Anatomy of the liver
General
The liver is the largest organ in the body,
weighing 1.5 kg in the average 70-kg male. Its position under the right hemi diaphragm
allows it to he protected from trauma by the costal margin. The liver parenchyma
is entirely covered by a thin capsule and by visceral peritoneum in all but the
posterior surface of the liver, termed the ‘bare area’. At first appearance
the liver is divided into two main lobes: a large right lobe which comprises
three-quarters of the liver parenchyma and a smaller left lobe the remaining
quarter. Surgical resection of these lobes would be termed a right or left
lobectomy.
Ligaments and
peritoneal reflections
The liver is fixed in the right upper quadrant
by peritoneal reflections which form ligaments. On the superior surface of the
left lobe is the left triangular ligament. Dividing the anterior and posterior
folds of this ligament allows the left lobe to mobilised from the diaphragm and
the left lateral wall of the inferior vena cava (IVC) to be exposed. Large veins
drain venous blood from the diaphragm to the hepatic veins and IVC at this
level. The right triangular ligament fixes the entire right lobe of the liver to
the undersurface of the right hemidiaphragm. Division of this ligament allows
the liver to be rnobilised from under the diaphragm and rotated to the left.
Another major supporting structure is the falciforrn ligament, which runs from
the umbilicus to the liver between the right and left lobes, passing into the
interlobar fissure. Embryologically this ligament contained the umbilical vein
which carried maternal nutrients to the liver of the foetus. From the fissure it
passes anteriorly on the surface of the liver attaching it
to the posterior aspect of the anterior abdominal wall. On the superior
aspect of the liver the falciform ligament divides into two leaves, between
which is loose areolar tissue and some small vessels. Division of this layer
allows exposure of the suprahepatic IVC lying within a thin sheath of fibrous
tissue. The final peritoneal reflection is between the stomach and the liver.
This lesser omentum is often thin and fragile but contains the hilar structures
in its free edge.
Liver blood
supply
The blood supply to the liver is unique, being
derived 80 per cent from the portal vein and 20 per cent from the hepatic
artery. The arterial blood supply in most individuals is derived from the
coeliac trunk of the aorta where the hepatic artery arises along with the
splenic artery. After branching to form the gastroduodenal artery it branches at
a very variable level to produce the right and left hepatic arteries. The right
artery supplies the majority of liver parenchyma and is therefore the larger of
the two arteries. There are many anatomical variations which are essential
knowledge for safe surgery on the liver. The blood supply to the right lobe of
the liver may be partly or completely supplied by a right hepatic artery arising
from the superior mesenteric artery. This vessel passes posterior to the
uncinate process and head of pancreas, and runs to the liver on the posterior
wall of the
Structures in
the hilum of the liver
The hepatic artery, portal vein and bile duct
are present within the free edge of the lesser omentum or the ‘hepatoduodenal
ligament’. To expose these structures requires the peritoneum overlying the
hilar triad to be divided followed by division of small vessels and an extensive
lymphatic plexus. The standard relationship of these three structures is for the
bile duct to be within the free edge, the hepatic artery
Division of
structures at the hilum
At the hilum the major structures are divided
into right and left branches. The right and left hepatic ducts arise from the
hepatic parenchyma and join to form the common hepatic duct. The left duct has a
longer extra hepatic course of approximately 2 cm. Once within the liver
parenchyma the duct accompanies the branches of the hepatic artery and portal
vein within a fibrous sheath. The portal vein often gives off two large branches
to the right lobe which are accessible outside the liver for a short length
before giving a left portal vein branch which runs behind the left hepatic duct.
Venous drainage
of the liver
The venous drainage of the liver is via the
hepatic veins into the IVC. The vena cava lies within a groove in the posterior
wall of the liver. Above the liver it immediately penetrates the diaphragm to
join the right atrium, whereas below the liver parenchyma there is a short
length of vessel before the insertion of the renal veins. The inferior hepatic
veins are short vessels which pass directly between the liver parenchyma and the
anterior wall of the IVC. The major venous drainage is through three large veins
which join the IVC immediately below the diaphragm. Outside the liver these
vessels are surrounded by a thin fibrous layer. The right hepatic vein can be
exposed fully outside the liver, but the middle and left veins usually join
within the liver parenchyma. Immediately adjacent to the retro hepatic IVC lies
the right kidney and adrenal gland. The right adrenal vein drains into the IVC
at this level, usually via one main branch. The IVC can be mobilised fully from
the retroperitoneal tissues and in the healthy state there are no significant
vessels in this tissue plane. The right inferior phrenic vein often drains to
the IVC via the right hepatic vein.
The internal
anatomy of the liver
Safe liver surgery has been enormously
facilitated by a better understanding of the internal anatomy of the liver which
has been extensively investigated by the French anatomist Couinaud. He described
the liver as being divided into eight segments (Fig. 52.2). Each of these segments
can be considered as functional units, each with a branch of hepatic artery,
portal vein and bile duct, and drained by a branch of the hepatic vein. The
overall anatomy of the liver is divided into a functional right and left along
the line between the gallbladder fossa and the middle hepatic vein. Liver
segments (v—viii) to the right of this line are supplied by the right hepatic
artery and the right branch of portal vein, and drain bile via the right hepatic
duct. The liver parenchyma to the left of this line (segments i—iv) is
functionally the left liver and is supplied by the left branch of the hepatic
artery and the left portal vein branch, and drains bile via the left hepatic
duct.
The hepatic
lobules
The functional units within the liver segments
are the liver lobules. These comprise plates of liver cells separated by the
hepatic sinusoids, large thin-walled venous channels which carry blood to the
central vein, a tributory of the hepatic vein from the portal tracts which
contains branches of the hepatic
Embryology
The liver is a foregut structure and forms as
a small endodermal bud early in gestation. The cell population within the bud
grows rapidly and forms two cell populations which differentiate into the liver
and extra hepatic biliary tree and gallbladder. The liver cells are bipotential
and may develop into hepatocytes or intrahepatic ductal cells. The liver
endothelium is derived from the vitelline and umbilical veins which merge with
the endodermal bud to form the liver sinusoids. The supporting connective
tissue, haemopoetic cells, which are important during intrauterine life, and the
Kupffer cells are derived from the mesoderm of the septum transversum.