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Liver Anatomy
The average adult liver weighs approximately 1,200 to
1,600 grams. The liver is located in the right upper quadrant beneath the rib cage. The inferior border of the liver
is the costal margin, and the superior portion lies just
beneath the diaphragm. The liver is located at the level
of the fifth rib on the right and the sixth rib on the left.
Most of the liver is encapsulated except for the gallbladder bed, the porta hepatis, and the posterior surface adjacent to the inferior vena cava (IVC). Ligaments are
formed from peritoneal reflections. The major ligaments
supporting the liver are the coronary ligaments (attach
liver to diaphragm), the triangular ligaments (attach liver
to diaphragm), the falciform ligament (connects liver to
diaphragm and anterior abdominal wall), the ligamentum
teres (contains the left umbilical vein), the gastrohepatic
ligament, the hepatoduodenal ligament (contains the
portal vein, common bile duct, and hepatic artery), the
hepatocolic ligament, and the hepatorenal ligament.
The functional unit of the liver is the hepatic lobule.
Lobules are composed of a central vein surrounded by
four to six terminal portal triads (portal vein, hepatic
artery, and bile duct) to form a polygon-shaped unit.
Hepatocytes are placed single file, radiating outward
from the central vein. Endothelial-lined sinusoids run
between each row of hepatocytes. The lateral walls of
the hepatocytes form bile canaliculi, which flow toward
the portal triads. The hepatocytes are divided into three
zones traveling from the perimeter of the lobule to
the center. Zone 1 (periportal zone) is located at the
perimeter closest to the portal triads. This zone is rich
in oxygen and nutrients and is the least susceptible to
hypoxic injury. Zone 2 (intermediate zone) is located
in the middle. Zone 3 (perivenular zone) is the most
distant from the portal triads and, thus, has the least
amount of oxygen and nutrients. It is this zone that is primarily affected during hypoxic insults (i.e., centrilobular
necrosis).
Blood is delivered to the liver from the portal venous
system and from the systemic system via the hepatic
artery. Blood is drained from the hepatic sinusoids to the
hepatic veins and then back to the systemic system
through the IVC.
The portal vein forms from the connection of the superior mesenteric vein and the splenic vein. The portal vein
runs in a superior direction behind the duodenum to
enter the posterior portion of the hepatoduodenal ligament. The portal vein branches into a right lobar and left
lobar branch at the porta hepatis. The right lobar branch
runs through the liver tissue and then makes anterior and
posterior divisions. These vessels divide further into
anterosuperior, anteroinferior, posterosuperior, and posteroinferior segments. The left lobar branches run
through the liver tissue for some distance before dividing
into superior and anteroinferior branches. The anteroinferior branch is the larger of the two. It continues to
divide into a lateral inferior branch and makes medial
superior and medial inferior divisions.
In most cases, the common hepatic artery originates
from the celiac axis off of the abdominal aorta. At the
superior edge of the duodenum, the common hepatic
artery gives off the right gastric artery as well as the gastroduodenal artery. From here, the common hepatic
artery continues on as the proper hepatic artery within
the anterior medial portion of the hepatoduodenal ligament. The proper hepatic artery divides into the left and
right hepatic arteries at the porta hepatis. The right
hepatic artery gives rise to the cystic artery before entering the hepatic parenchyma. The hepatic arteries tend to
follow the same course as the portal tributaries within
the liver itself. The medial hepatic artery arises from the
left hepatic artery where it enters the quadrate lobe.
Common variations in the arterial anatomy include the
origin of the right hepatic artery from the superior
mesenteric artery (17%) and the origination of the left
hepatic artery from the left gastric artery (23%).
The central hepatic veins of the hepatic lobule interconnect to form sublobular veins. The sublobular veins
coalesce to form collecting veins. The collecting veins
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unite to form three major hepatic vein conduits (i.e., the
left, right, and middle veins). The right hepatic vein drains
the right posterolateral segments and superior portion
of the anteromedial segments. The right hepatic vein
empties separately into the IVC. The left hepatic vein
drains the left superolateral and inferolateral segments of
the liver and then empties directly into the IVC. The
middle hepatic vein drains the inferior portion of the
right anteromedial segments as well as the left inferomedial portions of the liver. The middle vein unites with the
left hepatic vein in 60% of cases.
The division of the intrahepatic biliary system follows
the course of the portal vein divisions. On the right side,
the right anterosuperior and anteroinferior ducts unite to
form the right anterior bile duct. Likewise their posterior
counterparts combine to form the right posterior bile
duct. The right hepatic duct is formed when the two anterior and posterior segments unite before the porta
hepatis. The left hepatic duct is formed from the union of
the lateral duct and the medial duct. The lateral duct is
composed of the superior lateral and inferior lateral
ducts.The medial duct arises from the union of the medial
superior and inferior ducts. The right and left hepatic
ducts join outside of the liver at the porta hepatis to form
the common hepatic duct. The common hepatic duct
travels within the hepatoduodenal ligament and then
joins with the cystic duct to form the common bile duct.
Original descriptions of liver anatomy were based on
lobar divisions. The lobes were defined by ligamentous
attachments as well as fissures and grooves. Modern
Part XII. Gastrointestinal Disorders
descriptions organize the liver anatomy according to the
arrangement of hepatic vasculature and bile ducts
(Couinaud’s system). Couinaud’s system provides a
better anatomic road map for hepatic surgery. In this
system, the liver is divided into eight segments, each with
its own pedicle (portal vein, hepatic artery, and bile duct).
The segments are organized further into four sectors
determined by the hepatic veins. The sectors are separated by three scissurae that run in the course of the
hepatic veins. The four sectors can be grouped into components of the right and left liver. The middle scissura
(also called Cantlie’s line) runs anterior to posterior
between the gallbladder fossa to the left of the IVC. It
divides the liver into the left and right hemiliver. The
middle scissura contains the middle hepatic vein. The
right scissura contains the right hepatic vein and divides
the right liver into anterior and posterior segments. The
anterior portion contains liver segments V and VIII. The
posterior portion contains segments VI and VII. Likewise
the left liver is divided into anterior and posterior segments by the left scissura. The left scissura contains the
left hepatic vein and is located posterior to the ligamentum teres. The anterior portion is composed of segments
III and IV, and the posterior segment is made up of only
segment II. Segment I (the caudate lobe) is located on
the under surface of the liver anterior to the IVC. The
caudate lobe receives vascular inflow and biliary drainage
from both the left and right systems. The caudate lobe
drains venous blood directly into the IVC via multiple
small venules.