Liver- Structure
Introduction
The liver is the largest organ in the body and acts
as a well-equipped biochemical laboratory where practically metabolism of all
the nutritional substances, viz., carbohydrates, proteins, lipids, vitamins and
minerals takes place as well as heat is produced. The liver is both a secretory and an excretory
gland. The liver is an accessory digestive organ that produces bile, an alkaline fluid containing cholesterol and bile
acids, which helps the breakdown of fat.
liver's highly specialized tissue, consisting of mostly hepatocytes, regulates a wide variety of high-volume biochemical
reactions, including the synthesis and breakdown of small and complex
molecules, many of which are necessary for normal vital
functions. Estimates regarding the organ's total number of functions are
estimated to being around 500.
Anatomy
and histology
The liver is a reddish-brown,
wedge-shaped organ with two lobes of unequal size and shape. A human liver normally
weighs approximately 1.5 kg and has a width of about
15 cm. There is considerable size variation between individuals, with
the standard reference range as
follows-
|
·
Men |
970–1,860 g |
|
·
Women |
600–1,770 g |
It is both the heaviest internal
organ and the largest gland in the human body. Located in the right upper quadrant
of the abdominal cavity, it rests just below the diaphragm, to the right of the
stomach and overlies the gallbladder.
It is almost a solid organ consisting of several
lobes; each lobe is made up of numerous lobules. Under the microscope, each
lobule is found to be composed of rows of polygonal cells from the centre-like
the spokes, radiating of the lobule being delineated by the presence of portal
triads.
The periphery of a wheel is occupied by the central
(intra lobular) vein. The portal vein and hepatic artery enter into, and bile
duct and hepatic vein emerge out of the liver through a connective tissue
sheath (Glisson's capsule). After entering centre they branch repeatedly.
The liver cells are arranged in the form of plates
having thickness of single-cell diameter throughout this structure which makes
it a sponge like structure. The cell plates are tunneled by a communicating,
system of cavities or lacunae. These contain hepatic sinusoids, endothelial
cells and phagocytic cells of RE System called Kupffer cells. The narrow space
separating the sinusoidal wall from the liver cell plates is known as Disse's
space.
Gross Anatomy
The liver
is divided into two parts when viewed from above –
·
a
right lobe
·
a
left lobe
And four
parts when viewed from below-
·
Left
·
Right
·
Caudate
Important Relations
Anteriorly- Diaphragm, right and left costal
margins, right and left pleura and lower margins of both lungs, xiphoid
process, and anterior abdominal wall in the subcostal angle
Posteriorly- Diaphragm, right kidney, hepatic
flexure of the colon, duodenum, gallbladder, inferior vena cava, and esophagus
and fundus of the stomach Peritoneal Ligaments of the Liver
1. Falciform
Ligament- The falciform
ligament, which is a two-layered fold of the peritoneum, ascends from
the umbilicus to the liver. It has a sickle-shaped free margin that contains
the ligamentum teres, the remains of the umbilical vein. The falciform ligament
passes on to the anterior and then the superior surfaces of the liver and then
splits into two layers.
a. The right layer forms the upper layer of
the coronary ligament;
b.
the left layer forms the upper layer of the left triangular ligament.
The right extremity of the coronary ligament
is known as the right triangular ligament.
Bare Area of the Liver-
The peritoneal layers forming the coronary ligament are widely separated,
leaving an area of liver devoid of peritoneum. This area is called the bare area of the liver.
2.
Ligamentum Teres- The ligamentum teres passes into a fissure on the visceral surface of
the liver and joins the left branch of the portal vein in the porta hepatis.
3. Ligamentum Venosum. The ligamentum venosum, a fibrous band that is the remains of the ductus venosus, is attached to the
left branch of the portal vein and ascends in a fissure on the visceral surface
of the liver to be attached above to the inferior vena cava.
Liver Segments
A liver segment is
one of eight segments of the liver as described in the widely used Couinaud classification (named
after Claude Couinaud) in the anatomy of the liver. This system divides the lobes of the liver into eight
segments based on a transverse plane through
the bifurcation of the main portal vein, arranged in a
clockwise manner starting from the caudate lobe. There are four lobes of the liver. The Couinaud classification of
liver anatomy then further divides the liver into eight functionally
independent segments.
Each segment
has its own vascular inflow, outflow and biliary drainage. In the centre
of each segment there is a branch of the portal vein, hepatic artery and bile
duct. In the periphery of each segment there is vascular outflow through the
hepatic veins. The division of the liver into independent units means that
segments can be resected without damaging the remaining segments.
Lobes of Liver
Left lobe
The left
lobe is smaller and more flattened than the right. It is situated in
the epigastric, and left hypochondriac regions of the abdomen. Its upper surface is slightly convex and is molded on to
the diaphragm; its under surface presents the gastric impression and omental
tuberosity.
Right lobe
The right
lobe is six times the size of the left lobe. It occupies the right
hypochondrium, on
its posterior surface by the ligamentum
venosum for the
cranial (upper) half and by the ligamentum teres hepatis (Round
ligament of liver)
for the caudal (under) half. The ligamentum
teres hepatis turns
around the inferior margin of the liver to come out ventral in the falciform
ligament.
The right
lobe is functionally separated from the left lobe by the middle hepatic vein.
From a functional perspective (one that takes the arterial, portal venous, and
systemic venous anatomy into account) the falciform ligament separates the medial
and lateral segments of the left hepatic lobe.
The right
lobe is of a somewhat quadrilateral form. It’s under and posterior surfaces
being marked by three fossæ- the fossa for the portal vein, the fossa for the gall-bladder and the fossae for the inferior
vena cava. These
separate the right lobe into two smaller lobes on its left posterior part: the
quadrate lobe and the caudate lobe.
Quadrate lobe
The quadrate
lobe is an area of the liver situated
on the undersurface of the medial segment left lobe (Couinaud segment IVb),
bounded in front by the anterior margin of the liver, behind by the porta hepatis, on the right by the fossa for the gall-bladder, and on the left by the fossa for
the umbilical
vein. It is oblong
in shape, its anteroposterior diameter being greater than its transverse.
Caudate lobe
The caudate
lobe (posterior hepatic segment-I) is situated upon the
posterosuperior surface of the liver on the right lobe of the liver, opposite
the tenth and eleventh thoracic
vertebrae.
The caudate
lobe of the liver is bounded below by the porta hepatis, on the right by the fossa for
the inferior
vena cava, and on
the left by the fossa for the ductus venosus and the physiological division
of the liver, called the ligamentum
venosum. It looks
backward, being nearly vertical in position; it is longer from above downward
than from side to side, and is somewhat concave in the transverse direction. It
is situated behind the porta, and separates the fossa for the gall-bladder from the commencement of the
fossa for the inferior
vena cava.
The caudate lobe is named after the tail-shaped hepatic tissue (cauda; Latin, tail) papillary process
of the liver, which arise from its left side. It also has a caudate process
(that is not tail-like shaped) arising from its right side, which provides
surface continuity between the caudate lobe and the visceral surface of the
anatomical right lobe of the liver. The caudate process is a small
elevation of the hepatic substance extending obliquely and laterally, from the
lower extremity of the caudate lobe to the undersurface of the right lobe.
Caudate lobe
Segment I is the caudate lobe and is situated posterior
l. It may receive its supply from both the right and the left branches of
portal vein. It contains one or more hepatic veins which drain directly into
the inferior
vena cava. The caudate lobe is a separate structure which
receives blood flow from both the right- and left-sided vascular branches.
Left lobe
Segments II and III are
both medial to the falciform ligament
with II superior to the portal
venous supply and III inferior.
Segment IV is found lateral to the falciform ligament and is subdivided
into IVa (superior) and IVb (inferior).
Right lobe
Segments V to VIII make
up the right part of the liver.
·
Segment V -
the most medial and inferior
·
Segment VI -
located more posteriorly
·
Segment VII -
located above segment VI
·
Segment VIII -
above segment V in the superiomedial
position
Quadrate lobe
The fissure for the round
ligament of the liver (ligamentum
teres) separates the medial and lateral parts of segment IV. The inferior medial segment (IVb) is called the quadrate lobe.
Impressions
Several
impressions on the surface of the liver accommodate the various adjacent
structures and organs. Underneath the right lobe and to the right of the
gallbladder fossa are two impressions, one behind the other and separated by a
ridge. The one in front is a shallow colic
impression, formed by the hepatic flexure and the one behind is a deeper renal impression accommodating part of
the right kidney and part of the suprarenal gland.
The suprarenal impression is a small,
triangular, depressed area on the liver. It is located close to the right of
the fossa, between the bare area and the caudate
lobe, and immediately above the renal impression. The greater part of the
suprarenal impression is devoid of peritoneum and it lodges the right
suprarenal gland.
Medial to the
renal impression is a third and slightly marked impression, lying between it
and the neck of the gall bladder. This is caused by the descending portion of
the duodenum, and is known as the duodenal
impression.
The inferior
surface of the left lobe of the liver is present behind and to the left of the gastric impression. This is molded
over the upper front surface of the stomach, and to the right of this is a
rounded eminence, the tuber omentale, which fits into the concavity of
the lesser curvature of the stomach and lies in front of the anterior layer of
the lesser
omentum.
Borders and Surfaces
It has two
borders-
·
Anterior
border
·
Posterior
border
It has two
surfaces-
·
Superior
or diaphragmatic surface
·
Inferior
or visceral surface
On the
diaphragmatic surface, apart from a triangular bare area where it connects to
the diaphragm, the liver is covered by a thin, double-layered membrane,
the peritoneum that helps to reduce friction
against other organs. This surface covers the convex shape of the two
lobes where it accommodates the shape of the diaphragm. The peritoneum folds
back on itself to form the falciform ligament and the right and left triangular
ligaments.
The visceral
surface is uneven and concave. It is covered in peritoneum apart from where it
attaches the gallbladder and the porta hepatis. The fossa of gall bladder
lies to the right of the quadrate lobe, occupied by the gallbladder with its
cystic duct close to the right end of porta hepatis.
Functional Anatomy
The central
area or hepatic hilum, includes the opening known as the
porta hepatis which carries the common bile duct and common hepatic
artery, and the
opening for the portal vein. The duct, vein, and artery divide into left and
right branches, and the areas of the liver supplied by these branches
constitute the functional left and right lobes.
The
functional lobes are separated by the imaginary plane, Cantlie's line, joining
the gallbladder fossa to the inferior vena cava. The plane separates the liver
into the true right and left lobes. The middle hepatic vein also demarcates the
true right and left lobes.
The right
lobe is further divided into an anterior and posterior segment by the right hepatic
vein.
The left lobe
is divided into the medial and lateral segments by the left hepatic
vein.
The hilum of
the liver is described in terms of three plates that contain the bile ducts and blood vessels. The contents of the whole plate
system are surrounded by a sheath.
The three
plates are-
·
Hilar Plate
·
Cystic Plate
·
Umbilical Plate
The plate
system shows many anatomical variations to be found in the liver.
Blood Supply
Arteries
The hepatic artery, a branch of the
celiac artery, divides into right and left terminal branches that enter the
porta hepatis.
Veins
The portal vein divides into right and
left terminal branches that enter the porta hepatis behind the arteries. The hepatic veins (three or more) come
out from the posterior surface of the liver and drain into the inferior vena
cava.
Blood
Circulation through the Liver
The blood vessels conveying blood to the
liver are the hepatic artery (30%) and portal vein (70%). The hepatic artery
brings oxygenated blood to the liver, and the portal vein brings venous blood
rich in the products of digestion, which have been absorbed from the
gastrointestinal tract. The arterial and venous blood is conducted to the
central vein of each liver lobule by the liver sinusoids. The central veins
drain into the right and left hepatic veins, and these leave the posterior
surface of the liver and open directly into the inferior vena cava.
Lymph Drainage
The liver produces a large amount of
lymph—about one third to one half of all body lymph. The lymph vessels leave the
liver and enter several lymph nodes in the porta hepatis. The efferent vessels
pass to the celiac nodes. A few vessels pass from the bare area of the liver
through the diaphragm to the posterior mediastinal lymph nodes.
Nerve Supply
Sympathetic and parasympathetic nerves
form the celiac plexus. The anterior vagal trunk gives rise to a large hepatic branch,
which passes directly to the liver.
Histology of Liver
Microscopically, each
liver lobe is seen to be made up of hepatic
lobules. Lobules are the functional units of
the liver. Each lobule is made up of millions of hepatic
cells (hepatocytes), which are the
basic metabolic cells. The lobules are held together by a fine, dense,
irregular, fibroelastic connective tissue layer extending from the fibrous
capsule covering the entire liver known as Glisson's
capsule.
This extends into the
structure of the liver by accompanying the blood vessels, ducts, and nerves at
the hepatic hilum. The whole surface of the liver, except for the bare area, is covered in a serous coat derived from the peritoneum, and this firmly adheres to the inner Glisson's capsule.
The lobules are roughly
hexagonal, and consist of plates of hepatocytes, and sinusoids radiating from a
central vein towards an imaginary perimeter of interlobular portal
triads. The central vein joins to the hepatic vein to carry blood out from
the liver.
A distinctive component
of a lobule is the portal
triad, which can be found running along
each of the lobule's corners. The portal triad consists of the hepatic artery,
the portal vein, and the common bile duct.
Histology shows two major types of liver
cells-
·
Non
parenchymal cells
About 70–85%
of the liver volume is occupied by parenchymal hepatocytes. Non parenchymal
cells constitute 40% of the total number of liver cells but only 6.5% of its
volume. The liver sinusoids are lined with two types of cell, sinusoidal
endothelial cells,
and phagocytic Kupffer cells. Hepatic stellate
cells are non parenchymal
cells found in the perisinusoidal space, between a sinusoid and a hepatocyte. Additionally,
intrahepatic lymphocytes are also found.
Under the microscope, each lobule is found to be
composed of rows of polygonal cells from the centre-like the spokes, radiating
of the lobule being delineated by the presence of portal triads. The periphery
of a wheel is occupied by the central (intra lobular) vein. The portal vein and
hepatic artery enter into, and bile duct and hepatic vein emerge out of the
liver through a connective tissue sheath (Glisson's capsule). After entering
centre they branch repeatedly.
The liver cells are arranged in the form of plates
having thickness of single-cell diameter throughout this structure which makes
it a sponge like structure. The cell plates are tunneled by a communicating,
system of cavities or lacunae. These contain hepatic sinusoids, endothelial
cells and phagocytic cells of RE System called Kupffer cells. The narrow space
separating the sinusoidal wall from the liver cell plates is known as Disse's
space.
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