The Liver

STRUCTURE OF THE LIVER OF A MAMMAL
The liver is the largest organ in the body, weighing about 1.5kg, which if about three to four precent of the total body mass. It is supplied with blood by the hepatic portal vem and hepatic artery, which is a branch of the dorsal aorta, and transports oxygenated blood to the liver. The hepatic portal vein transports blood rich in disgested food materials from the gut to the liver. The hepatic vein drains deoxygenated blood containing glucose, amino acids, lipids, plasma proteins, urea, cholesterol and Carbon (IV) oxide, evolved from cellular respiration, from liver to the inferior vena cava. The liver has an excellent blood supply.

The liver is made up of numerous lobules, which are roughly cylindrical in shape and measures about one millimetre in diameter. Each lobule is lilled with structurally identical liver cells, which are arranged in rows radiating from the centre towards the periphery. Running alongside each lobule are branches of the hepatic artery, hepatic portal vein and bile duct. The first two are located between adjacent lobules and are referred to as interlobular blood vessels. In the centre of each lobule is a branch of the hepatic vein, which is referred to as central or interlobular vein. The interlobular vessels are connected with the central or intralobular vein by a system of capillary-like sinusoids. The sinusoids run parallel to, and come into close contact with, the chains of liver cells. The liver cells are surrounded by fine channels called canaliculi, which connect with the bile ducts at the edge of the lobule. Therefore, the liver cells have an intimate association with the sinusoids and canaliculi.


Attached to the walls of the sinusoids are specialized kupffer cells. These are large phagocytic macrophage, which destroy old and worn-out red blood cells and remove bacteria and other foreign particles from the blood flowing through the liver.



FUNCTlONS OF THE LIVER
1. Regulation of Glucose: The liver converts excess glucose to glycogen with the help of the hormone, insulin. Glycogen stored in liver cells can be converted back to glucose with the help of the Glucagon, when the level of glucose in the blood falls.

2. Regulation of Lipids: The liver cells remove lipids from the blood and either oxidised them to release energy or modified and stored in fat depots.

3. Deamination: The liver cells deaminate excess amino acids. in this process, the amino group (-NHZ) is separated from the amino acid and converted to ammonia. which is then converted to urea through a series cyclical of reactions called the ornithine cycle. The urea is excreted from the body by the kidneys. The residue of the amino acid, after deamination, is oxidised to release energy.

4. Detoxification: The liver converts toxic substances into less toxic or harmful compounds. For instance, Hydrogen peroxide is converted to water and oxygen and ammonia into urea. The phagocytic cells or macrophages lining blood vessels in the liver, remove foreign particles from the blood stream.

5. Production of Heat: The liver has a high metabolic rate. which results in the production of a large amount of heat. The excellent blood supply and large size of the liver enable the liver to distribute the heat in the body. This helps to regulate the body temperature.

6. Production of Bile: Bile is synthesized by the liver cells and stored in the gall bladder. The bile salts play an important part in digestion by emulsifying fats intc small fat droplets.

7. Elimination of Sex Hormones: after they have performed their functions. Some are modified chemically by the liver cells, some are sent to the kidney to be excreted while others are eliminated through the bile.

8. Formation of Red Blood Cells: In the foetus, red blood cells are formed in the liver, but this function is gradually taken over by the bone marrow. In adults however, the liver stores Vitamin B12, required for the formation of red blood cells.

9. Formation of Cholesterol: Cholesterol is a lipidlike substance, some of which is required as an important constituent of cell membranes, such as those of nerve cells. Excess cholesterol is eliminated through the bile.

10. Elimination of Haemoglobin: from old or worn-out red blood cells. The cells are destroyed by phagocytic cells lining blood vessels of the liver, spleen and bone marrow. The haemoglobin is broken down by liver cells into bile pigment. biliverdin. which is then reduced to bilirubin, another bile pigment. These are eliminated through the bile.

11.Smrage of Blood:_ The liver acts as a reservoir for blood. It contains many capillaries and blood spaces which can hold a large volume of blood.

12. Storage of Vitamins: The liver stores vitamins such as, vitamins A, D, K and BIZ.

13. Storage of Mineral Salts: The liver stores minerals such as, Potassium, Iran and Copper.

14. Synthesis of Plasma Proteins: The liver produces essential plasma proteins like, librinogen, heparin and prothrombin. These proteins play an important part in the clotting of blood.

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