Difference between revisions of "Liver - Anatomy & Physiology"

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<big><center>[[Alimentary - Anatomy & Physiology|'''BACK TO ALIMENTARY - ANATOMY & PHYSIOLOGY''']]</center></big>
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{{OpenPagesTop}}
 
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[[Image:Pig Liver Topography.jpg|thumb|right|300px|Liver Topography (Pig) - Copyright Nottingham 2008]]
 
 
 
==Introduction==
 
==Introduction==
 
 
The liver (hepar) is an extremely important organ in the body of mammals and vertebrates as it provides functions essential for life. It is the largest internal organ and has numerous functions including production of bile and protein, fat and carbohydrate metabolism. During foetal development, the liver has an important haemopoetic function, producing red and white blood cells from tissue between the hepatic cells and vessel walls.
 
The liver (hepar) is an extremely important organ in the body of mammals and vertebrates as it provides functions essential for life. It is the largest internal organ and has numerous functions including production of bile and protein, fat and carbohydrate metabolism. During foetal development, the liver has an important haemopoetic function, producing red and white blood cells from tissue between the hepatic cells and vessel walls.
  
The size of the liver varies due to its role in metabolism. In carnivores the liver weighs about 3-5% of body weight, in omnivores 2-3% and in herbivores 1.5%. the liver is much heavier in young animals than older animals as it atropies with age.
+
The size of the liver varies due to its role in metabolism. In carnivores the liver weighs about 3-5% of body weight, in omnivores 2-3% and in herbivores 1.5%. the liver is much heavier in young animals than older animals as it atrophies with age.
  
 
The liver is derived from an outpocketing of endoderm epithelium on the ventral duodenum from the caudal part of the foregut. The connection to the gut narrows to become the bile duct. The parenchymal tissue of the liver is formed from proliferating epithelial cords or strands which integrate with the blood sinuses of the umbilical and vitelline veins.
 
The liver is derived from an outpocketing of endoderm epithelium on the ventral duodenum from the caudal part of the foregut. The connection to the gut narrows to become the bile duct. The parenchymal tissue of the liver is formed from proliferating epithelial cords or strands which integrate with the blood sinuses of the umbilical and vitelline veins.
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==Structure==
 
==Structure==
  
[[Image:Topography of the Liver.jpg|thumb|right|150px|Topography of the Liver (Dog)- Copyright RVC 2008]]
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[[Image:Topography of the Liver.jpg|thumb|right|250px|Topography of the Liver (Dog)- Copyright RVC 2008]]
*Cranial part of the abdomen
 
 
 
*Immediately caudal to the diaphragm
 
 
 
*Cranial to the stomach and intestines
 
 
 
*Generally the bulk of the liver on the right of the midline
 
 
 
*Divided into lobes by fissures
 
 
 
*Cranially the liver is convex, called the diaphragmatic surface
 
 
 
*Caudally the liver is concave, called the visceral surface
 
 
 
*Caudate lobe has a renal impression from the right kidney
 
  
*Gastric impression occupies the whole of the left half of the visceral face
+
The liver is located in the cranial part of the abdomen. It is immediately caudal to the diaphragm and cranial to the [[Monogastric Stomach - Anatomy & Physiology|stomach]] and intestines. Generally the bulk of the liver is on the right of the midline. It is divided into lobes by fissures. Cranially the liver is convex, called the diaphragmatic surface. Caudally the liver is concave, called the visceral surface. The caudate lobe has a renal impression from the right [[Urinary System Overview - Anatomy & Physiology#Upper Urinary Tract|kidney]]. The gastric impression occupies the whole of the left half of the visceral face. The [[Duodenum - Anatomy & Physiology|duodenal]] impression at the junction of the right and quadrate lobes continues onto the right lateral and caudate lobes. Passages or notches on the median plane allow the caudal vena cava and [[Oesophagus - Anatomy & Physiology|oesophagus]] to pass by. The [[Gall Bladder - Anatomy & Physiology|gall bladder]] is located between the right medial and quadrate lobes. '''Reticular fibres''' (collagen type III, proteoglycans and glycoproteins) support the hepatocytes and walls of the sinusoids. Interlobular spaces support bile ducts and blood vessels. The lesser omentum (often fat filled) is on the visceral surface between the left lateral lobe, heptic porta and lesser curvature of the [[Monogastric Stomach - Anatomy & Physiology|stomach]]. There is a oesophageal notch where the [[Oesophagus - Anatomy & Physiology|oesophagus]] passes over the liver.
  
*Duodenal impression at the junction of the right and quadrate lobes continuing onto the right lateral and caudate lobes
+
==Divisions of the Liver==
  
*Passages or notches on the median plane allow the caudal vena cava and oesophagus to pass by
+
The liver can be divided into lobes, lobules, hepatocytes and sinusoids.
 
 
*The [[Gall Bladder - Anatomy & Physiology|gallbladder]] is located between the right medial and quadrate lobes
 
 
 
*Reticular fibres (collagen type III, proteoglycans and glycoproteins) support the hepatocytes and walls of the sinusoids
 
 
 
*Interlobular spaces support bile ducts and blood vessels
 
 
 
*Lesser omentum (often fat filled) on the visceral surface between the left lateral lobe, heptic porta and lesser curvature of the stomach
 
 
 
*Oesophageal notch where oesophagus passes over the liver
 
 
 
===Divisions of the Liver===
 
 
 
*Lobes
 
 
 
*Lobules
 
 
 
*Hepatocytes and sinusoids
 
  
 
===Lobes of the Liver===
 
===Lobes of the Liver===
  
*Left lateral
+
The lobes of the liver include the left lateral, left medial, right lateral, right medial, quadrate, caudate and papillary.
  
*Left medial
+
==Ligaments==
  
*Right lateral
+
The '''coronary ligament''' attaches the liver (from the diaphragmatic surface) to the diaphragm. It is an irregular fold of peritoneum. It surrounds the triangular base of the diaphragmatic surface. It is continuous with outer most layer of the caudal vena cava.
 
+
The '''falciform ligament''' is ventral to the coronary ligament. It is a fat filled embryological remnant of the fetal blood vessels from the placenta. It causes problems for surgical entry into the abdomen. It is located cranial to the umbilicus and is a vestige of the umbilical vein.
*Right medial
+
The '''triangular ligament''' is on the right and left sides of the coronary ligament.
 
 
*Quadrate
 
 
 
*Caudate
 
 
 
*Papillary
 
 
 
===Ligaments===
 
 
 
*Coronary ligament  
 
**Between the liver and diaphragm on the diaphragmatic surface of the liver
 
**Irregular fold of peritoneum
 
**Surounds the triangular base of the diaphragmatic surface
 
**Continuous with outer most layer of the caudal vena cava
 
 
 
*Falciform ligament
 
**Ventral to the coronary ligament
 
**Fat filled embryological remnant of the fetal blood vessels from the placenta
 
**Causes problems for surgical entry into the abdomen
 
**Located cranial to the umbilicus
 
 
 
*Triangular ligament
 
**Right and left sides of the coronary ligament
 
  
 
==Function==
 
==Function==
  
*Production of bile
+
Production of bile see <big>'''[[Bile Formation]]'''</big>.
 
 
*Nearly all the blood circulated around the adbomen flows back through the portal vein to the liver where it comes in contact with the liver cells, ensuring the products of digestion are presented to the hepatic cells before entering the general circulation.
 
 
 
*Carbohydrate metabolism
 
**Glycogenesis
 
**Glyconeolysis
 
**Gluconeogenesis
 
**Breakdown of insulin and other hormones
 
 
 
*Protein metabolism
 
**Produces soluble mediators of the clotting cascade
 
**Albumin
 
**Hormone transporting globulins
 
 
 
*Lipid metabolism
 
**Lipogenesis
 
**Synthesis of cholesterol
 
 
 
*Hormonal control
 
**Insulin and glucagon
 
**Glucocortocoids
 
**Catecholamines
 
 
 
*Immunoregulation
 
**Kupfer cells
 
**Complement synthesis and metabolism
 
 
 
*Storage
 
**Water soluble vitamins
 
**fat soluble vitamins
 
**Iron
 
**Triglyceride
 
**Glycogen
 
  
*Breaks down haemoglobin
+
Nearly all the blood circulated around the abdomen flows back through the portal vein to the liver where it comes in contact with the liver cells, ensuring the products of digestion are presented to the hepatic cells before entering the general circulation. Other functions include carbohydrate metabolism, glycogenesis, glyconeolysis, gluconeogenesis and the breakdown of insulin and other [[Hormones - Anatomy & Physiology|hormones]]. Protein metabolism produces soluble mediators of the clotting cascade, Albumin and hormone transporting globulins. The liver is also involved in lipid metabolism, lipogenesis and the synthesis of cholesterol.
  
*Breaks down toxic substances through drug metabolism
+
The liver has a role in hormonal control of the following; Insulin and glucagon, Glucocortocoids, Catecholamines and the synthesis of other important [[Hormones - Anatomy & Physiology|hormones]] (see [[Liver Endocrine Function - Anatomy & Physiology|Endocrine System]]). It also has a role in immunoregulation via kupfer cells and the complement synthesis and metabolism.
  
*Converts ammonia to urea
+
The liver is important in storage of water soluble vitamins, fat soluble vitamins, iron, triglyceride and glycogen.
  
*Management of endogenous waste, e.g haem (Hb, cytochromes, Mb) and ammonia (amino acids)
+
The liver breaks down haemoglobin and toxic substances through drug metabolism. It converts ammonia to urea and allows the management of endogenous waste, e.g haem (Hb, cytochromes, Mb) and ammonia (amino acids).
  
 
==Vasculature==
 
==Vasculature==
  
[[Image:Blood Flow In and Away from the Liver.jpg|thumb|right|150px|Blood Flow in the Liver - Copyright nabrown RVC 2008]]
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[[Image:Blood Flow In and Away from the Liver.jpg|thumb|right|250px|Blood Flow in the Liver - Copyright nabrown RVC 2008]]
*Dual blood supply
 
**70-80% hepatic portal vein (nutrient rich)
 
**20-30% hepatic artery (oxygen rich)
 
 
 
*Large blood supply (nearly a 1/3 of cardiac output passes through the liver)
 
 
 
*Hepatic artery
 
 
 
*Branch of the caeliac artery
 
 
 
*Portal vein
 
**Formed by tributaries draining the spleen, pancreas and digestive tract
 
  
*Intrahepatic arteries combine with portal vein branches to supply the connective tissue and hepatic sinusoids of the liver
+
The liver has a dual blood supply. 70-80% via the hepatic portal vein (nutrient rich) and 20-30% via the hepatic artery (oxygen rich). It has a large blood supply (nearly a 1/3 of cardiac output passes through the liver).
  
*Blood flows from the portal areas into the central vein
+
The '''hepatic artery''' is a branch of the caeliac artery. The '''portal vein''' is formed by tributaries draining the [[Spleen - Anatomy & Physiology|spleen]], [[Pancreas - Anatomy & Physiology|pancreas]] and digestive tract. '''Intrahepatic arteries''' combine with '''portal vein''' branches to supply the connective tissue and hepatic sinusoids of the liver. Blood flows from the portal areas into the central vein. The central vein is lined by simple squamous epithelium. The bile duct, blood vessels (including the important hepatic vein) and nerves enter and leave the liver at the hepatic porta. Blood from the central vein opens into the caudal vena cava. Liver circulation is controlled by interarterial, intervenous, arteriovenous, and by sphincter mechanisms, allowing careful regulation.
 
 
*Central vein lined by simple squamous epithelium
 
 
 
*the bile duct, blood vessels (including the important hepatic vein) and nerves enter and leave the liver at the hepatic porta
 
 
 
*Blood from the central vein open into the caudal vena cava
 
 
 
*Liver circulation controlled by interarterial, intervenous and arteriovenous, by sphincter mechanisms allowing carefull reglulation
 
  
 
==Innervation==
 
==Innervation==
  
*Sympathetic nerves from periarterial plexuses
+
The liver is innervated by sympathetic nerves from the '''periarterial plexuses''' and '''parasympathetic nerves''' from the vagal trunk.
 
 
*Parasympathetic nerves from vagal trunk
 
 
 
  
 
==Lymphatics==
 
==Lymphatics==
  
*Effernt vessels pass to hepatic nodes around the hepatic porta
+
Efferent vessels pass to hepatic nodes around the hepatic porta. The lymph drains into the visceral cysterna chyli. Some lymph travels to the accessory hepatic and caudal mediastinal lymph nodes on the caudal vena cava.
 
 
*The lymph drains into the visceral cysterna chyli
 
 
 
*Some lymph travels to the accessory hepatic and caudal mediastinal lymph nodes on the caudal vena cava
 
 
 
 
 
==Histology==
 
 
 
[[Image:Lobule Histology.jpg|thumb|right|150px|Liver Lobule Histology - Copyright RVC 2008]]
 
*Connective tissue capsule around each lobule
 
 
 
*Thin mesothelium covers connective tissue layer
 
 
 
*The larger liver cells are called lobules
 
**Each lobule contains an opening for the central vein and contains portal areas
 
**Lobules composed of liver cords called hepatocytes
 
**Sinusoids present between hepatocytes containing red blood cells
 
 
 
*Portal area present in lobules
 
**Hepatic artery- thick walls, small diameter
 
**Hepatic vein- thin walls, large and irregular shape
 
**Bile ducts- cuboidal or columnar epithelium
 
**Lymphatics- small and delicate
 
[[Image:Portal Triad Histology.jpg|thumb|right|150px|Portal Triad Histology in a Lobule- Copyright RVC 2008]]
 
 
 
*Hepatocytes are the smaller liver cells in the lobules
 
**Contain glycogen granules
 
**Pink stain as are eosinophilic
 
**Spherical nucleus
 
**Forms cords called branching plates (lamellae)
 
**Upper and lower margins are tight junctions
 
**3 functioning surfaces
 
***Adjacent to sinusoid contains microvilli
 
***Between adjacent cells are smooth
 
***Adjacent cells have short microvilli, membranes diverge, explanded intracellular spaces and bile canaliculi
 
 
 
*Kupfer macrophages present near the lining of the sinusoids
 
 
 
  
 
==Hepatic Duct Systems==
 
==Hepatic Duct Systems==
  
*Canaliculi within lobules
+
There are '''canaliculi''' within lobules. Canaliculi open into larger ductules then into a few large hapatic ducts. Before and shortly after leaving the hepatic porta, the ducts combine into a single trunk which runs to the [[Duodenum - Anatomy & Physiology|duodenum]]. The cystic duct runs from common trunk to the [[Gall Bladder - Anatomy & Physiology|gall bladder]] transporting bile from the liver to the [[Gall Bladder - Anatomy & Physiology|gall bladder]]. Distal to the cystic duct is the bile duct (ductus choledochus) which transports bile from the [[Gall Bladder - Anatomy & Physiology|gall bladder]] into the [[Duodenum - Anatomy & Physiology|duodenum]]. There are no valves, so bile may flow in either direction.
  
*Canaliculi opein into larger ductules then into a few large hapatic ducts
+
[[Image:Formation of Bile Acids.jpg|thumb|right|250px|Formation of Bile Acids - Copyright RVC 2008]]
  
*Before and shortly after leaving the hepatic porta the ducts combine into a single trunk which runs to the [[Duodenum - Anatomy & Physiology|duodenum]]
 
 
*The cystic duct runs from common trunk to the [[Gall Bladder - Anatomy & Physiology|gall bladder]] transporting bile from the liver to the [[Gall Bladder - Anatomy & Physiology|gall bladder]]
 
 
*Distal to the cystic duct is the bile duct (ductus choledochus) which transports bile from the gall bladder into the [[Duodenum - Anatomy & Physiology|duodenum]]
 
 
*No valves so bile may flow in either direction
 
 
[[Image:Formation of Bile Acids.jpg|thumb|right|150px|Formation of Bile Acids - Copyright RVC 2008]]
 
 
==Bile Acids==
 
==Bile Acids==
  
*Composed of cholesterol, bile acids and steroids
+
Bile acids are composed of cholesterol, bile acids and steroids. The main bile acid is '''cholic acid''' (C24). Conjugated to taurine or glycine in the liver to reduce pKa so they exist in an ionised form as bile salts. Bile salts conjugate with cholesterol and phospholipids and are then secreted into the bile.  They emulsify fats which helps absorb fat soluble vitamins. In aqueous solution, they form micelles which are amphiphilic and can transport free fatty acids across the brush border.
 +
==Species Differences==
 +
[[Image:Canine Liver Topography.jpg|thumb|right|250px|Liver Topography (Dog) - Copyright Nottingham 2008]]
  
*Main bile acid is cholic acid (C24)
 
 
*Conjugated to taurine or glycine in the liver to reduce pKa so they exist in an ionised form as bile salts
 
 
*Bile salts conjugate with cholesterol and phospholipids and are then secreted into the bile
 
 
*95% are recycled in enterohepatic circulation
 
 
*Emulsify fats
 
 
*Helps absorb fat soluble vitamins
 
 
*In aqueous solution form micelles which are amphiphilic and can transport free fatty acids across the brush border
 
 
 
==Species Differences==
 
[[Image:Canine Liver Topography.jpg|thumb|right|150px|Liver Topography (Dog) - Copyright Nottingham 2008]]
 
 
===Canine & Feline===
 
===Canine & Feline===
  
*Both left and right lobes subdivided
+
Both the left and right lobes are subdivided. Complete obstruction of the hepatic artery is fatal. The liver is almost entirely intra-thoracic. An enlarged caudate process contacts the right [[Urinary System Overview - Anatomy & Physiology#Upper Urinary Tract|kidney]].
 
 
*Complete obstruction of the hepatic artery is fatal
 
 
 
*Almost entirely intra-thoracic
 
 
 
*Enlarged caudate process contacts right kidney
 
  
 
===Equine===
 
===Equine===
  
*Contained entirely within the rib cage
+
The liver is contained entirely within the rib cage, to the right of the midline. It is less lobated. There is no [[Gall Bladder - Anatomy & Physiology|gall bladder]] and the left lobe is subdivided. There is no papillary lobe. In the foal, the liver is larger and more symmetrical. The bile duct opens into the [[Duodenum - Anatomy & Physiology|duodenum]] at the same papillae as the major pancreatic duct. Bile is constantly secreted.
 +
[[Image:Pig Liver Topography.jpg|thumb|right|250px|Liver Topography (Pig) - Copyright Nottingham 2008]]
  
*To the right of the midline
+
===Porcine===
  
*Less lobated
+
The liver has deep interlobular fissures and a large amount of interlobular connective tissue. It has a mottled appearance. A Deep interlobular fissure divides the liver into 4 lobes- the left, right, medial and lateral. There is a small caudate lobe (which does not contact the [[Urinary System Overview - Anatomy & Physiology#Upper urinary Tract|kidney]] so no renal impression). It is mostly on the right of the midline and has no papillary lobe.
  
*No [[Gall Bladder - Anatomy & Physiology|gall bladder]]
+
===Ruminants===
  
*Left lobe subdivided
+
The liver is entirely displaced to right of the midline. It has fused lobes.
  
*No papillary lobe
+
'''Small Ruminants'''
 +
Sheep have a deeper umbilical fissure than cows. Sheep also have a smaller caudate lobe than cows and have two papillary processes.
  
*In the foal the liver liver is larger and more symmetrical
+
===Avian===
  
*Bile duct opens into the [[Duodenum - Anatomy & Physiology|duodenum]] at the same papillae as the major pancreatic duct
+
See [[Avian Liver - Anatomy & Physiology|avian liver]]
  
*Bile is constantly secreted
+
==Histology==
[[Image:Pig Liver Topography.jpg|thumb|right|150px|Liver Topography (Pig) - Copyright Nottingham 2008]]
+
[[Image:Lobule Histology.jpg|thumb|right|250px|Liver Lobule Histology - from [[Gastrointestinal Tract Histology resource|GIT-2 tutorial]] ]]
===Porcine===
+
The larger liver cells are called '''lobules'''. Each lobule contains an opening for the '''central vein''' and contains portal areas. The lobules are composed of liver cords called '''hepatocytes'''. Sinusoids are present between hepatocytes containing red blood cells. There is a connective tissue capsule around each liver lobule. A thin '''mesothelium''' covers the connective tissue layer.
  
*Deep interlobular fissures
+
The '''portal area''' present in the lobules contains the '''hepatic artery''', which has thick walls and a small diameter and the '''hepatic vein''', which has thin walls and a large and irregular shape. It also contains '''bile ducts''', with cuboidal or columnar epithelium and '''lymphatics''' that are small and delicate.
 +
[[Image:Portal Triad Histology.jpg|thumb|right|250px|Portal Triad Histology in a Lobule- from [[Gastrointestinal Tract Histology resource|GIT-2 tutorial]] ]]
  
*Large amount of interlobular connective tissue
+
'''Hepatocytes''' are the smaller liver cells in the lobules. They contain glycogen granules and have a spherical nucleus. They form cords called '''branching plates''' (lamellae). The upper and lower margins are tight junctions. They have 3 functioning surfaces. '''Kupfer macrophages''' are present near the lining of the sinusoids. The hepatocytes stain pink as they are '''eosinophilic'''.
  
*Mottled appearance
 
  
*Deep interlobular fissure divides the lived into 4 lobes- the left, right, medial and lateral
+
===Acinus===
  
*Small caudate lobe (which does not contact the kidney so no renal impression)
+
*the smallest functional unit of the [[Liver - Anatomy & Physiology|liver]] - this unit stresses the dependence of the [[Liver - Anatomy & Physiology|liver]] on its afferent blood vessels and efferent bile ducts
 +
*a roughly diamond shaped parenchymal mass surrounding and supplied by the portal area
 +
*consists of 3 zones:
 +
**Zone 1 - periportal (centroacinar), around the portal areas
 +
**Zone 2 - midzonal
 +
**Zone 3 - centrilobular (periacinar), bordering the hepatic venules
 +
NB: blood enters from Zone 1 to Zone 3 and thus becomes less and less oxygenated
  
*Mostly on the right of the midline
+
==Links==
 
 
*No papillary lobe
 
 
 
===Ruminants===
 
  
*Entirely displaced to right of midline
+
'''Click here for information on [[:Category:Liver - Pathology|pathology of the Liver]]'''
  
*Fused lobes
+
'''Click here for information on [[Hepatic Stellate Cells]]'''
  
'''Small Ruminants'''
+
'''Click here for information on [[Bile Formation]]'''
*Sheep have a deeper umbilical fissure than cows
 
  
*Sheep have a smaller caudate lobe than cows
+
'''Click here for information on [[Portosystemic Shunt|portosystemic shunting]]'''
 
 
*Sheep have two papillary processes
 
 
 
===Avian===
 
 
 
*See [[Avian Liver - Anatomy & Physiology|avian liver]]
 
 
 
==Links==
 
  
[[Liver|Pathology of the Liver]]
 
  
[[Liver - Developmental#Portosystemic shunt|Portosystemic Shunting]]
+
{{Learning
 +
|dragster =[[Comparative Liver Anatomy Dragster resource|Compare liver structure in different species]]
 +
|flashcards = [[Liver - Anatomy & Physiology - Flashcards]]
 +
|videos =[https://wikivet.mediacore.tv/media/bovine-liver-potcast Bovine liver potcast]<br>[https://wikivet.mediacore.tv/media/bovine-liverpng Bovine liver potcast 2]<br>[https://wikivet.mediacore.tv/media/ovine-liver-with-hepatitis-cysticercosa-potcast Ovine liver with hepatitis cysticercosa potcast]<br>[https://wikivet.mediacore.tv/media/equine-liver-with-hydatid-cysts-potcast Equine liver with hydatid cysts potcast]<br>[https://wikivet.mediacore.tv/media/equine-liver-potcast Equine liver potcast]<br>[http://www.onemedicine.tuskegee.edu/DigestiveSystem/Acessory_Organs/Access_Equine.html Liver lobes of the Horse]<br>[http://www.onemedicine.tuskegee.edu/DigestiveSystem/Acessory_Organs/Access_Ruminants.html Liver lobes of the Cow]<br>
 +
|powerpoints = [[Gastrointestinal Tract Histology resource|Histology of the liver - see part 2]]
 +
|Vetstream = [https://www.vetstream.com/canis/Content/Disease/dis01079.asp Acute Liver disease]
 +
}}
  
[http://stream2.rvc.ac.uk/Anatomy/bovine/Pot0061.mp4 Pot 61 The Bovine Liver]
+
==Webinars==
 +
<rss max="10" highlight="liver hepatopathies diarrhoea">https://www.thewebinarvet.com/gastroenterology-and-nutrition/webinars/feed</rss>
  
[http://stream2.rvc.ac.uk/Anatomy/equine/pot0228.mp4 Pot 228 The Equine Liver]
 
  
<big><center>[[Alimentary - Anatomy & Physiology|'''BACK TO ALIMENTARY - ANATOMY & PHYSIOLOGY''']]</center></big>
+
[[Category:Liver and Gall Bladder - Anatomy & Physiology]]
 +
[[Category:To Do - AimeeHicks]]

Latest revision as of 15:39, 27 October 2022


Liver Topography (Pig) - Copyright Nottingham 2008

Introduction

The liver (hepar) is an extremely important organ in the body of mammals and vertebrates as it provides functions essential for life. It is the largest internal organ and has numerous functions including production of bile and protein, fat and carbohydrate metabolism. During foetal development, the liver has an important haemopoetic function, producing red and white blood cells from tissue between the hepatic cells and vessel walls.

The size of the liver varies due to its role in metabolism. In carnivores the liver weighs about 3-5% of body weight, in omnivores 2-3% and in herbivores 1.5%. the liver is much heavier in young animals than older animals as it atrophies with age.

The liver is derived from an outpocketing of endoderm epithelium on the ventral duodenum from the caudal part of the foregut. The connection to the gut narrows to become the bile duct. The parenchymal tissue of the liver is formed from proliferating epithelial cords or strands which integrate with the blood sinuses of the umbilical and vitelline veins. The mesoderm of the septum transversum forms the venous sinosoids and connective tissue of the liver.

Structure

Topography of the Liver (Dog)- Copyright RVC 2008

The liver is located in the cranial part of the abdomen. It is immediately caudal to the diaphragm and cranial to the stomach and intestines. Generally the bulk of the liver is on the right of the midline. It is divided into lobes by fissures. Cranially the liver is convex, called the diaphragmatic surface. Caudally the liver is concave, called the visceral surface. The caudate lobe has a renal impression from the right kidney. The gastric impression occupies the whole of the left half of the visceral face. The duodenal impression at the junction of the right and quadrate lobes continues onto the right lateral and caudate lobes. Passages or notches on the median plane allow the caudal vena cava and oesophagus to pass by. The gall bladder is located between the right medial and quadrate lobes. Reticular fibres (collagen type III, proteoglycans and glycoproteins) support the hepatocytes and walls of the sinusoids. Interlobular spaces support bile ducts and blood vessels. The lesser omentum (often fat filled) is on the visceral surface between the left lateral lobe, heptic porta and lesser curvature of the stomach. There is a oesophageal notch where the oesophagus passes over the liver.

Divisions of the Liver

The liver can be divided into lobes, lobules, hepatocytes and sinusoids.

Lobes of the Liver

The lobes of the liver include the left lateral, left medial, right lateral, right medial, quadrate, caudate and papillary.

Ligaments

The coronary ligament attaches the liver (from the diaphragmatic surface) to the diaphragm. It is an irregular fold of peritoneum. It surrounds the triangular base of the diaphragmatic surface. It is continuous with outer most layer of the caudal vena cava. The falciform ligament is ventral to the coronary ligament. It is a fat filled embryological remnant of the fetal blood vessels from the placenta. It causes problems for surgical entry into the abdomen. It is located cranial to the umbilicus and is a vestige of the umbilical vein. The triangular ligament is on the right and left sides of the coronary ligament.

Function

Production of bile see Bile Formation.

Nearly all the blood circulated around the abdomen flows back through the portal vein to the liver where it comes in contact with the liver cells, ensuring the products of digestion are presented to the hepatic cells before entering the general circulation. Other functions include carbohydrate metabolism, glycogenesis, glyconeolysis, gluconeogenesis and the breakdown of insulin and other hormones. Protein metabolism produces soluble mediators of the clotting cascade, Albumin and hormone transporting globulins. The liver is also involved in lipid metabolism, lipogenesis and the synthesis of cholesterol.

The liver has a role in hormonal control of the following; Insulin and glucagon, Glucocortocoids, Catecholamines and the synthesis of other important hormones (see Endocrine System). It also has a role in immunoregulation via kupfer cells and the complement synthesis and metabolism.

The liver is important in storage of water soluble vitamins, fat soluble vitamins, iron, triglyceride and glycogen.

The liver breaks down haemoglobin and toxic substances through drug metabolism. It converts ammonia to urea and allows the management of endogenous waste, e.g haem (Hb, cytochromes, Mb) and ammonia (amino acids).

Vasculature

Blood Flow in the Liver - Copyright nabrown RVC 2008

The liver has a dual blood supply. 70-80% via the hepatic portal vein (nutrient rich) and 20-30% via the hepatic artery (oxygen rich). It has a large blood supply (nearly a 1/3 of cardiac output passes through the liver).

The hepatic artery is a branch of the caeliac artery. The portal vein is formed by tributaries draining the spleen, pancreas and digestive tract. Intrahepatic arteries combine with portal vein branches to supply the connective tissue and hepatic sinusoids of the liver. Blood flows from the portal areas into the central vein. The central vein is lined by simple squamous epithelium. The bile duct, blood vessels (including the important hepatic vein) and nerves enter and leave the liver at the hepatic porta. Blood from the central vein opens into the caudal vena cava. Liver circulation is controlled by interarterial, intervenous, arteriovenous, and by sphincter mechanisms, allowing careful regulation.

Innervation

The liver is innervated by sympathetic nerves from the periarterial plexuses and parasympathetic nerves from the vagal trunk.

Lymphatics

Efferent vessels pass to hepatic nodes around the hepatic porta. The lymph drains into the visceral cysterna chyli. Some lymph travels to the accessory hepatic and caudal mediastinal lymph nodes on the caudal vena cava.

Hepatic Duct Systems

There are canaliculi within lobules. Canaliculi open into larger ductules then into a few large hapatic ducts. Before and shortly after leaving the hepatic porta, the ducts combine into a single trunk which runs to the duodenum. The cystic duct runs from common trunk to the gall bladder transporting bile from the liver to the gall bladder. Distal to the cystic duct is the bile duct (ductus choledochus) which transports bile from the gall bladder into the duodenum. There are no valves, so bile may flow in either direction.

Formation of Bile Acids - Copyright RVC 2008

Bile Acids

Bile acids are composed of cholesterol, bile acids and steroids. The main bile acid is cholic acid (C24). Conjugated to taurine or glycine in the liver to reduce pKa so they exist in an ionised form as bile salts. Bile salts conjugate with cholesterol and phospholipids and are then secreted into the bile. They emulsify fats which helps absorb fat soluble vitamins. In aqueous solution, they form micelles which are amphiphilic and can transport free fatty acids across the brush border.

Species Differences

Liver Topography (Dog) - Copyright Nottingham 2008

Canine & Feline

Both the left and right lobes are subdivided. Complete obstruction of the hepatic artery is fatal. The liver is almost entirely intra-thoracic. An enlarged caudate process contacts the right kidney.

Equine

The liver is contained entirely within the rib cage, to the right of the midline. It is less lobated. There is no gall bladder and the left lobe is subdivided. There is no papillary lobe. In the foal, the liver is larger and more symmetrical. The bile duct opens into the duodenum at the same papillae as the major pancreatic duct. Bile is constantly secreted.

Liver Topography (Pig) - Copyright Nottingham 2008

Porcine

The liver has deep interlobular fissures and a large amount of interlobular connective tissue. It has a mottled appearance. A Deep interlobular fissure divides the liver into 4 lobes- the left, right, medial and lateral. There is a small caudate lobe (which does not contact the kidney so no renal impression). It is mostly on the right of the midline and has no papillary lobe.

Ruminants

The liver is entirely displaced to right of the midline. It has fused lobes.

Small Ruminants Sheep have a deeper umbilical fissure than cows. Sheep also have a smaller caudate lobe than cows and have two papillary processes.

Avian

See avian liver

Histology

Liver Lobule Histology - from GIT-2 tutorial

The larger liver cells are called lobules. Each lobule contains an opening for the central vein and contains portal areas. The lobules are composed of liver cords called hepatocytes. Sinusoids are present between hepatocytes containing red blood cells. There is a connective tissue capsule around each liver lobule. A thin mesothelium covers the connective tissue layer.

The portal area present in the lobules contains the hepatic artery, which has thick walls and a small diameter and the hepatic vein, which has thin walls and a large and irregular shape. It also contains bile ducts, with cuboidal or columnar epithelium and lymphatics that are small and delicate.

Portal Triad Histology in a Lobule- from GIT-2 tutorial

Hepatocytes are the smaller liver cells in the lobules. They contain glycogen granules and have a spherical nucleus. They form cords called branching plates (lamellae). The upper and lower margins are tight junctions. They have 3 functioning surfaces. Kupfer macrophages are present near the lining of the sinusoids. The hepatocytes stain pink as they are eosinophilic.


Acinus

  • the smallest functional unit of the liver - this unit stresses the dependence of the liver on its afferent blood vessels and efferent bile ducts
  • a roughly diamond shaped parenchymal mass surrounding and supplied by the portal area
  • consists of 3 zones:
    • Zone 1 - periportal (centroacinar), around the portal areas
    • Zone 2 - midzonal
    • Zone 3 - centrilobular (periacinar), bordering the hepatic venules

NB: blood enters from Zone 1 to Zone 3 and thus becomes less and less oxygenated

Links

Click here for information on pathology of the Liver

Click here for information on Hepatic Stellate Cells

Click here for information on Bile Formation

Click here for information on portosystemic shunting



Liver - Anatomy & Physiology Learning Resources
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Canis, Felis, Lapis or Equis
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Compare liver structure in different species
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Histology of the liver - see part 2


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