Hepatobiliary Apparatus: Gross Anatomy and Histology of the Liver and the Gallbladder


Hepatobiliary System

Liver

The liver is the largest of the abdominal viscera and occupies a substantial portion of the upper abdominal cavity. It is an essential organ that performs a wide range of metabolic activities required for homeostasis, nutrition and immunity. It is mainly composed of epithelial cells (hepatocytes) originating from the endoderm of primitive foregut. These cells are bathed in blood derived from the hepatic portal veins and hepatic arteries, thereby facilitating continuous chemical exchange between the cells and the blood. Hepatocytes are also associated with an extensive system of minute canals, which form the biliary system into which products are secreted. 

The liver essentially has the following functions:

  • removes and breaks down of toxic, or potentially toxic, materials from the blood
  • regulates blood glucose and lipids,
  • stores of certain vitamins, iron, and other micronutrients
  • breaks down or modification of amino acidsView drug information
  • provides thermal energy to body especially at rest
  • removes particulates from the blood stream by phagocytic macrophages
  • acts as organ of hemopoiesis in fetal life
Topographic representation of Liver
Topographic representation of the liver. RH - Right Hypogastrium, EG- Epigastrium, LH- Left Hypogastrium, RL- Right Lumbar Region, U - Umbilical Region, LL - Left Lumbar Region, RI - Right Iliac Fossa, HG - Hypogastrium and LI - Left Iliac Fossa

Location and Shape of the Liver
The liver is located in the upper part of the abdominal cavity just beneath the diaphragm. The greater part of the liver is situated under cover of the right costal margin and also extends to the left to reach the left hemidiaphragm. The diaphragm separates liver from pleura, lungs, pericardium and heart. Thus it occupies most of the right hypochondrium and epigastrium and also a small portion extends into the left hypochondrium.


Click here to view the 3D image of liver from http://www.healthline.com/


The liver has an overall wedge shape, which is in part determined by the form of the upper abdominal cavity into which it grows. For example, the superior and right lateral aspects are shaped by the anterolateral abdominal and chest wall as well as the diaphragm whereas the inferior aspect is shaped by the adjacent viscera.
Liver In-situ
Liver In-situ (anterior thoracic and abdominal walls removed)

It is covered by liver capsule which plays an important part in maintaining the integrity of its shape. Once the capsule is lacerated, the liver tissue is easily parted. This, in combination with its high vascular supply, makes the liver prone to potentially lethal injuries if it is split open.

Presenting parts of Liver

Surfaces
Superior surface: Features and Relations

It is the largest surface, convex in shape and is molded to the undersurface of the domes of the diaphragm. This surface is covered by peritoneum except for a small triangular area between the two diverging layers of falciform ligament. The majority of the superior surface lies beneath the right dome (related to the right diaphragmatic pleura and base of the right lung). Centrally there is a shallow cardiac impression corresponding to the position of the heart above the central tendon of the diaphragm and is related to the pericardium. The left side of the superior surface lies beneath part of the left dome of the diaphragm and is related to part of the left diaphragmatic pleura and base of the left lung.

Anterior surface: Features and Relations

Triangular and convex in shape, the anterior surface is covered by peritoneum except at the attachment of the falciform ligament. Much of it is in contact with the anterior attachment of the diaphragm. Separated by the diaphragm, the anterior surface is related on the right with the pleura and 6th to 10th ribs and their cartilages and with 7th and 8th costal cartilages on the left. The medline area of the anterior surface lies behind the xiphoid process and the anterior abdominal wall in the infracostal angle.

Right Surface: Features and Relations

Covered by peritoneum, the right surface lies adjacent to the right dome of the diaphragm which separates it from the right lung and pleura and the seventh to eleventh ribs. The right lung and basal pleura between the diaphragm and the seventh and eighth ribs lie above and lateral to upper third of this surface. The diaphragm, the costodiaphragmatic recess, and the ninth and tenth ribs are related lateral to the middle third of the right surface. In the lower third, the diaphragm and thoracic wall are in direct contact.

The superior, anterior and right surfaces are collectively referred as diaphragmatic surface.
Surfaces of the Liver
Surfaces of the Liver. Left Top - Superior view showing most of the superior surface. Right Top - Posterior View - Posterior and inferior surface of the liver are seen, Left Bottom - Anterior View - Superior, anterior and part of the right surfaces are seen and Right Bottom - Inferior View, Inferior and posterior surfaces are seen.
Major impression on the liver surface
Relations of the Liver. Left Top - Superior view. Right Top - Posterior View. Left Bottom - Anterior View and Right Bottom - Inferior View.

Posterior surface

The posterior surface is convex, wide on the right, but narrow on the left. Much of the posterior surface is attached to the diaphragm by loose connective tissue in the region of the 'bare area'.

Features and relations:

From the left to the right
  1. The posterior surface over the left lobe presents a shallow oesophageal impression which is related to the abdominal part of the oesophagus. The posterior surface of the left lobe to the left of this impression is related to part of the fundus of the stomach.

  1. A deep median concavity near the attachment of ligamentum venosum is related to vertebral column.

  1. Fissure for ligamentum venosusm separates the caudate lobe of liver from the left lobe. The lips of the fissure give attachment to the two layers of the lesser omentum. The floor of the fissure lodges the ligamentum venosum.

  1. Caudate lobe (explained later)

  1. Groove for inferior venacava lies in the medial end of the ‘bare area’ and to the left it is related to the caudate lobe. The groove lodges the inferior venecava.

  1. The ‘bare area’ of the liver is devoid of peritoneum and is connected to the diaphragm by loose areolar tissue. Inferolateral angle of the 'bare area' presents suprarenal impression that is related with the upper pole of the right suprarenal gland.
  2. Inferior Surface


Inferior surface

The inferior surface is irregular and bounded separated from the anterior and right surface by inferior border.

Features and Relations:

  1. The inferior surface of the left lobe of liver presents gastric impression which is related inferiorly with the fundus of stomach and upper lesser omentum.

  1. Fissure for ligamentum teres lodges the ligamentum teres.

  1. Quadrate lobe (explained later) is related to pylorus, first part of duodenum and lower part of lesser omentum.

  1. Fossa for gallbladder lodges the gallbladder.

  1. To the right of the fossa for the gall bladder, inferior surface of liver is related with the first part of duodenum, the hepatic flexure of colon, the right kidney and right suprarenal gland.

The posterior and inferior surfaces are together called poeteroinferior surface or visceral surface of the liver.


Porta Hepatis:

The porta hepatis is hilum of the liver present in the inferior surface. It provides a passageway to the neurovascular and biliary structures, except the hepatic veins. It is anteriorly bounded by the quadrate lobe and the caudate process posteriorly. The portal vein, hepatic artery and hepatic nervous plexus ascend into the parenchyma of the liver. The right and left hepatic bile ducts and some lymph vessels emerge from it. At the porta hepatis, the hepatic ducts, the hepatic artery with its branches and the portal vein are arranged in that order from before backwards. The margin of porta gives attachment to the lesser omentum.
Porta Hepatis
Structures entering or leaving the liver through porta hepatis

Cross Section of Porta Hepatis
Cross section through Porta Hepatis to show the arrangement of structures of portal triad.
Structures in the Porta Hepatis
Part of the liver removed to show the structures forming the portal triad. Note that the structures of portal triad are enclosed within the free margin of lesser omentum.


Lobes

Liver has a larger right and smaller left ‘anatomical’ lobes. The line of attachment of falciform ligament on anterior and superior surfaces of liver and the fissure for ligamentus venosum and fissure for ligamentum teres on the posteroinferior surface of the liver separate the two lobes.

The right lobe also presents quadrate lobe and caudate lobe.
Lobes of the liver
Figure showing the different lobes of the liver.

Right Lobe

The right lobe is largest and contributes to all surfaces.
The line of attachment of the falciform ligament, the fissure for the ligamentum teres, the groove for the ligamentum venosum, and the attachment of the lesser omentum separate it from the left lobe.

The inferior border of the right lobe, to the right of the gallbladder, may present a bulge of tissue, which when pronounced, is referred to as Riedel's lobe.

Quadrate lobe

Present in the inferior surface
Anatomically is the part of right lobe and functionally belongs to left lobe

Boundaries
Anteriorly – inferior border of the liver
Posteriorly – porta hepatis
To the right – fossa for the gallbladder
To the left – fissure for ligamentum teres

Relations:
Pylorus and first part of duodenum


Caudate lobe
Present in the posterior surface of the liver
Anatomically is the part of right lobe and functionally belongs to left lobe

Boundaries
Above continuous with superior surface
Below porta hepatis
To the right groove for inferior vencava
To the left fissure for ligamentum venosum

Caudate process: Below and to the right, caudate lobe present a narrow strip called the caudate process. Caudate process bounds porta hepatis posteriorly and forms the upper boundary of epiploic foramen.

Left lobe

The left lobe is smaller and ends in a thin apex pointing into the left upper quadrant. Since it is substantially thinner than the right lobe it is more flexible.

Functional lobes and segments of the liver

The functional right and left lobes of the liver are separated by an imaginary plane passing along the floor of fossa for gall bladder and the groove for inferior venacava (cholecysto-caval line). On the anterosuperior surface of the liver the plane passes little right to the attachment of falciform ligament. The functional right and left lobes of the liver are of more or less equal in size.

The liver is further divided into segments. Each segment is supplied by a principal branch hepatic artery and portal vein and the bile of these segments are collected by a principal branch of hepatic duct. Each functional lobe of the liver consists of four segments. The segments I, II, III and IV belong to the functional LEFT lobe and segments V, VI, VII and VIII to the functional RIGHT lobe.
Segments of the Liver
Liver segments - Exploded View

The lobes can be further divided as right anterior (Segments V and VIII) and right posterior (Segments VI and VII) sections/sectors in the right functional lobe and left medial (Segment IV) and left lateral (Segments II and III) sections/sectors in the left functional lobe. The hepatic veins lie in liver parenchyma between the sections.  

Segment I corresponds to the gross anatomical caudate lobe and segment IV to the quadrate lobe.


Peritoneal ligaments of the liver

Falciform ligament

Falciform ligament develops from ventral part of ventral mesogastrium. It attaches the liver to the anterior abdominal wall. It is a two-layered fold of peritoneum that ascends from the umbilicus to along the posterior surface of anterior abdominal wall. On reaching the superior surface of the liver, the ligament splits into right and left layers. The right layer turns laterally and forms the upper layer of the coronary ligament, (the extreme of which is called the right triangular ligament) whereas the left layer turns medially and forms the anterior layer of left triangular ligament.

Falciform ligament has a sickle shaped free margin that contains ligamentum teres hepatis or round ligament of liver. Ligamentum teres represents the obliterated umbilical vein which extends from umbilicus and ascends upward in free margin of falciform ligament. On reaching the lower border of liver,  ligamentum teres passes into the fissure on the visceral  (inferior) surface of the liver and joins the left branch of the portal vein in the porta hepatis.

Ligaments of the liver - Anterosuperior View
Ligaments of the liver. Note the ligamentum teres enclosed in the free margin of falciform ligament - Anterosuperior View.


Coronary Ligament

The coronary ligament is formed by the reflection of the peritoneum from the diaphragm onto the posterior surfaces of the right lobe of the liver. It consists of the upper/superior and lower/inferior layers. Between the two layers of this ligament bounded laterally by groove for inferior venacava, there is a large triangular area of liver devoid of peritoneal covering. This triangular area in the liver is called the 'bare area' of the liver. This part of liver is attached to the diaphragm by areolar tissue. The two layers of coronary ligament meet on the right to form the right triangular ligament. The upper layer of the coronary ligament is reflected superiorly onto the inferior surface of the diaphragm and inferiorly onto the right and superior surface of the liver. The lower layer of the coronary ligament reflects inferiorly over the right suprarenal gland and right kidney, and superiorly onto the inferior surface of the liver.
Ligaments of the Liver Posterosuperior View
Ligaments of the liver. Also note the 'bare area' of the liver - Posterosuperior View.


Triangular Ligaments

The left triangular ligament represents double layer of peritoneum which extends from diaphragm to a variable length over the superior border/surface of the left lobe of the liver. As already mentioned the anterior layer of the ligament is formed by the left layer of falciform ligament whereas the posterior layer is continuous with the left layer of lesser omentum.

The right triangular ligament that forms the apex of the ‘bare area’ of liver is formed at the extreme of coronary ligament by the union of its two layers. It connects the right surface of the liver to the diaphragm.
Ligaments of the liver -Posteroinferior View
Ligaments of the Liver - Posteroinferior View


Lesser Omentum

The lesser omentum is also the peritoneal fold that is attached on the margin of the porta hepatis and the fissure for the ligamentum venosum and passes down to the lesser curvature of the stomach and proximal part of duodenum. Thus lesser omentum has two components- hepatogastric and hepatoduodenal. The attachment to the liver is L-shaped. The vertical component follows the line of the fissure for the Ligamentum venosum (Ligamentum venosum, the remains of ductus venosus, is attached to the left branch of the portal vein below and the inferior venacava above and runs in the fissure on the inferior surface of the liver.) The horizontal component attaches on the margin of porta hepatis. At its upper end, the superior or left layer of lesser omentum is continuous on the left with the posterior layer of the left triangular ligament, and the inferior or right layer is continuous on the right with the coronary ligament as it encloses the inferior vena cava. At its lower end, it presents free anterior margin where the two layers diverge to enclose the portal vein, bile duct and hepatic artery.
Ligaments of the liver - Viewd from behind
Ligaments of the Liver - Viewed from behind
Portal triad in free margin of lesser omentum
Note the structures of the portal triad (portal vein, hepatic artery and bile duct) in the free margin of lesser omentum.



Neurovascular supply of liver

The portal vein and hepatic artery ascend in the lesser omentum and enter the liver at the porta hepatis, where each bifurcates into right and left branches. The hepatic bile duct and lymphatic vessels leave the liver at porta hepatis and descend through the same omentum. The hepatic veins directly drain into the inferior vena cava after leaving the liver through it posterior surface.

Hepatic Artery:
It arises from the coeliac trunk of abdominal aorta.
The artery may be subdivided into the common hepatic artery - from the coeliac trunk to the origin of the gastroduodenal artery - and the hepatic artery 'proper' - from that point to its bifurcation.

Course:
After its origin, it passes anteriorly and laterally to the upper surface of first part of the duodenum. It then moves forward beneath the peritoneal floor (of epiploic foramen) and reaches the free border of the lesser omentum. It then ascends in the free omental margin in front of epiploic foramen. Here, the artery lies left to the common bile duct and anterior to the portal vein. At the porta hepatis, it divides into right and left hepatic arteries.

Major Branches:
Right Gastric artery
Gastroduodenal artery
Cystic artery from right hepatic artery
Hepatic artery and its branches
Note the hepatic artery and its branches

Veins
The liver has two venous systems. The portal venous system that conveys venous blood from the majority of the gastrointestinal tract and its associated organs to the liver and the hepatic venous system that drains blood from the liver parenchyma into the inferior vena cava

Hepatic Veins
The hepatic veins convey blood from the liver to the inferior vena cava. The tributaries arise within the parenchyma of the liver and emerge from the posterior hepatic surface to open directly into the inferior vena cava in its groove on the posterior hepatic surface.

Hepatic veins are arranged in upper and lower groups.

The veins in upper group are usually large and commonly referred to as the right, middle and left hepatic veins. The right hepatic vein drains segments V, VI, VII and VIII. The middle hepatic vein lies between segments IV and VIII and drains both these segments and segment V. The left hepatic vein drains segments II and III with some drainage from segment IV.

The lower groups vary in number and extent of distribution. They drain segment I and occasionally from segments VII and VIII.
Hepatic Veins
Showing the Hepatic Veins

Lymphatic drainage

The lymphatic from the liver are arranged into two sets:
Superficial lymphatics run in subserous areolar tissue and drain into:

  1. Subdiaphragmatic lymph nodes (most of the posterior surface, surface of caudate lobe and posterior part of inferior surface)
  2. Hepatic lymph nodes (most of inferior surface, anterior and most of the superior surface)
  3. Coeliac lymph nodes (few lymphatics from right surface)
  4. Paracardiac lymph nodes ()few lymphatics from posterior surface of left lobe)

Deep lymphatics

Most of the liver parenchyma is drained by deep lymphatic vessels present within the substance of the liver. The lymph vessels partly accompany the hepatic veins and drain into supra-diaphragmatic lymphnodes and partly accompany portal vein draining into the hepatic lymph nodes.



Nerve supply

The liver parenchyma is supplied by hepatic nerves, which arise from the hepatic plexus and contain sympathetic and parasympathetic (vagal) fibres. They enter the liver at the porta hepatis and largely accompany the hepatic arteries and bile ducts.

The capsule is supplied by branches of the lower intercostal nerves, which also supply the parietal peritoneum.

The hepatic plexus is the largest derivative of the coeliac plexus. It also receives branches from the anterior and posterior vagus nerves. It accompanies the hepatic artery and portal vein and their branches into the liver, where its fibres run close to the branches of the vessels. These branches supply vasomotor fibres to the hepatic vessels and biliary tree.

Branches to the gallbladder form a delicate cystic plexus. Multiple fine branches from the plexus supply the common and hepatic bile ducts directly. The vagal fibres are motor to the musculature of the gallbladder and bile ducts and inhibitory to the sphincter of the bile duct.  
Nerve supply of the Liver
Showing the innervation of the liver

Ducts of liver

Bile ducts of the liver:

The bile ducts of the liver consist of right and left hepatic ducts, the common hepatic duct, the bile duct, the gall bladder and the cystic duct.

The formation of intrahepatic bile duct is described in the section structure of the liver. The Intrahepatic bile ducts at porta hepatis of the liver form the right and left hepatic ducts. The right hepatic duct drains the right lobe of the liver whereas the left hepatic duct drains the left, caudate and quadrate lobe of the liver.

Ducts of the Liver
Figure showing the ducts of the liver (Underlined with red lines)

Hepatic Duct

After the short course the right and left hepatic ducts unite to form common hepatic duct which is about 1½ inches (4cm) long and descends within the free margin of lesser omentum. It is joined on its right side by the cystic duct thereby forming the bile duct.

Bile duct

The bile duct or Common bile duct is about 3 inches (8cm) long. In the first part of its course, it lies in the right free margin of the lesser omentum whereas in the second part of its course, it lies behind the first part of the duodenum. Further in the third part of its course it lies in a groove on the posterior surface of the head of the pancreas. The bile duct ends by piercing the wall of the second part of the duodenum where it is usually joined by main pancreatic duct and together they open into the ampulla of Vater.

Gall Bladder

Location

It is a pear shaped sac lying on the inferior surface of the liver. It has a capacity of about 30-50 ml and stores bile.

Presenting parts of gall bladder

Fundus, body and neck

Fundus is rounded and comes in contact with the anterior abdominal wall at the level of tip of the ninth costal cartilage.

The body lies in contact with the liver surface and is directed upward, backward and to the left.

The neck becomes continuous with the cystic duct which turns into the lesser omentum to join the right side of the common hepatic duct to form the bile duct.

Structure of the liver

The histological features of the liver are summarized below.
         Structural component of liver include
        Connective tissue stroma
        Parenchyma
        Portal triad
        Sinusoids
        Bile canaliculi

         There are three way to describe the structure of liver
        The classic lobule
        The portal lobule
        Portal acinus

Classical lobule

         Connective tissue stroma
     liver is covered by a thin connective tissue capsule (Glisson's capsule) and extends into its interior as numerous branching septa.
        Blood vessels, nerves, lymphatic vessels and bile ducts travel within these septa.

         Parenchyma

    In cross section, the substance of liver appears to be made up of hexagonal areas that constitute  the hepatic lobules. In certain animals (eg, pigs micrograph b in adjacent figure), the lobules are separated by connective tissue septa (C). But in human (micrograph c in adjacent figure) the septa are not distinct and the lobules appear to merge with each other making it difficult to establish the exact limits between different lobules. 

     Each lobule consists of interconnected plate of liver cells (hepatocytes). These plates are one cell thick that branch and anastomose with each other to form network.

        The center of the lobule is occupied by a central vein (V) which is a tributary of hepatic vein

   Along the periphery of each lobule there are angular intervals filled by connective tissue. These intervals are called portal canal/tract (T). Each portal tract contains a venule (a branch of the portal vein), an arteriole (a branch of the hepatic artery), a duct (part of the bile duct system).These three structures collectively form a portal triad.

   venule contains blood coming from the superior and inferior mesenteric and splenic veins.
   The arteriole contains oxygen-rich blood coming from the celiac trunk of the abdominal aorta.
  The duct, carries bile synthesized by the hepatocytes and eventually empties into the hepatic duct
Liver structure - Photomicrographs
Photomicrographs showing the section of the liver at low magnification: Top - pig liver and Below - Human liver
Liver structure - high magnification
Photomicrograph showing structure of the liver at higher magnification: V - Central vein.

Bile canaliculi

    Each liver cell is cuboidal presenting six surfaces, out of which two surfaces are related to the sinusoids and the remaining surface are in contact with the wall of adjacent hepatocyte. Wherever two hepatocytes are in contact, they make a tubular space between them known as the bile canaliculus
        Bile is synthesised by hepatocytes and secreted into bile canaliculi
        Bile canaliculi of adjacent hepatocyte plates merge to form canals of Hering or bile ductules which latter finally drain into the bile duct in the portal canal
Bile canaliculus
Schematic diagram of structure of the liver. Note the formation of bile canaliculus (labeled inside the box)

Portal lobule

    The portal lobule emphasizes the exocrine functions of the liver
   The portal lobule includes the territory of liver tissue centered around a portal triad, and is drawn by joining the central veins of the three adjacent liver lobules
    The territory of portal lobule includes those portion of three classic lobules that secrete the      bile that drains into its axial bile duct.

Portal lobule and liver acinus
Schematic diagram of the liver structure to show Portal Lobule (hexagonal outline) and Liver Acinus (diamond shaped outline)

Liver acinus

    Liver acinus is the structural unit that provides the best correlation between blood perfusion and metabolic activity
         It consists of the area of liver tissue supplied by one hepatic arteriole running along the line of junction of two hepatic lobule. Two central veins lie at the ends of the acinus.

Structure of Gall Bladder

The histological features of the gallbladder are summarized below.


  consists of a mucosa composed of simple columnar epithelium and lamina propria, fibromuscular layer, and a serous membrane


         Mucous membrane:
   lined by is a single layer of tall columnar absorptive cells bearing numerous short irregular microvilli . Goblet cells are absent.
      Mucosa is highly folded, and the folds are called rugae.The submucosa (S) is relatively loose. 

     The fibromuscular coat: is composed of fibrous tissue (A) mixed with smooth muscle cells (M)  which are arranged loosely in longitudinal, circular and oblique bundles but do not form distinct layers. 


Gallbladder Histology - Photomicrograph
Photomicrograph of the Gallbladder. S - Submucosa, M - Smooth Muscles layer and A - Fibrous Adventita



References
Gray's Anatomy
K. L. Moore's Clinically Oriented Anatomy
R. Snell's Clinical Anatomy
Wheater's Functional Histology

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