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The normal  Liver

Contents:
1.  Gross Anatomy of the Liver (Click Here)
2. The Vessels of the Liver (Click Here)
3. Microscopic Anatomy of the Liver (Click Here)
4. Functions of the Liver (Click Here)

1. Gross Anatomy of the liver:

• The liver is wedge-shaped, and covered by a network of connective tissue (Glisson's capsule).
• It is located in the upper right region of the abdominal cavity.
• The liver is divided into two main lobes by the falciform ligament, which is a mesentery attached to the anterior mid-abdominal wall.
• The right lobe is six times larger than the left lobe.
• The right lobe is situated over the right kidney.
• The left lobe lies over the stomach.
• The liver has the remarkable property of self-regeneration. If a part of the liver is removed, the remaining parts can regrow back to its original size and shape.

2. The Vessels of the liver:

• Hepatic artery: a blood vessel which supplies the liver with oxygenated arterial blood flow to the liver. It supplies 20% of the liver's blood.
• Hepatic portal vein: a blood vessel which drains venous blood into the liver from the entire gastrointestinal tract. It supplies the remaining 80% of the liver's blood.
• Hepatic vein: a blood vessel which drains blood from the liver into the inferior vena cava.
• Bile duct: channels through which bile secreted by the hepatocytes drain into. Bile in the bile ducts then drain into the left and right hepatic ducts.
• Common bile duct: the duct formed after the left and right hepatic ducts converge with the cystic duct from the gallbladder.

3. Microscopic Anatomy of the Liver:

•  Hepatocytes: a.k.a. liver cells.
•  Lobules: the hexagonally shaped functional units of the liver, made up of hepatocytes arranged in one-cell-thick platelike layers that radiate from the central vein to the edge of the lobule.
• Portal area: situated at he corner of each lobule, it is a complex composed of branches of the hepatic portal vein, hepatic artery, bile duct, and nerve.
• Sinusoids: delicate blood channels between the radiating rows of hepatocytes which transport blood from the branches of the hepatic portal vein and hepatic artery from the portal areas to the central vein.
• Bile canaliculi: channels through which bile secreted by hepatocytes drain into the bile ducts in the portal areas.
• Central vein: a blood vessel in the middle of each lobule which receives blood from the hepatic portal vein and hepatic artery via the sinusoids and drains the blood into the hepatic vein.

4. Functions of the Liver:

• It is estimated that the liver carries out several hundred separate functions involving thousands of different chemical reactions.
• The liver and the kidney are the major organs responsible for regulating the steady state of blood metabolites and the composition of blood tissues.
• All food materials absorbed from the gastrointestinal tract pass directly into the liver where they are stored or converted into some other form as required by the body at that time.

1. Carbohydrate Metabolism:

• The optimun level of glucose in a person's blood is approximately 90 mg of glucose per 100cm³ of blood.
• The liver prevents the blood glucose level from fluctuating too much and so prevents damage to tissues that cannot store glucose, such as the brain.
• All hexose sugars, including galactose and fructose are converted into glucose in the liver and stored as the insoluble polysaccharide, glycogen.
• When blood glucose level drops below 60 mg cm^-3 , the pancreas will secrete glucagon, which stimulates the liver to undergo glycogenolysis to restore the blood glucose level back to optimum level.
• When blood glucose level exceeds 90 mg cm^-3,such as after a carbohydrate-rich meal, the pancreas will secrete insulin, which stimulates the liver to undergo glycogenesis to bring the blood glucose level down to its optimum level.
• When the demand for glucose has exhausted the glycogen store in the liver,  the liver undergoes gluconeogenesis to obtain more glucose.
• Carbohydrates in the body which cannot be utilized or stored as glycogen are converted into fats and stored.

2. Protein Metabolism:

• The body is unable to store absorbed amino acids, and those not immediately required for protein synthesis or gluconeogenesis are deaminated in the liver.
• Deamination: the enzymatic removal of the amino group (-NH₂) from the amino acid with the simultaneous oxidation of the reminder of the molecule to form a carbohydrate which is utilized in respiration.
• The nitrogenous product of deamination is ammonia (NH₃).
• Ammonia is toxic and dangerous if accumulated in large amounts. The ammonia produced by deamination is converted in the liver into the soluble excretory product urea via the ornithine cycle.
• The liver is also able to produce some amino acids which are deficient in the diet via transamination, e.g. glutamic acid.
• Transamination: the synthesis of amino acids by the enzymatic transfer of the amino group from an amino acid to a carbohydrate in the form of a keto acid.

3. Plasma Protein production:

• A majority of plasma proteins are synthesized in the liver.  The most common being plasma albumin.
• Plasma globulins are also produced.
• Other plasma proteins are blood-clotting factors, prothrombin, and fibrinogen.

4. Fat Metabolism:

• The liver is involved in the processing and transport of fats rather than their storage.
• The liver converts excess carbohydrates to fat,
• removes cholesterol and phospholipids from the blood and breaks them down, and synthesizing them if necessary.

5. Vitamins and Mineral storage:

• The liver stores mainly the fat-soluble vitamins, like vitamins A, D, E, and K.
• Elements such as copper, zinc, cobalt,molybdenum, iron and potassium are also stored in the liver.
• Most of the iron in the liver comes from the breakdown of old erythrocytes and is stored as ferritin in the liver for later use in the formation of new erythrocytes in the bone marrow.

6. Formation of Erythrocytes in foetus:

• The foetal liver is responsible for the initial formation of erythrocytes until this function is gradually taken over by the bone marrow.

7. Breakdown of Erythrocytes in adults:

• The adult liver takes on the opposing role of breaking down old erythrocytes and haemoglobin.
• The erythrocytes are broken down by phagocytic macrophage cells in the liver.
• Haemoglobin is broken down into haem and globin.
• The globin is reduced to its constituent amino acids and is used according to demand.
• Haem is eventually converted into iron and bilirubin, which is a yellow component of bile.

8. Bile production.

• Bile is a viscous, greenish yellow fluid secreted by hepatocytes.
• It is involved in digestion, the absorption of fats and is a means of excretion of bile pigments.
• However, bile pigments have no function and their presence is purely excretory.
• Bile is made up of bile salts, which are derivatives of  cholesterol, synthesized in hepatocytes. The most common bile salts are sodium glycocholate and sodium taurocholate. They are secreted with cholesterol and phospholipids as micelles.
• The bile micelles exist as small droplets of lipids, because cholesterol and phospholipids hold the bile pigments together so that all the hydrophobic ends of the molecules are orientated in the same way.
• The small droplets greatly increases the surface area for pancreatic lipase to convert lipids into glycerol and fatty acids, so these can then be absorbed from the gut.

9. Detoxification:

• The liver is the first major organ which encounters toxins from the gastrointestinal tract.
• The major toxic substance in the blood is ammonia.
• Toxins are rendered harmless by oxidation, reduction, methylation, or by combining molecules together.
• The detoxified substances are then excreted by the kidney.
• The popular analgesic, paracetamol, if taken in excess is changed into a substance which affects enzyme systems and can cause damage to the liver and other tissues.

References:
Hwa Chong Junior College, 1996, Biology lecture on "Homeostasis" 

For more Information :
Dr Sing Kong Yuen

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