a right accessory artery arising from the left hepatic artery running behind the portal vein [17], an accessory right hepatic artery passing inferolaterally to supply the caudal elements of segments 5 and 6 [23], and an accessory right artery arising from the left hepatic artery high in the porta and passing behind the portal vein confluence [18].

Table 1.2  Bile duct variation

Source:  Frequencies and concept taken from Blumgart L, Han L, in Jarnagin WR (ed.), Blumgart’s Surgery of the Liver and Biliary Tract, 5th ed., Elsevier Saunders, Philadelphia, 2012, pp. 31–57.

Note:  The ‘normal’ arrangement, as described in the text, is Type A.

1.1.7  Hepatic veins

There are three named hepatic veins: right, middle and left, which lie in intersectional planes and drain from either side. The middle hepatic vein lies in the principal plane separating the right from the left hemiliver, and hence receives sinusoidal blood from segments 4, 5 and 8. The middle hepatic vein usually joins the left just short of the cava to form a common trunk in up to 95% of subjects [19,20]. The left is somewhat more horizontal, with segments 2 and 3 on either side. The right hepatic vein is usually single, draining segments 5 and 6 by an anterior trunk, and mainly segment 8 by a posterior trunk. It often has a short extrahepatic course and relatively few terminal branches, making outflow venous ligation in a right hemihepatectomy a practical and safe technique. There may also be short accessory dorsal veins, draining segments 6 and 7 directly into the cava on the right side [11]. From one to five phrenic veins also join the inferior vena cava at the level of hepatic vein confluence [23].

Variations of the above description are common, and include separate venous drainage of segment 3 directly into the vena cava or into the middle hepatic vein. The right and middle veins also have a reciprocal relationship in terms of size, with the right usually being larger. Smaller right hepatic veins therefore are usually associated with larger middle hepatic veins that take on most of the venous outflow for the right lobe.

1.1.8  Bile ducts

1.1.8.1  INTRAHEPATIC BILE DUCTS

Working backwards from the level of the porta hepatis, the common hepatic duct divides into right and left hepatic ducts (named as the first-order division by Healey and Schroy in 1953 [5]). The right hepatic duct enters the liver parenchyma, dividing into anterior and posterior sectional ducts (formerly known as sectorial ducts) (i.e. second-order division), before subdividing into ducts to segments 5 and 8 (anterior), and segments 6 and 7 (posterior) (i.e. third-order division).

The left hepatic duct lies within the umbilical recess, having a much more horizontal course and being much more exposed than the right, and curves around the left portal vein, with sectional ducts on the left side to segments 2 and 3 and on the right to segment 4.

1.1.8.2  EXTRAHEPATIC BILE DUCTS

Now working forward from the porta hepatis, the common hepatic duct runs down the right side of the portal pedicle to be joined at a variable point by the cystic duct to become the common bile duct. This passes behind the superior aspect of the duodenum into the head of the pancreas, or at least in a pancreatic groove, to drain into the medial aspect of the wall of the second part of the duodenum at the ampulla of Vater.^* In about 10%–15% of cases, the termination is within the third part of the duodenum.

The main pancreatic duct joins the common bile duct on its medial aspect, typically just at the entry to the duodenum. There is then a short intramural dilatation (the ampulla of Vater), which is surrounded by a distinct muscular sphincter (of Oddi^†) (see Section 1.2.4). This arrangement ensures that the bile and pancreatic secretions are separated prior to the intestinal lumen. This junction may occur somewhere short of the duodenal wall, and is then termed a common pancreatobiliary channel.

The blood supply to the extrahepatic biliary tree is predominantly arterial, axial and bidirectional. Thus, an arterial arcade supplies the supraduodenal duct from both above (~40%) and below (~60%), via the right hepatic and gastroduodenal arteries, respectively [31,32]. A small nonaxial supply (~2%) is derived from the adjacent common hepatic artery. There are two predominant paraductal vessels, termed the three and nine o’clock arteries, that contribute to a superficial plexus visible on most bile ducts [24,25]. In addition, there is a retroportal artery, which is derived directly from the coeliac axis or SMA and supplies part of the supraduodenal duct having passed behind the portal vein [24]. Intrahepatic ducts also retain a vascular plexus with frequent cross-collateralisation between the right and left [25]. For instance, ligation of the right hepatic artery at the hilum may be tolerated by collaterisation via a defined hilar plate arterial plexus.

The venous drainage of the common bile duct occurs via two plexuses: the epicholedochal venous plexus (of Saint^‡) and the paracholedochal plexus (of Petren^§). The former is a reticular meshwork of fine venules surrounding the external wall, while the latter runs along the common bile duct and drains into the gastric, pancreaticoduodenal and portal venous systems.

1.1.8.3  BILE DUCT VARIATIONS

Variations are common and clinically important in all types of biliary surgery. The textbook description above is probably only found in 55%–60% of cases. Blumgart [26] has described five main types (A–E), with Types C, D and E subdivided into two, making nine distinct variations (Table 1.2).

The cystic duct itself may run parallel with the common bile duct before having a low union (~20%). This may be made more complicated by the sharing of a common wall, hence resembling the double barrel of a shotgun. Occasionally, the entry of the cystic duct is on the left side (<5%).

Further anatomical detail is given in relation to liver resection (Chapter 21), split liver and living donor anatomy (Chapter 36), and shunt surgery (Chapter 24).

1.1.9  Gallbladder

The parts of the gallbladder include a fundus, body and neck, within which is a distinct pouch (Hartmann’s pouch^¶) just before narrowing down as the cystic duct. Characteristically, the folds of the mucosa converging on the cystic duct have a spiral appearance and are known as the valve of Heister.^* The organ is situated on the undersurface of the anatomical right lobe of the liver, to which it is usually adherent by a layer of fascia from the fundus to the neck. Uncommonly, the peritoneal investment is complete, forming a mesentery, which may rarely be subject to a volvulus. Conversely, the gallbladder can be completely enclosed within the liver parenchyma, making removal more difficult.

Many variations in the morphology of the gallbladder have been described, and the first comprehensive account of these was published by Robert Gross in 1936 [27]. These included agenesis (in the absence of biliary atresia), duplication, bilobulation and septation, and left-sided gallbladders (in the absence of situs inversus).

What is widely known as Calot’s^† triangle can now be defined as the space between the common hepatic duct, cystic duct and neck of the gallbladder and the undersurface of the liver, although it was first described with the cystic artery as the superior side.

The cystic artery commonly arises from the right hepatic artery and lies behind the common hepatic duct crossing Calot’s triangle to supply the gallbladder from a surface network. Variations include a double artery (15%) and originating from the common hepatic (1%) or left hepatic (5%) arteries. Sometimes the right hepatic artery takes a sinuous course to the gallbladder, with a very short cystic artery arising from its apex, known as Moynihan’s ‘hump’.^‡ There is no actual cystic vein, as the venous drainage is through multiple venules into the bed of the gallbladder.

1.1.9.1  ACCESSORY DUCT OF LUSCHKA^§ AND CHOLECYSTOHEPATIC DUCTS

These two entities are frequently confused but are distinct [28]. Luschka described an aberrant biliary duct found in the gallbladder fossa in 1863 [29]. This is a small bile duct running along the fossa between the gallbladder and liver parenchyma, but which does not enter the lumen or connect the two. It is not accompanied by an artery or vein and does not drain the parenchyma itself.

Conversely, cholecystohepatic ducts drain liver parenchyma, traversing the fossa directly into the lumen of the gallbladder or cystic duct. Both, of course, may be damaged by cholecystectomy and be a cause of a bile leak. The former is supposed to be a relatively common finding (~30% if looked for according to McQuillan et al. [30]), while the latter are much rarer. Vaginali ductuli are small communications between two bile ducts or between a bile duct and a cystic duct.

1.1.10  Lymphatic drainage

The lymphatic system of the liver has received less detailed study than that of its vascular supply or biliary drainage [31]. It has been suggested that about half of the total lymph of the body is formed in the liver, initially within the microscopic space of Disse.^¶ This then empties into two interconnected systems, a superficial subcapsular plexus and a deep intrahepatic plexus.

Generally, drainage is directed towards the thorax via the bare area of the liver and retroperitoneal attachments, and in a more circuitous route, via the preaortic coeliac nodes. Lymphatic vessels can be identified traversing the caval hiatus, and anteriorly along the nodes of the internal thoracic chain (via the hiatus of Morgagni).

The deep system drains towards the porta hepatis, within the Glissonian sheath, to where there are a series of nodes lying in a loose relationship with the portal triad. Three main chains have been described corresponding to the three portal elements. The gallbladder drains specifically to a fairly constant node (of Lund^**) within Calot’s triangle and on to two further nodes adjacent and anterior to the epiploic foramen.

1.1.11  Nerve supply

The nerve supply is derived from the hepatic plexus, which is both sympathetic from T7–10 and parasympathetic via the vagus nerve, with the former predominating. The para-sympathetic input, predominantly from the right vagal nerve, has hepatic branches travelling directly to the porta via the lesser omentum. Preganglionic sympathetic fibres travel via the greater splanchnic nerve to the coeliac ganglion, with postganglionic fibres reaching the liver usually via the hepatic arterial system. Some sympathetic input also reaches the liver directly from the right adrenal gland. There is an afferent nerve network with some visceral sensory nerve elements from the peritoneal coverings passing through the diaphragm, which are carried with the right phrenic nerve.

Two possible functions of hepatic innervation have been described: vascular autoregulation and perhaps some elements of liver metabolism. Thus, hepatic nerve stimulation in animal models causes increased hepatic vascular resistance, a rise in hepatic artery pressure and a reduction in its flow, and conversely, denervation causes an impaired response to hypovolaemia with an exacerbation of the reduction in portal blood flow. Similarly in animals, hepatic nerve stimulation consistently causes hyperglycaemia.

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