Functions of Liver

 Functions of Liver

Liver is an essential organ of body. Its functions are numerous which are briefly summarized below:

(I) Functions related to blood and circulation

(i) R. B. C. formation-in fetal life.

(ii) R. B. C. destruction in adult life.

(iii) Storehouse of blood and regulates blood volume.

(iv) In relation with blood clotting-

(a) Manufactures prothrombin (with the help of vitamin K) and fibrinogen and thus essential for clotting.

(b) Mast cells form heparin and prevent intravascular clotting.

(v) Related to activity of its R. E. system in immune mechanism.

(vi) It transfers blood from portal to systemic circulation.  

(vii) Manufactures all plasma proteins.

(viii) Stores iron, haematinic factor also known as vitamin B₁, and copper, and thus helps in the formation of red cells and hemoglobin.

(ix) Hepatic and portal circulation. The total flow of blood in the liver through both hepatic artery and portal vein is about 1-5 litre/minute. 20-30% of which is carried by the hepatic artery and 100% saturated with O₂, whereas the remaining is carried by portal vein which is 80-90%saturated with O_2.

Portal Venous blood is derived mainly (60%) from superior mesenteric vein and remaining from splenic vein. The bloods of two streams are well mixed in human before they are supplied to the cell and thereby every cell receives the same blood.

The pressure in the portal system is normally between 5 and 13 mm of Hg. Due to high pressure in arterial blood of hepatic sinusoids, the portal blood mixes with it freely. The hepatic vein formed from the central vein in which hepatic venous pressure is about 6 mm of Hg and normally 60% saturated with O₂

In vascular components of the liver there are sphincters which regulate local contributions of hepatic artery, portal vein, total liver blood flow and the capacity of total venous bed.

Thus the liver can accommodate blood up to one-third of the total blood volume. The liver produces 75 ml of lymph per minute and its protein content is 90% of that of plasma. Through hepatic lymphatics, 40% of the total plasma protein is returned to the blood every 24 hours.

II. Bile secretion. Bile is secreted continuously from the liver cells and stored in the gall-bladder. Bile is the product of both secretion and excretion. Cholesterol is synthesized in the liver from active acetate. Cholesterol is also excreted from the liver. Bile acids-cholic acid, deoxycholic acid and lithocholic acid have been considered to be the derivatives of cholanic acid and formed in the liver.

The bile acids in conjugation with glycine and taurine form the compounds- Taurocholic acid Glycocholic acid respectively. Bile salts are the Na salts of taurocholic acid and glycocholic acid.

Bile salts have got important functions in absorption of fats and also for the emulsification of fats with the concurrent production of a great surface area to enable lipase and other enzymes to act more efficiently.

Bile pigments are the biliverdin and bilirubin. These are the excretory products of hemoglobin of broken down R.B.C. and are formed in the R. E. system in the various parts of the body. Bone-marrow, liver and spleen have been considered to be the site of formation of the bile pigments.

III. Functions relation with carbohydrate metabolism

(i) Converts non-glucose monosaccharide into glucose (and glycogen).

(ii) Converts lactic acid, pyruvic acid and glycerol into glucose and also glycogen.

(iii) Stores carbohydrate in the form of glycogen (Glycogenesis) and when blood sugar tends to be low, it mobilizes glycogen (Glycogenolysis).

(iv) Takes an important part in blood sugar regulation,

(v) It is the seat of neoglucogenesis.

(vi) Manufactures fats from carbohydrates, etc.

(vii) Glucose is metabolized here through TCA cycle and other alter

(viii) Glucuronic acid is formed from uridine diphosphate glucose (UDPG) which plays an important role in the conjugation of bilirubin and other substances.

(ix) Alcohol metabolism. The liver is the main seat of alcohol metabolism. A zinc-containing enzyme, alcohol dehydrogenase catabolizes acetaldehyde. A coenzyme acetaldehyde dehydrogenase converts acetaldehyde to acetyl CoA. Acetaldehyde can be further catabolized to acetate. Acetate may be oxidized to CO₂, and H₂O or converted to other biochemical compounds including fatty acids through TCA cycle.

When alcohol is converted to acetaldehyde and then to acetyl CoA, NAD acts as a cofactor, i.e., hydrogen acceptor. ATP, generated by the oxidation of NADH through electron transport chain (ETC) of oxidation, provides energy for the synthesis of fatty acids.

The reduction of activity of TCA cycle substrates is due to decreased fatty acid oxidation. On the other hand, NADH+H⁺, being hydrogen carrier, is utilized for the conversion of pyruvate to lactate and for this reason alcohol raises blood lactate level.

This mechanism explains post-alcoholic hypoglycemia. Hepatic microsomes help in the esterification of fatty acids to triglycerides rather than phospholipids. The direct effect of alcohol may be alcoholic fatty liver which is the cause of increased hepatic fatty acid synthesis as well as decreased hepatic fatty acid oxidation.

IV. Functions in relation with fat metabolism  

(i) It stores fats. Liver contains about 3% of fat (with mixed diet).

(ii) It helps in the oxidation of fat, releasing energy in the form of ATP.

(iii) Site of synthesis of cholesterol from acetate.

(iv) Synthesizes phospholipids.

(v) Synthesizes fats from carbohydrates and proteins.

(vi) It is the seat of ketone body formation.

(vii) Unused free fatty acid (FFA) released from fat depot, is converted to triglycerides and other lipids to meet energy requirement.

(viii) Glycerol is oxidized here via the pathway of carbohydrate Metabolism.

(viii) In a carbohydrate deficiency, the fat metabolism in the liver is increased and fat is partially converted to glucose or glycogen.

(ix) Fat-soluble vitamins, e.g., A, D, E, K, are stored here.

(x) It is the seat of ketone body formation.

V. Functions in Relation with protein metabolism

(i) It is the seat of specific dynamic action of protein.

(ii) Chief seat of deamination.

(iii) Main seat of urea and uric acid formation.

(iv) Synthesis of some amino acids takes place here.

(v) Plasma proteins are manufactured here except immunoglobulin.

(vi) Coagulation factors in addition to fibrinogen and prothrombin are manufactured here.

(viii) It is the seat of nitrogen metabolism as evident from deamination process, urea synthesis, etc.

VI. Hormone metabolism:

(i) Reduces the circulating adrenal cortical and sex hormones degradation and conjugation.

(ii) Steroid hormones like oestrogen, Cortisol, testosterone, etc., are insoluble and solubility is increased and made excretable after being conjugated with glucuronic acid and sulphuric acid in the liver.

(iii) Inactivation of other hormones also occurs here.  Inactivation of Insulin, glucagon, antidiuretic hormone (ADH) and anterior pituitary trophic hormones, etc also occur here

VII. Functions in Relation with vitamins

(i)It manufactures prothrombin with the help of vitamin K.

(ii) It forms vitamin A from carotene and stores vitamins A and D.

(iv)The liver converts folate to its active form-tetrahydrofolate. Tetrahydrofolate is the storage form of folic acid.

(v) The liver is the principal storage organ for vitamin B₁₂.

 

VIII. Excretory functions

(i)Various toxins, heavy metals, bacteria and pharmacological compounds are excreted by liver.

(ii) Cholesterol and bile pigments are excreted through bile.

IX. Detoxicating and protective functions

The liver does the detoxication of different toxic substances either produced in the body or taken along with food. Detoxication is the process by which toxic substances are rapidly made excretable through different biochemical changes. This is done by the following processes-

·        Oxidation

·        Reduction

·        Hydrolysis.

·        Conjugation

X Heat regulation-liver produces a large amount of heat and takes part in heat regulation.

XI. It is the storehouse of Fe, fat-soluble vitamins, glycogen, labile protein, fat, etc.