Chapter 24

### Segments

The liver develops as an embryologic outpouching from the duodenum by a process described in Chapter 25. The liver is one of the largest organs in the body, representing 2% of the total body weight. In classic descriptions, the liver was characterized as having four lobes: right, left, caudate, and quadrate; however, this is an overly simplistic view that fails to consider the much more complex segmental anatomy, which is depicted in Figure 24–1.

###### Figure 24–1.

Segmental anatomy of the liver is shown, with each of the individual segments numbered. Segment I (caudate) is indicated at the back of the liver, posterior to the middle hepatic vein. The most common major hepatic resections performed and the segments removed with each are indicated.

The liver is divided into eight segments based on the branching of the portal triads and hepatic veins. The structures of the portal triad (hepatic artery, portal vein, and biliary duct) are separate extrahepatically but enter the hepatic hilus ensheathed within a thickened layer of the Glisson capsule. The three main hepatic veins divide the liver into four sectors, each of which is supplied by a portal pedicle. The caudate lobe is an exception because its venous drainage is directly into the vena cava and therefore independent of the major hepatic veins. The four sectors delimited by the hepatic veins are called the portal sectors, and these portions of the parenchyma are supplied by independent portal pedicles arising from the right or left main pedicles. The divisions separating the sectors are called portal scissurae, within each of which runs a hepatic vein. Further branching of the pedicles subdivides the sectors into segments. The liver is thus subdivided into eight segments, with the caudate lobe designated as segment I. Segments I–IV comprise the left liver, and segments V–VIII, the right. Each segment is supplied by an independent portal pedicle, which forms the basis of sublobar segmental resections.

The anatomical right and left hemilivers are separated by an imaginary line running from the medial aspect of the gallbladder fossa to the inferior vena cava, running parallel with the fissure of the round ligament. This division is known as the Cantlie line or the principal plane and marks the course of the middle hepatic vein. The right hepatic vein further subdivides the right liver into anterior (segments V and VIII) and posterior (segments VI and VII) sectors, while the umbilical fissure subdivides the left liver into the medial sector (segment IV) and left lateral segment (segments II and III). The relationship of the liver to the other abdominal organs is shown in Figure 24–2.

###### Figure 24–2.

Relationships of the liver to adjacent abdominal organs.

### Portal Circulation

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