CHAPTER 68 Liver
The liver is the largest of the abdominal viscera, occupying a substantial portion of the upper abdominal cavity. It occupies most of the right hypochondrium and epigastrium, and frequently extends into the left hypochondrium as far as the left lateral line (Fig. 68.1). As the body grows from infancy to adulthood the liver rapidly increases in size. This period of growth reaches a plateau around 18 years and is followed by a gradual decrease in the liver weight from middle age. The ratio of liver to body weight decreases with growth from infancy to adulthood. The liver weighs approximately 5% of the body weight in infancy and it decreases to approximately 2% in adulthood. The size of the liver also varies according to sex, age and body size. It has an overall wedge shape, which is in part determined by the form of the upper abdominal cavity into which it grows. The narrow end of the wedge lies towards the left hypochondrium, and the anterior edge points anteriorly and inferiorly. The superior and right lateral aspects are shaped by the anterolateral abdominal and chest wall as well as the diaphragm. The inferior aspect is shaped by the adjacent viscera. The capsule is no longer thought to play an important part in maintaining the integrity of the shape of the liver.
Fig. 68.1 The ‘bed’ of the liver. The outline of the liver is shaded green. The central bare area is unshaded.
Throughout life the liver is reddish brown in colour, although this can vary depending upon the fat content. Obesity is the most common cause of excess fat in the liver (also known as steatosis): the liver assumes a more yellowish tinge as its fat content increases. The texture is usually soft to firm, although it depends partly on the volume of blood the liver contains and the fat content.
The liver performs a wide range of metabolic activities required for homeostasis, nutrition and immune defence. For example, it is important in the removal and breakdown of toxic, or potentially toxic, materials from the blood and the regulation of blood glucose and lipids, the storage of certain vitamins, iron, and other micronutrients, and in breaking down or modifying amino acids. It is involved in a plethora of other biochemical reactions. Since the majority of these processes are exothermic, a substantial part of the thermal energy production of the body, especially at rest, is provided by the liver. The liver is populated by phagocytic macrophages, components of the mononuclear phagocyte system capable of removing particulates from the blood stream. It is an important site of haemopoiesis in the fetus.
SURFACES OF THE LIVER
The liver is usually described as having superior, anterior, right, posterior and inferior surfaces, and has a distinct inferior border (Figs 68.2, 68.3). However, the superior, anterior and right surfaces are continuous and no definable borders separate them. It would be more appropriate to group them as the diaphragmatic surface, which is mostly separated from the inferior, or visceral surface, by a narrow inferior border. At the infrasternal angle the inferior border is related to the anterior abdominal wall and is accessible to examination by percussion, but is not usually palpable. In the midline, the inferior border of the liver is near the transpyloric plane, about a hand’s breadth below the xiphisternal joint. In women and children the border often projects a little below the right costal margin.
Fig. 68.2 The surfaces and external features of the liver. A, superior view; B, posterior view; C, anterior view; D, inferior view.
Fig. 68.3 Relations of the liver. A, superior view; B, posterior view; C, anterior view; D, inferior view.
The superior surface is the largest surface and lies immediately below the diaphragm, separated from it by peritoneum except for a small triangular area where the two layers of the falciform ligament diverge. The majority of the superior surface lies beneath the right dome, but there is a shallow cardiac impression centrally that corresponds to the position of the heart above the central tendon of the diaphragm. The left side of the superior surface lies beneath part of the left dome of the diaphragm.
The anterior surface is approximately triangular and convex and is covered by peritoneum except at the attachment of the falciform ligament. Much of it is in contact with the anterior attachment of the diaphragm. On the right the diaphragm separates it from the pleura and sixth to tenth ribs and cartilages, and on the left from the seventh and eighth costal cartilages.
The right surface is covered by peritoneum and lies adjacent to the right dome of the diaphragm which separates it from the right lung and pleura and the seventh to 11th ribs. The right lung and basal pleura both lie above and lateral to its upper third, between the diaphragm and the seventh and eighth ribs. The diaphragm, the costodiaphragmatic recess lined by pleura, and the ninth and tenth ribs all lie lateral to the middle third of the right surface. Lateral to the lower third, the diaphragm and thoracic wall are in direct contact. Rarely, the hepatic flexure and proximal transverse colon may lie on a long mesentery over the right and superior surfaces of the liver, referred to as Chilaiditi syndrome.
The posterior surface is convex, wide on the right, but narrow on the left. A deep median concavity corresponds to the forward convexity of the vertebral column close to the attachment of the ligamentum venosum. Much of the posterior surface is attached to the diaphragm by loose connective tissue, forming the triangular ‘bare area’. The inferior vena cava lies in a groove or tunnel in the medial end of the ‘bare area’. To the left of the caval groove the posterior surface of the liver is formed by the caudate lobe, and covered by a layer of peritoneum continuous with that of the inferior layer of the coronary ligament and the layers of the lesser omentum. The caudate lobe is related to the diaphragmatic crura above the aortic opening and the right inferior phrenic artery, and separated by these structures from the descending thoracic aorta.
The fissure for the ligamentum venosum separates the posterior aspect of the caudate lobe from the main part of the left lobe. The fissure cuts deeply in front of the caudate lobe and contains the two layers of the lesser omentum. The posterior surface over the left lobe bears a shallow impression near the upper end of the fissure for the ligamentum venosum that is caused by the abdominal part of the oesophagus. The posterior surface of the left lobe to the left of this impression is related to part of the fundus of the stomach. Together these posterior relations make up what is sometimes referred to as the ‘bed’ of the liver (Fig. 68.1).
The inferior surface is bounded by the inferior edge of the liver. It blends with the posterior surface in the region of the origin of the lesser omentum, the porta hepatis and the lower layer of the coronary ligament, and is marked near the midline by a sharp fissure which contains the ligamentum teres (the obliterated fetal left umbilical vein). The gallbladder usually lies in a shallow fossa, but this is variable: it may have a short mesentery or be completely intrahepatic and lie within a cleft in the liver parenchyma. The quadrate lobe lies between the fissure for the ligamentum teres and the gallbladder.
The inferior surface of the left lobe is related inferiorly to the fundus of the stomach and the upper lesser omentum. The quadrate lobe lies adjacent to the pylorus, first part of the duodenum and the lower part of the lesser omentum. Occasionally the transverse colon lies between the duodenum and the quadrate lobe. To the right of the gallbladder, the inferior surface is related to the hepatic flexure of the colon, the right suprarenal gland and right kidney, and the first part of the duodenum.
GROSS ANATOMICAL LOBES
Historically, the liver has been considered to be divided into right, left, caudate and quadrate lobes by the surface peritoneal and ligamentous attachments.
The right lobe is the largest in volume and contributes to all surfaces of the liver. It is divided from the left lobe by the falciform ligament superiorly and the ligamentum venosum inferiorly. On the inferior face to the right of the groove formed by the ligamentum venosum there are two prominences separated by the porta hepatis: the caudate lobe lies posterior, and the quadrate lobe anterior, to the porta hepatis. The gallbladder lies in a shallow fossa to the right of the quadrate lobe.
The left lobe is the smaller of the two main lobes, although it is nearly as large as the right lobe in young children.
It lies to the left of the falciform ligament with no subdivisions, and is substantially thinner than the right lobe, having a thin apex that points into the left upper quadrant.
The quadrate lobe is visible as a prominence on the inferior surface of the liver, to the right of the groove formed by the ligamentum venosum (and thus is incorrectly said to arise from the right lobe although it is functionally related to the left lobe). It lies anterior to the porta hepatis and is bounded by the gallbladder fossa to the right, a short portion of the inferior border anteriorly, the fissure for the ligamentum teres to the left, and the porta hepatis posteriorly.
The caudate lobe is visible as a prominence on the inferior and posterior surfaces to the right of the groove formed by the ligamentum venosum: it lies posterior to the porta hepatis. To its right is the groove for the inferior vena cava. Above, it continues into the superior surface on the right of the upper end of the fissure for the ligamentum venosum. In gross anatomical descriptions this lobe is said to arise from the right lobe, but it is functionally separate.
FUNCTIONAL ANATOMICAL DIVISIONS
Current understanding of the functional anatomy of the liver is based on Couinaud’s division of the liver into eight (subsequently nine) functional segments, based upon the distribution of portal venous branches and the location of the hepatic veins in the parenchyma (Couinaud 1957). Further understanding of the intrahepatic biliary anatomy, especially of the right ductal system, was enhanced by contributions from Hjortsjo (1948) and Healey & Schroy (1953) using the biliary system as the main guide for division of the liver (Fig. 68.4).
Fig. 68.4 The fissures and sectors of the liver. (Right lateral = right posterior; right medial = right anterior.)
The liver is divided into four portal sectors by the four main branches of the portal vein. These are right lateral, right medial, left medial and left lateral (sometimes the term posterior is used in place of lateral and anterior in place of medial). The three main hepatic veins lie between these sectors as intersectorial veins. These intersectoral planes are also called portal fissures (scissures). The fissures containing portal pedicles are called hepatic fissures. Each sector is sub-divided into segments (usually two) based on their supply by tertiary divisions of the vascular biliary sheaths.
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).
The main fissure extends from the tip of the gallbladder back to the midpoint of the inferior vena cava and contains the middle (main) hepatic vein. It separates the liver into right and left hemi-livers. Segments V and VIII lie to the right and segment IV to the left of the fissure.
The left fissure divides the left hemi-liver into medial (anterior) and lateral (posterior) sectors. It extends from the mid point of the anterior edge of the liver between the falciform ligament and the left triangular ligament to the point which marks the confluence of the left and middle hepatic veins. It contains the left hepatic vein and separates the left anterior and left posterior sectors: segment III lies anteriorly and segment II posteriorly.
The right portal fissure divides the right hemi-liver into lateral (posterior) and medial (anterior) sectors. The plane of the right fissure is the most variable amongst the main fissures and runs approximately diagonally through the gross right lobe from the lateral end of the anterior border to the confluence of the left and middle hepatic veins. The fissure divides the right anterior sector to its left (segments V and VIII) from the right posterior sector to its right (segments VI and VII), and contains the right hepatic vein. The right fissure marks the thickest point of liver parenchyma which is commonly transected during liver resection.
The umbilical fissure separates segment III from segment VI within the left anterior sector and contains a main branch of the left hepatic vein (the umbilical fissure vein). It is marked by the attachment of the falciform ligament and sometimes covered by a ridge of liver tissue extending between the segments: it is often avascular and can be divided safely with diathermy during a surgical approach. It contains the umbilical portion of the left portal vein and the final divisions of the left hepatic duct and the left hepatic artery branches. The umbilical portion of the left portal vein is an important landmark: access to this vein and mobilization of the left portal vein are essential steps in surgery for hilar cholangiocarcinoma. A knowledge of the arrangement of the portal vein, hepatic artery and bile duct within the umbilical fissure is also essential when splitting the liver for an adult and paediatric recipient and for live donor liver transplantation for a child recipient.
The venous fissure is a continuation of the umbilical fissure on the under surface of the liver and contains the ligamentum venosum. It lies between the caudate lobe and segment IV. The deeper continuation of this plane is the dorsal fissure.
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 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 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 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 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 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 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 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 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.
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