Liver

Liver

PKGhatak, MD

The liver is an amazing organ. It is the largest solid abdominal organ but hardly noticed. The liver is tucked away underneath the dome of the diaphragm and surrounded by a rib cage. Doctors want to examine the liver have to make it descend down with breathing (breathing out) by asking patients to breathe deep in and out and pushing his/her fingers deep underneath the right side of the rib cage. If the liver is palpable a normal liver is like the relaxed bicep muscle, a cirrhotic liver feels like ear cartilage.

The liver is the largest chemical factory of the body. It gets its raw materials from the GI tract, the digested food and drinks via the portal veins. Then the liver manufactures proteins, fat, cholesterol, glycogen from sugar and glucose from amino acids and fatty acids, hormones, enzymes, and other various chemicals; filters the waste products, detoxifies, absorbs toxins, and then releases these substances at the appropriate time in the blood. The liver is an endocrine cum exocrine gland and much more.

Embryology of Liver:

Liver growth starts early, as early as 2 weeks following fertilization of the ovum. An invagination of endoderm of the foregut, close to the Septum Trasvestum, occurs and that becomes the site of liver development. A liver bud develops and the cells of the head end of the liver bud start to grow in an empty space reserved for the heart and blood vessels. The developing cells destined to be liver cells grow like finger projections and orient in alternating rows with the endothelium of the blood vessels which later become the terminal capillary branches of the portal vein. The septum trasvestum becomes the capsule of the liver and fibrous tissues from the capsule enter the developing liver cells and become the scaffolding and provide stability of the liver. The tail end of the same liver bud forms the gall bladder and extrahepatic bile ducts.

Liver lobule:

The liver is a mixture of the exocrine gland, endocrine gland, vascular organ and storehouse of many things including every Stem Cell line.

The structural and functional unit of the liver is a lobule – a tiny acinus. The total number of lobules in a liver is estimated to be 300 billion. An understanding of microscopic anatomy is essential to study the physiology of the liver.

A liver lobule is a three dimensional unit. It looks hexagonal under the microscope but in a living state, a lobule is more or less like a circular structure. At the periphery, there is an arcade formed by a branch of the portal vein. Straight branches, the venules of the portal vein – often referred as a lake, originate from the arcade and travel toward the center of the lobule and converse on a central vein. The appearance resembles the spokes of a wheel. Columns of liver cells follow closely the straight portal venules. The other side of the liver cells column is a draining tube – a tiny bile duct. This pattern is repeated in all lobules. The interlobar space is interlaced with fine fibrous tissues attached to the liver capsule. This space contains branches of the hepatic artery and lymph vessels and various mesenchymal cells, including the well known Kupffer's cells.

Branches of the hepatic artery, bile duct and lymphatics follow the branches of the portal vein. a hepatic artery branch to the lobule opens into the lake. The plexus of fine branches of the hepatic artery supplies all the cells of the bile duct and other structures in a lobule. The central vein joins with adjoining veins and ultimately forms 2 to 4 Hepatic veins and hepatic veins terminate in the Superior Vena Cava.

In essence, each hepatic cell is bathed by the blood that is a mixture of arterial blood and venous blood returning from the GI tract. Intrahepatic bile ducts are as profuse as blood vessels are and lobules are drained continuously.

A Hepatocyte:

A liver cell is a large cell and rich in the cytoplasm and nuclear materials. The nucleus is surrounded by a well demarcated nuclear wall. The nuclear chromatin is very prominent and generally contains two nucleoli. The cytoplasm is loaded with mitochondria, smooth and rough endoplasmic reticulum, Golgi apparatus, lysosome, peroxisome, inclusion bodies, stored glycogen, and lipid molecules.

Blood supply: The liver is a very vascular organ. It weighs just 2 % of the body weight but gets 25 % of cardiac output. Of the total blood inside the liver, 75 % is from the portal vein, and the rest is arterial. The liver stores 15 % of total body blood and in hemorrhage, hepatic blood is made available.

Bile secretion: In a day about 1 liter of bile is produced. Bile is produced continuously and finds its way to the gall bladder first and is stored there. After a fatty meal gall bladder discharges bile into the intestine.

Liver Function: It is said that the liver completes 500 different tasks.

The following are just to name some of these functions.

Metabolism of Carbohydrate: Every aspect of carbohydrate metabolic processes from energy production, breakdown of glycogen, making glucose from fat and amino acids and conjugation of the carbohydrate moiety to toxic substances take place in the liver.

Fat metabolism: Cholesterol is synthesized in the liver. The liver converts fatty acid to fat molecules and breaks down fat and releases fatty acid in starved conditions.

Protein synthesis: Albumin and ferritin are made in the liver, so also all the transport proteins - just too many to mention here.

Acute phase positive proteins are about 30 in number. Some important are C- reactive protein, procalcitonin, alpha antitrypsin, heptidine,  IL-1 receptor antagonist, D- dimer. The negative acute phase proteins are albumin, transferrin, transthyretin. 

Blood coagulation: Several anticoagulants and clotting factors are made in the liver. Anticoagulants are protein S and protein C, Antithrombin. Coagulants are fibrinogen, prothrombin, factors- V, VII, IX, X, XI, XII. Von Willebrand and factor VIII are made in the liver but not by the hepatocytes.

Hormone production: Thrombopoietin, Somatomedin, Angiotensinogen, Hepcidin.

Vitamins production: Vitamin D 25-OH. The liver acts as a store of fat-soluble vitamins- A, D, E, K. and B12.

Storage function: Iron, Glycogen, fat, copper.

Detoxification: Most toxins are made less toxic or harmless by conjugation wand made sulfate, glucuronide and simply converted to inert substances.

Execratory function: Bile acids and bilirubin.

Digestive function: Bile acids and bicarbonates.

Myeloid Metaplasia and liver: Several hematological conditions end up destroying bone marrow and replaced it with fibrous tissue, a condition known as myelofibrosis. In such conditions liver and spleen attempt to generate deficient blood cells.

Regenerative power: The liver carries all stem lines. At little as 15 % of the liver remaining from an accident or liver donation, the remaining liver will regenerate to the normal size.

Liver transplant began in 1967, it has become an essential operation in congenital biliary atresia. A liver transplant is performed for liver failure due to many causes including alcoholic cirrhosis and viral hepatitis C.

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