Human Liver : Structure and Functions


Human Liver : Structure and Functions

Liver is not only the largest organ of our body, but also the largest and most important gland. It is about 15 to 22 cm long and wide and weighs about 1.5 kg. It is a large, soft, but solid, bilobed organ of chocolate color, held by the mesentery, in the abdominal cavity, just below the diaphragm. A falciform ligament connects its right and left lobes.

This muscle works to keep the liver straight from the diaphragm. The left lobe is very small and the right lobe is large. The right lobe is divided into three lobes by slight furrows—the main right lobe, the quadrate lobe and the caudate lobe.

Located in a shallow pit of the liver, towards the front of the main right lobe and quadrate lobe, there is a gall bladder about 7 to 10 cm long. Downward from the gallbladder, there is another shallow depression on the surface of the liver called the porta, because it is in this area that the hepatic artery and the hepatic portal vein enter the liver. The hepatic vein and hepatic duct exit the liver.

Human Liver : Structure and Functions

Functions of Liver
Organelles like ribosomes, endoplasmic reticulum, mitochondria, lysosomes etc. are present in relatively large numbers in liver cells. This gives evidence of metabolic activity in these cells. That is why vacuoles filled with glycogen particles, fat globules, enzymes, crystals of iron containing substances etc. are found in them. These cells are so active because the liver is not only a digestive gland, but it performs many important functions for the efficiency of the body, which are many of the following-

1. Secretion of Bile: Continuous secretion of bile is the main function of the liver. Bile is a greenish-brown alkaline liquid. It contains substances like bile salts, bile pigments, cholesterol, lecithin etc. Its salts include sodium bicarbonate, glycocholate and taurocholate. 

The major pigment substances are biliverdin and bilirubin, which are formed by the breakdown of hemoglobin in the blood. Although bile does not contain digestive enzymes, bile salts are essential for the digestion of food, especially fats. Bile prevents food from rotting and destroys harmful bacteria present in it. Being alkaline, it neutralises the acidity of gastric juice in the chyme and makes it alkaline so that pancreatic juice can react with it in the small intestine.

2. Carbohydrate Metabolism: The hepatic portal vein, which brings the digested substances from the stomach and intestines, goes to the liver instead of going to the heart. Therefore, the digested substances other than lipids enter the liver before reaching other parts of the body. Liver cells, like efficient duodenums, take up excess sugar from the blood and store it by converting it to glycogen. This is called glycogenesis.

Stored glycogen is converted back to sugar and released into the blood when there is a decrease in blood sugar. This is called glycogenolysis. Not only this, liver cells also synthesize glucose from other substances like amino acids, fatty acids, glycerol etc. This is called gluconeogenesis.

3. Fat Metabolism: Liver cells also take an important part in fat metabolism. They synthesize some fats and other lipids and store some fats. It breaks down some fatty acids into acetyl coenzyme A. They produce lipoproteins for the transport of lipids in the blood.

4. Deamination of Amino Acids: Liver cells take more amino acids from the blood and break them down into pyruvic acid and ammonia (NH3). This is called deamination of amino acids. Pyruvic acid is used in the synthesis of glucose under energy production or gluconeogenesis.

5. Synthesis of Urea: The ammonia produced in the process of deamination and the ammonia produced as a result of protein metabolism in the body cells from the blood and the liver cells combine with the CO2 taken from the blood, with the help of an enzyme called urease, to synthesize urea. Kidneys excrete this urea from blood to urine along with -

                      2NH3 + CO2 → CO(NH2)2 + H2O

6. Excretion of excretory substances: In addition to synthesis of urea, liver also helps in excretion of some other waste substances (cholesterol, metals and hemoglobin breakdown products etc.). These substances mix with bile and reach the duodenum and pass out with feces.

7. Detoxification of toxins: The bacteria present in the intestine make some toxic substances which mix with the blood of the hepatic vein and reach the liver. Liver cells destroy or neutralize these toxins and convert them into harmless substances. Similarly, harmful prussic acid produced as a result of metabolism throughout the body is also removed from the blood by the liver cells.

8. Formation and breakdown of blood cells: In the embryonic stage of vertebrates, the liver is haemopoietic, that is, red blood cells are formed in it. In the adult stage, on the contrary, the Kupffer cells of the liver break down inactive and dead red blood cells and convert their hemoglobin into bile pigments, which reach the duodenum with bile and are excreted in the feces. In this process the iron of hemoglobin gets accumulated in the liver cells.

9. Storage of inorganic substances: In addition to iron, other substances like copper etc. are also stored by liver cells.

10. Secretion of enzymes: Liver cells secrete certain enzymes which take an important part in the metabolism of proteins, fats, carbohydrates etc. in the body.

11. Synthesis of blood proteins: Most of the proteins of blood plasma are synthesized in liver cells only. Prominent among these are albumin, globulins, prothrombin and fibrinogen.

12. Secretion of heparin: Hepatic cells also synthesize a protein called heparin and release it into the blood. This protein prevents the clotting of blood in the blood vessels.

13. Eating of bacteria: Kupfer cells of the liver feed on the harmful bacteria present in the blood and destroy them. This action is called phagocytosis.

14. Synthesis and storage of vitamins: Vitamin A is probably synthesized by liver cells. Vitamin D is activated by skin, liver and kidney together. Vitamins A, B12, D, E and K are also stored for a short time in the liver cells.

15. Lymph-production and blood-storage: The blood sinusoids present in the liver work for the storage of blood.

Liver tissue damage is called hepatitis. This damage is caused by the infection of some viruses or by the consumption of some medicines or toxic substances. As a result of this, jaundice often occurs. Nowadays, due to the excessive consumption of alcohol, the disease of hepatitis is increasing. The condition of acute hepatitis is called cirrhosis. In this, due to extensive liver damage, the patient often dies.

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