Digestive System Dr.Dr. Rand Abdulateef Lecture 5 Lecture 1
Liver is the largest organ in the body; it is situated in the abdominal cavity beneath the diaphragm. Most of it's blood supply comes from the portal vein (70%) and a small percentage come from the hepatic artery. All the absorbed material comes from the intestine through the portal vein, except the lipid which passes through the lymphatic vessels. Liver is an exocrine gland which secrete bile through the bile duct and also endocrine gland which it's secretion pass directly to the blood stream.
The liver is covered by a C.T. capsule.
Histological structure :
The main structural unit of the liver is the "Hepatocyte " , these cells are grouped in plates which are interconnected in such away to appear in light microscope as polygonal mass called ( liver lobule ), in certain animals these lobules are separated from each other by a layer of C.T., this does not occur in humans where it is difficult to determine the exact limit between the lobules . In some regions however the lobules are separated by C.T. containing blood vessels. These regions are called "portal spaces or portal areas or portal canals", each containing a venule which is a branch of the portal vein, arteriole which is a branch of the hepatic artery, a duct (part of the bile duct system) & lymphatic vessels.
The hepatocytes are radially arranged as plates in the liver lobules, they are piled up forming a layer of one cell thick in a fashion similar to the bricks of a wall, these plates are directed from the periphery to the center of the lobule, between the plates lies sinusoidal capillaries which are lined by two types of cells, the endothelial cells and the Kupffer cells which are typical macrophages to phagocytose the aged RBCs.
The hepatocytes are separated from the sinusoids by a space (space of Disse) which consist of extracellular fluid with reticular fibers and the microvilli of the hepatocyte float in it. The sinusoids arise at the periphery where the hepatic artery and portal vein are emptying and these sinusoids run to end in to the central vein of the lobule.
The hepatocytes are polyhedral with six or more surfaces, each containing one or two nuclei, in sections stained with H&E the cytoplasm appear eosinophilic due to the presence of mitochondria, smooth and rough endoplasmic reticulum, Golgi, lysosomes & free ribosomes.
TheFunctional units of the liver are:
1- Classical lobule.
2- Portal lobule: which at it's center is the portal canal, it is triangular in shape, surrounding the portal canal is parts of three adjoining hepatic lobules, so all of which drain bile into one portal canal. The central veins lies at the tip of the angles of the triangle.
3- Hepatic Acinus:it is the region which irrigated by the terminal branch of the interlobular veins. it is diamond in shape, in addition to the vein there is the terminal branch of the hepatic artery & the ductule, they lie between 2 classical liver lobules.
In section the liver is formed of a masses ( Acini ).The area which close to the vessels is called Zone 1(periobular),so it has a largest amount of blood supply&receive more o2 , nutrient &it is the first zone affected by toxins in the blood ا
Zone 11 (middle) which lie between zone 1 & 111 which has less blood supply than zone 1.
Zone 111 (pericentral) which is near the central vein & has the least blood supply and the last zone affected by toxins in the blood.
The hepatocyte has three important surfaces , the hepatocyte surfaces are important because they have a role in a transfer of substances between the hepatocytes blood vessel and bile canaliculi :
Sinusoidal surface: separated from a sinusoids by a space of Disse this account for 70% of a total hepatocyte surface, the surface is covered by short microvilli and it is a site where materials transferred between the sinusoids and hepatocytes.
Canalicular surface: account for 15% of a hepatocyte surface across which the bile drains from the hepatocytes into the canaliculi which are 0.5 – 2.5 in diameter and lined by irregular microvilli , the cytoplasm of hepatocytes close to the canaliculi is rich in " actin filaments " which influence the diameter of the canaliculi and a rate of flow of bile.
Intercellular surface: is the surface between the adjacent hepatocytes , account for 15% of the hepatocyte surface , they are specialized for cell to cell communication by communicating junctions.