Liver
The liver is the second largest organ in the body ,(the largest is the skin) and the largest gland. weighing about 1-1.5 kg. It is situated in the abdominal cavity beneath the diaphragm.The liver is the organ in which nutrients absorbed in the digestive tract are processed and stored for use by other parts of the body . It thus an interface between the digestive system and the blood.
Most of its blood (70%-80%) comes from the portal vein, arising from the stomach , intestines , and spleen ; the smaller percentage (20%-30%) is supplied by the hepatic artery.
All the materials absorbed via the intestines reach the liver through the portal vein, except the complex lipids (chylomicrons), which are transported mainly by lymph vessels.
The position of the liver in the circulatory system is optimal for gathering , transforming , and accumulating metabolites and for neutralizing and eliminating toxic substances.
Elimination occurs in the bile , an exocrine secretion of the liver that is important for lipid digestion. The liver also has the very important function of producing plasma proteins , such as albumin, other carrier proteins , coagulation factors, and growth factors.
The liver is covered by a thin connective tissue capsule Glison's capsule that becomes thicker at the hilum , where the portal vein and the hepatic artery enter the organ and where the right and left hepatic ducts and lymphatics exit. .
These vessels are surrounded by connective tissue all the way to their termination or origin in the portal spaces between the liver lobules. At this point, a delicate reticular fiber network that supports the hepatocytes and sinusoidal endothelial cells of the liver lobules is formed.
Schematic drawing of the structure of liver.
The liver lobule: The basic structural component of the liver is the liver cells , or the hepatocytes . These epithelial cells are grouped in interconnected plates and constitute two-thirds of the mass of the liver.In light – microscope sections , structural units called liver lobules can be seen . The liver lobules is formed of a polygonal mass of tissue about 0.7Ч2 mm in size , with portal spaces at the periphery and a vein (central or centrolobular vein), in the center .
Portal spaces , regions located in the corners of the lobules , contain connective tissue , bile ducts , lymphatics , nerves, and blood vessels
The human liver contains three to six portal spaces per lobule , each with a venule (a branch of the portal vein) , an arteriol ( a branch of hepatic artery) , a duct ( part of the bile duct system), and lymphatic vessels.
The 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 , lined by cuboidal epithelium , carries bile synthesized by the hepatocytes and eventually empties into the hepatic duct. One or more lymphatics carry lymph , which eventually enters the blood circulation.
In certain animals as in pigs , the lobules are separated by a layer of connective tissue. This is not the case in humans, where the lobules are in close contact along most of their length , making it difficult to establish the exact limits between different lobules.
The hepatocytes in the liver lobule are radially disposed and are arranged like the bricks of a wall . These cellular plates are directed from the periphery of the lobule to its center and anastomose freely, forming a labrythine and sponglike structure . The spaces between these plates contains capillaries , the liver sinusoids. .
The sinusoidal capillaries are : irregularly dilated vessels composed solely of a discontious layer of fenestrated endothelial cells.The fenestrate `are about 100 nm in diameter, have no diaphragm , and are grouped in clusters.
These are also spaces between the endothelial cells , which , together with the cellular fenestrate and a discontinous basal lamina, give these vessels great permeability.
A subendothelial space known as space of Disse separates the endothelial cells from the hepatocytes. The fenestrae and discontinuity of the endothelium allow the free flow of plasma but not of cellular elements into the space of Disse, permitting an easy exchange of molecules from the sinusoidal lumen to the hepatocytes and vice versa.
Scanning electrone micrograph of liver showing fenestration in the wall of sinusoidal capillary .note the microvilli of the hepatocyte in space of Disse.
This exchange is physiologically important not only because of the large number of macromolecules (eg, lipoproteins , albumin, fibrinogen) secreted into the blood by hepatocytes but also because the liver takes up and catabolizes many of these large molecules . The basolateral side of the hepatocyte , which lines the space of Disse , contains many microvilli and demonstrates endocytic and pinocytic activity.
The sinusoid is surrounded and supported by a delicate sheath of reticular fibers.
In addition to the endothelial cells, the sinusoids contain macrophages known as Kupffer cells. These cells are found in the luminal surface of the endothelial cells, within the sinusoids.Their main functions are: To metabolize aged erythrocytes . Digest hemoglobin Secrete proteins related to immunological processes . And destroy bacteria that eventually enter the portal blood through the large intestine . Kupffer cells account for 15% of the liver cell population. Most of them are located in the periportal region of the liver lobule, where they are very active in phagocytosis .
.In the space of Disse (perisinusoidal space), fat-storing cells, called Ito's cells or stellate cells, contain vitamin A-rich lipid inclusions. In healthy liver , these cells play: A role in storage of retinoids. Syntheseis of some proteins . Secretion of growth factors.
In chronic liver disease Ito's cells are activated by factors from the hepatocytes and Kupffer cells , so they proliferate and that may lead to liver fibrosis which may be irreversible and called cirrhosis.
The surface of each hepatocyte is in contact with the wall of the sinusoids, through the space of Disse , and with the surface of other hepatocytes. The surface of the hepatocyte that faces the space of Disse contains many microvilli that protrude into that space ; but there is always a space between them and the cells of the sinusoidal wall.
Wherever two hepatocytes abut, they delimit a tubular space between them known as the bile canaliculus.
They are limited only by the plasma membrane of two hepatocytes and have a small number of microvilli in their interiors.
Scanning electrone micrograph of of bile canaliculi in liver with microvilli lining the internal surface.
The cell membranes near these canaliculi are firmly joined by tight junctions . The bile canaliculi form a complex anastomosing network progressing along the plates of the liver lobule and terminating in the region of the portal spaces.
The bile flow therefore progresses in direction opposite to that of the blood (from the center of the lobule to the periphery).
At the periphery , bile enters the bile ductules , or Hering's canals .After a short distance , the ductules cross the limiting hepatocytes of the lobule and end in the bile ducts in the portal spaces
. Bile ducts are lined by cuboidal or columnar epithelium and have a distinct connective tissue sheath. They gradually enlarge and fuse, forming right and left hepatic ducts, which subsequently leave the liver.
The hepatocytes has one or two rounded nuclei with one or two nucleoli. The hepatocyte has an abundant endoplasmic reticulum (RER and SER).
Several proteins (blood albumin , fibrinogen )are synthesized on polyribosomes in the RER. SER is responsible for the process of oxidation , methylation , and conjucation required for inactivation or detoxification of various substances before their excretion from the body.
One of the main processes occurring in SER is the conjucation of hydrophobic (water insoluble )toxic bilirubin by glucoronyletransferase to form water soluble nontoxic bilirubin glucorunide which is excreted into the bile. If it is not excreted , various diseases characterized by jaundice can result. One of these cases is neonatal hyperbilirubinemia.
The hepatocytes contains glycogen , mitochondria, lipid droplets, lysosomes, peroxisomes (enzyme-containing organelles), , golgi complex .
There is an inherited disorder called x-linked adrenoleukodystrophy which leads to deterioration of myelin sheaths of the neurons due to the defects in the metabolism of fatty acids which is resulted from the mutation in peroxisomes structure.
Protein synthesis and carbohydrates storage in the liver.
Gallbladder function: 1. Storage and concentration of bile: The bile exits the liver through the common hepatic duct.The gallbladder has a storage capacity of 40 to 70 ml.
2. Expulsion of bile: When a meal is ingested, the sphincter of Oddi relaxes and the smooth muscle cells in the wall of the gallbladder contract to expel the bile through the cystic and common hepatic duct into the duodenum.Structure of gallbladder wall: A. The wall of the gallbladder consists of a mucosa that is composed of a simple columnar epithelium of absorptive cells and a lamina propria.a thin irregular layer of smooth muscle cells (responsible for the contraction and expulsion of the stored bile in response to cholecystokinin).
a well developed perimuscular layer. connective tissue layer, and the serosa.
B. The mucosa is extensively folded when the gallbladder is contracted and modestly folded when it is relaxed. However, it does not contain villi or tubular glands (crypts). C. The underlying connective tissue is richly vascularized.
Cholelithiasis: Formation of gallstones (from cholesterol and bile acids) within the gallbladder as a consequence of an abnormal bile composition (80% due to an excess of cholesterol). Loose gallstones can cause obstruction of the main bile ducts and consequently a backup of bile into bile canaliculi and hepatocytes.
This in turn leads to the appearance of bile components in the blood circulation. Gallstones blocking the major duodenal papilla impair also the flow of pancreatic fluid. This can lead to an inflammation of the pancreas (pancreatitis).