Liver, Gallbladder and Pancreas

Gastrointestinal system II | Main Anatomy Index | Lymphatics system

Last updated 30 March 2006

This page was contributed by David Boshell.

Liver, Gallbladder, and Pancreas

The Liver

• This is the largest glandular mass of tissue, and the largest internal organ of the body.
• It receives substances from the digestive tract via the portal vein, including metabolites, nutrients, and toxins.
• The liver normally conjugates (degrades) toxins, but may be overwhelmed by them.

• The liver has both exocrine and endocrine functions:

• Exocrine functions include the production of bile from metabolic conversions of substrates from the digestive tract, pancreas and spleen.
• Secretion occurs via bile ductules draining into the hepatic duct, the gallbladder, the cystic duct, the common bile duct and, finally, into the duodenum.

• Endocrine functions include the release of substances produced by liver cells into blood, including albumin, lipoprotein, globulins, liver glycogen, and T3 (thyroid hormone).

Blood supply to the liver

• The liver's unique blood supply arises from two sources:

1. 75% of the liver's blood comes from the hepatic portal vein, carrying deoxygenated blood from the small intestine, pancreas and spleen. This blood contains absorbed nutrients and toxins from the intestine, blood products from the spleen, and endocrine secretions of the pancreas.
2. 25% comes from the hepatic artery, a branch of the celiac trunk, carrying oxygenated blood.

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Structural organisation of the liver

• Structural components of the liver include:

1. Hepatocytes, organised as plates or lamellae
2. Connective tissue stroma
3. Vessels, nerves, lymphatics and bile ducts
4. Sinusoidal capillaries (sinusoids) between plates of hepatocytes

Liver lobules

• Structural components of the liver are organised into lobules, which can be described in three ways: the classic lobule, the portal lobule and the liver acinus.

Classic Lobules of the Liver

• These are roughly hexagonal blocks of tissue.
• They consist of stacks of anastomosing plates of hepatic cells, separated by anastomosing sinusoids that perfuse the cells with mixed portal and arterial blood.

• These plates outwardly radiate from a central vein: the terminal hepatic venule, into which the sinusoids drain.
• At the angles of the periphery, connective tissue surrounds portal canals containing portal triads.

Portal Lobules of the Liver

• These are centred around the interlobular bile ducts of the portal triads.
• All blood in the portal lobule originates from the portal triad in the centre.
• This is to emphasise the liver's exocrine functions.

• Borders of the portal lobules are formed by the three surrounding central veins, creating a triangular shape.

The Liver Acinus

• This organisational unit allows description and interpretation of degenerative patterns, regeneration, and toxic effects in liver parenchyma according to vascular perfusion of hepatic cells.

• It has a short axis between two portal triads and a long axis between two central veins.

• Three zones surround the short axis:

• Zone 1 - closest to the axis.

• Zone 3 - farthest from the axis, closest to the central veins.

• Zone 2 - lies between zone 1 and zone 3.

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Hepatocytes

• Hepatocytes a large polygonal cells approximately 25 microns wide.
• They make up the anastomosing plates of the liver lobules.

• They have large, central, spherical nuclei, and may be binucleate.
• Hepatocytes have a life span of about 5 months, although they are capable of rapid regeneration.

• They contain lipid droplets and large deposits of glycogen, along with organelles.

• Two surfaces of the cell face the sinusoidal lumen: the basal (free) surfaces.
• Other surfaces face neighbouring hepatocytes, and may have bile canaliculi between them, forming lateral and apical surfaces, respectively.

Major functions of Hepatocytes

• Bile production and secretion;
• Metabolism (including detoxification) of lipid soluble drugs and steroids as well as cholesterol synthesis;
• Lipoprotein synthesis;
• Carbohydrate synthesis;
• Urea formation from ammonium ions.

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Portal Canals

• These are loose stromal connective tissue (continuous with the fibrous capsule of the liver), at the angles of the periphery of lobules.
• They contain portal triads and autonomic nerves, and are bordered by the outermost hepatocytes of the lobules.

• The space of Mall, between the connective tissue stroma and the outermost hepatocytes, is thought to be one of the sites where lymph originates in the liver.

Portal Triads

• These are groups of 4 structures in the portal canals:

1. Branches of the portal vein (the largest structure);
2. Branches of the hepatic artery;
3. Bile ductules
4. Lymphatic vessels (small)

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Blood vessels of the parenchyma

• Portal and hepatic arterial blood flow through the interlobular vessels of the portal triads, then into distributing branches from the periphery which supply the sinusoids.
• From the sinusoids, blood flows centrally to the central vein, which empties into sublobular veins and, eventually, the hepatic veins and inferior vena cava.

• The central vein is thin walled, with circularly arranged connective tissue fibres surrounding its endothelium.

Sinusoids

• Hepatic sinusoids are lined with a thin, discontinuous, fenestrated endothelium containing silver binding reticular fibres and stellate sinusoidal macrophages or Kupffer cells.
• Processes of Kupffer cells may span the sinusoidal lumen, and they may be involved in the final breakdown of red blood cells from the spleen.

Perisinusoidal Space (Space of Disse)

• This space lies between the basal surfaces of the hepatocytes and the basal surfaces of the endothelium lining the sinusoids.
• Numerous microvilli extend from the hepatocytes into the space of Disse, where materials are exchanged between blood and liver cells.
• This forms the pathway for endocrine secretions of the liver.

• Adipocytes with vitamin A are also found here.

• Plasma in this space drains to the space of Mall, then enters lymphatic capillaries that travel with components of the portal triads.
• Thus, lymph moves towards the portal canals and, eventually the hilum of the liver, draining into the thoracic duct.

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The Biliary Tree

• This is the conduit system by which bile flows from hepatocytes to the gallbladder, and then to the intestine.

• It begins with the canaliculi, grooves between hepatocytes, into which they secrete bile.
• These bile canaliculi form a ring about the hepatocytes, and drain into small bile ductules, the canals of Hering.
• These drain outwardly, opposite to the flow of blood, from the central vein, into the interlobular bile ducts of the portal canals.
• These ductules are lined by cuboidal epithelium, whilst the interlobular (intrahepatic) bile ducts are lined by cuboidal epithelium near the lobules that gradually becomes columnar nearer to the porta hepatis.

• The interlobular ducts then fuse to form left and right hepatic ducts that join at the hilum to form the common hepatic duct.
• Extrahepatic ducts now carry the bile to the gallbladder and intestine.

• The common hepatic duct fuses with the cystic duct from the gallbladder and becomes the common bile duct (or just "bile duct") which opens into the wall of the duodenum at the hepatopancreatic ampulla (of Vater).

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The Gallbladder

• The gallbladder is a pear-shaped sac with a volume of about 50 mL, attached to the visceral surface of the liver.

• The function of the gallbladder is to store the dilute bile it receives from the hepatic duct, concentrate it, and discharge the concentrated bile into the common bile duct.

Mucosa of the Gallbladder

• The interior surface of the gallbladder has numerous mucosal folds of simple columnar epithelium, resembling the absorptive cells of the intestine.
• The lamina propria of the mucosa is highly vascular, but there are no lymphatic vessels.
• It also contains many lymphocytes and plasma cells.

• The lamina propria is similar to that of the colon; it is specialised for the absorption of electrolytes and water (thus concentrating the bile).

Muscularis Externa, Adventitia and Serosa of the Gallbladder

• External to the lamina propria is a muscularis externa containing numerous collagen and elastic fibres, amongst randomly oriented bundles of smooth muscle cells.

• The gallbladder does not have a muscularis mucosae or a submucosa.

• Contraction of the smooth muscle forces bile out through the cystic duct.

• External to the muscularis externa is a thick layer of dense connective tissue, adventitia, containing large blood vessels, lymphatics, autonomic nerves, as well as elastic fibres and adipose tissue.
• On the visceral surface of the gallbladder, visceral peritoneum (serosa) consisting of mesothelium and loose connective tissue lines the adventitia.

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The Pancreas

• The pancreas is an exocrine and endocrine gland, dividing these functions into two structurally distinct components:

1. A large, serous, exocrine component, continuous throughout the organ, that synthesises and secretes digestive enzyme precursors.
2. A small, endocrine component, dispersed throughout the organ as islets of Langerhans, that mainly synthesises insulin and glucagon and secretes them into the blood.

Exocrine Pancreas

• The exocrine pancreas closely resembles the parotid gland.
• It has serous, secretory acini that are formed by a simple epithelium of pyramidal cells.
• These serous cells produce digestive enzyme precursors (proenzymes).

• Pancreatic acini are unique, in that the intercalated duct actually begins within the acinus.
• The duct cells located inside the acinus are the centroacinar cells.

• The acinar cells have acidophilic zymogen granules in their cytoplasm, which contain a variety of digestive proenzymes, including:

• Trypsinogen, pepsinogen and procarboxylase for digestion of proteins
• Amylase for carbohydrates
• Lipase for lipids
• Deoxyribonulcease and ribonuclease for nucleic acids

• The zymogen granules are released by exocytosis into lumenal spaces between the apical surfaces of the acinar cells.
• The basophilic cytoplasm of the acinar cells have extensive rough endoplasmic reticulum (rER) and free ribosomes, indicating high levels of protein synthesis.

Duct system

• This begins with squamous, centroacinar cells, which are intercalated duct cells located in the acinus.
• These cells are continuous with the short intercalated duct lying outside the acinus.
• The intercalated ducts drain into intralobular collecting ducts.

• There are no striated ducts in the pancreas.

• The intralobular collecting ducts drain into the larger interlobular ducts which, in turn, drain directly into the main pancreatic duct.
• This runs the length of the gland and opens into the hepatopancreatic ampulla (of Vater) with the common bile duct.
• The accessory pancreatic duct also arises in the head of the pancreas, opening into the minor duodenal papilla.

Hormonal control of exocrine secretion

• Acidic chyme in the duodenum stimulates the release of the following hormones:

1. Secretin, which stimulates the duct cells to secrete large volumes of fluid with little or no enzyme content.
2. CCK, which causes acinar cells to secrete their proenzymes.

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Endocrine Pancreas

• The endocrine pancreas is a diffuse organ.
• The islets of Langerhans, most numerous in the tail, comprise about 1-2% of the volume of the pancreas.

• They secrete hormones that regulate blood glucose.
• Islets contain varying numbers of polygonal cells arranged in short, irregular cords, profusely invested with fenestrated capillaries (for hormonal release), and stain less intensely than the surrounding serous acini.

• Three principal cell types are found in the islets of Langerhans:

1. A Cells, about 15-20% of the islet population, found in the periphery. They stain red with the Mallory-Azan method, and secrete glucagon from densely packed cytoplasmic granules.
2. B Cells, about 70% of the islet population, located in the central portion. They stain brownish-orange with the Mallory-Azan method, and secrete insulin from large secretory granules.
3. D Cells, about 5-10% of the islet population, located in the periphery. They stain blue with the Mallory-Azan method, and secrete somatostatin from the largest secretory granules in the islet.

• Other pale staining cells, about 5% of the islet population, also secrete hormones, including pancreatic polypeptide (PP).

Functions of Hormones

• Hormones secreted by the endocrine pancreas regulate metabolic functions.

• Insulin, the major hormone secreted by the islet tissue, stimulates the uptake, utilisation, and storage of glucose.

• Glucagon, the second largest secretion, stimulates the mobilisation of glucose.

• Somatostatin, when released from the hypothalamus, inhibits growth hormone (GH or somatotropin) release from the anterior pituitary gland.
• In the islets, it inhibits both insulin and glucagon secretion.

Blood Supply of the Pancreas

• The blood supply of the pancreas provides a cascading perfusion of the islets and acini.
• Blood from arterioles at the periphery of the islets, will perfuse by A and D cells before reaching B cells in the centre.

• Larger vessels invading the islets are accompanied by A and D cells, so that blood reaching B cells has always perfused the A and D cells (for the self-regulatory mechanisms of, say, somatostatin).

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