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
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.
Relationships of the liver to adjacent abdominal organs.