Anatomy and Physiology of the Liver

Modern Anatomic Approach to Liver Surgery

The liver is a single organ that can be functionally regarded as two hemilivers. The parenchyma can be further subdivided into several regions sharing common arterial, portal, and biliary supply and venous drainage. The portal and venous systems define these regions that are named sectors and segments, respectively ( Figs. 118.2–118.5 ). The liver has a rather constant anatomic pattern, the knowledge of which allows for a safe surgical approach. Nevertheless, there are some anatomic irregularities, and in particular instances, exact knowledge of the anatomy specific to the individual patient being examined or operated on is necessary (live donors, left extended or central hepatectomies, caudate lobe masses). In these cases a computed three-dimensional reconstruction of each anatomic detail is possible following an accurate computed tomographic or magnetic resonance imaging contrast scan. Several software packages are currently available that allow for the mapping of the individual anatomy, as well as for the calculation of volumes corresponding to the whole liver, liver sectors, and segments (Hepavision, MeVis-Germany, Hitachi-Japan, Hepavis-Slovenia, Université de Strasbourg-France). The reliability of virtual three-dimensional reconstructions based on standard anatomic landmarks for both surgical planning and graft volume calculations has been demonstrated ( Fig. 118.6 ). For standard liver surgery, the operating surgeon should be familiar with the basic anatomic pattern of the liver and the most frequent variations that have been described.

Embryology of the Liver

The liver primordium, also known as diverticulum hepatis or liver bud, arises from endoderm in weeks 3 through 4 of embryologic development and invades the septum transversum, vitelline (omphalomesenteric) veins, and umbilical veins. Its connection to the embryologic duodenum (foregut) will eventually become the bile duct. Embryologically the liver receives blood from both portal and umbilical veins, themselves connected by the left portal vein. Although the primitive portal veins arise from the caudal part of the vitelline veins, the primitive hepatic veins arise from the cranial part of the vitelline veins. In humans and many other mammals the inferior vena cava (IVC), ductus venosus, and umbilical vein are initially surrounded by liver parenchyma and become extrahepatic only in later stages of embryologic development. The arteries develop in conjunction with the bile ducts at a later period than the veins. On the right side, arteries and bile ducts follow the trajectory of the portal venous branches. On the left side, although arterial and biliary branching follows a symmetric pattern similar to that of the right side, the portal vein branches do not. During early stages of development, there are three hepatic arteries: (1) a left hepatic artery arising from the left gastric artery, (2) a middle hepatic artery arising from the celiac trunk, and (3) a right hepatic artery arising from the superior mesenteric artery. Although in most cases the middle artery is the only one that persists, variations in regression and origin of these three early arteries account for the so-called accessory and replaced variants. A complete ductal system is present by the 10th week of intrauterine life. The mesoderm of the septum between liver and abdominal wall develops into the falciform ligament . The surface of the developing liver in contact with the diaphragm is devoid of peritoneum, and the so-called bare area is a reminder of such association. By week 10 of development, the liver is involved in hematopoietic function, an activity that diminishes markedly during gestational months 8 and 9. By week 12 of development, the liver is already producing bile. However, hepatocytes only attain single-cell plate configuration by the age of 5 years. Several events take place at birth. The ductus venosus, which optimized venous return from the placenta to the fetus by connecting the left umbilical and common hepatic vein, closes and becomes the ligamentum venosus . Also at birth, the extrahepatic umbilical vein closes and becomes the ligamentum teres.

True anomalies of the liver are relatively infrequent. Prolongations of liver tissue from either the right (Riedel lobe) or left lobes usually present as incidental abdominal masses. In other instances, hepatic tissue connected by an isthmus to the liver is found in the chest. Small accessory collections of tissue attached to the liver by a pedicle are also occasionally encountered.

Hepatic Divisions

Several nomenclatures and topographic divisions have been proposed. According to Couinaud, the right and left hemilivers are supplied by first-order branches. Sectors are supplied by second-order branches. Segments are supplied by third-order branches. Subsegments are supplied by fourth-order or other branches. Segments are numbered in a counterclockwise fashion, from I to VIII. A main portal fissure, a right portal fissure, and a left portal fissure are grossly or conceptually defined because they may not always be anatomically present. Left and right paramedian sectors are adjacent to the main portal fissure. Left and right lateral sectors are located on the outer side of the corresponding paramedian ones (see Figs. 118.2–118.5 ). During our discussion, we follow this nomenclature with some modifications.

Vasculobiliary Sheaths

Couinaud referred to the vasculobiliary sheaths that envelop the portal elements as “the most important structure of liver anatomy.” They seem to have been described initially by Walaeus in 1640, and thus some have used the terms walaean pedicles or walaean sheaths . Glisson published his description of the liver “tunic” in 1642, and Laennec did so in 1803. The elements of the portal pedicle (hepatic artery, portal vein, bile ducts, nerves, and lymphatics) are surrounded throughout their trajectory to the parenchymal plates by connective tissue. Not so the hepatic veins. The hilar plate is located over the left and right pedicles, and the portal division, on the hilum of the liver. The umbilical plate is found in continuity with the hilar plate and the round ligament, covering the left paramedian pedicle in its upper surface. According to Couinaud and others, dissection of the hilar plate allows for the detachment of the hilar contents. Exposure of the umbilical plate is the gateway to the segmental and sectoral pedicles of the left liver. Because dissection at the level of the plates can lead to complications, an approach to the sheaths is recommended. This strategy has been applied by Lazorthes et al. in the so-called suprahilar approach for anatomic hepatectomies and segmentectomies. Sheaths originate at the right edge of the hilum, at the umbilical plate, and at the posterior margin of the hilar plate. From these sites they will reach the right liver, the left liver, and the caudate area, respectively. In cases of narrow hila or hila of difficult access, consideration should be given to dividing the anterior portion of the main portal fissure. This maneuver does not damage any structures of significance and allows better exposure of the hilar elements.

Couinaud recognized the following three types of approaches to a portal pedicle :

• 1

  Intrafascial—dissection within the sheath, where the elements are identified

• 2

  Extrafascial—dissection around the pedicle sheath

• 3

  Extrafascial and transfissural—dissection of the sheaths at their origin from the hilar and umbilical plates (considered the safest approach, especially for second- and third-order branches)

Arterial and Biliary Systems

According to Couinaud and Houssin, the most frequent arterial and biliary configurations, accounting for almost 90% of cases in their series, are the following :

• A unique artery and bile duct on right and left (24%)

• Two right bile ducts (17%)

• Two left arteries (26%)

• Two right ducts and two left arteries (22%)

Bile ducts are usually located above the portal branches and arteries below the corresponding veins. The bile ducts derive their blood supply predominantly from arterial branches. However, preliminary results from our observations in live-donor liver transplantation would point to some differences in the classically accepted (see Fig. 118.3 ) anatomic similarities among arteries and bile ducts.

Portal Vein and Portal Vein Anomalies

The left portal, left paramedian, left lateral, and right paramedian veins are constant structures within the liver architecture. The absence of the bifurcation of the portal vein can be an extremely dangerous situation. In such cases the portal vein follows a curvilinear trajectory within the liver, arching from right to left, and giving off collateral branches along the way until it reaches the caudate lobe. Ligation of the presumed right portal vein branch leads to complete interruption of portal blood into the liver. The classically accepted portal venous branching is illustrated in Fig. 118.4 .

Hilum, Plates, Fissures, and Other Structures

Couinaud reminded us that “ hilum meant in Latin a tiny black point seen in beans” and that anatomists in antiquity referred to that region as porta hepatis , or gateway of the liver. It contains the bifurcation of the portal elements, with the short right and the long left branches. In approximately 23% of cases the right portal vein is not present but rather is replaced by two sectoral branches. In 47% of cases the right hepatic duct is not present as such.

The location of the main portal fissure, described by Rex, may vary (see Fig. 118.2 ). It is identified by the posterior extremity of the cystic plate and, in cases of normal right portal vein anatomy, tends to be located to the right of the portal vein, less frequently at the site of the bifurcation of the portal vein, or even less frequently to its left. In cases of right portal vein variants the fissure is almost always at the level of the bifurcation or at the left portal vein. Its topographic location on the liver is not outlined by superficial markings in humans and can be traced from the gallbladder fossa to the left anterior surface of the IVC. Furthermore, it has been noted that when the main portal fissure lies on the left, the biliary confluence is located in more than 70% of cases in front of the left portal vein.

The hilar plate (see Fig. 118.2 ) is detached from the liver parenchyma by dissecting in between the left portal pedicle and liver tissue. The left hepatic duct is the structure located in the superior aspect of the portal elements. No major vessels or biliary ducts are encountered in this pathway. Only in the posterior region are there branches to the caudate lobe.

The umbilical fissure and plate (see Fig. 118.2 ) is the site of origin of segmental and sectoral pedicles to the left liver. Its anatomic landmarks are the falciform ligament and the left longitudinal sulcus. The left paramedian pedicle and the umbilical plate can be identified by following the round ligament in continuity with the left portal vein. No walaean pedicles cross the umbilical fissure. This structure divides the left lobe from the rest of the liver and is a landmark point for the evaluation and performance of left lobectomies and trisectorectomies (trisegmentectomies in the classic diction).

The sulcus of Rouvière is an irregular fissure in continuity with the right hilum (see Fig. 118.2 ). It represents the extrahepatic anatomic landmark of the right fissure, usually buried in liver parenchyma. Following this structure leads to the pedicles of segments V and VI and further deeply and posteriorly to the pedicles of segments VII and VIII. The maneuver of isolating these structures is advantageous in the difficult procedures of right sectorectomy (segments VI to VII resection) or left trisectorectomy (trisegmentectomy) (segments I-II-III-IV-V-VIII resection).

The right paramedian portal pedicle is, according to Couinaud, “one of the most constant vessels of the liver.”

The parabiliary venous system of Couinaud is an accessory venous system with collateral branches to the duodenum, pancreas, and stomach, located within the hilar plate. It is associated with liver parenchyma, especially in the caudate and quadrate lobes, as well as with cystic veins. It may act as a collateral pathway in cases of portal hypertension and may serve as a connection between the right and left livers.

The cystic vein(s) usually drain into the right portal vein but may also drain into the right liver, the left liver, and/or enter the parabiliary venous system.

In 20% to 50% of cases, small ducts that are not part of a portal pedicle and do not communicate with the gallbladder are encountered in the cystic fossa. These ducts, described by Luschka, represent part of the “vasa aberrantia.” They are different from the cystohepatic ducts , which are true biliary ducts that traverse from liver tissue to the gallbladder.

Hepatic, Suprarenal, and Phrenic Veins

There are three main hepatic veins that drain into the IVC (see Fig. 118.5 ): the right hepatic vein (RHV), middle hepatic vein (MHV), and left hepatic vein (LHV). Accessory, inferior, right inferior, right middle, or dorsal hepatic veins also drain directly into the IVC. The MHV and LHV show a relative lack of anatomic diversity, whereas the RHV exhibits multiple variants.