Physiological functions of the liver

2021-06-17 04:00 PM

The liver synthesizes fatty acids from glaucid, Protide and from lipid degradation products, fatty acids are cyclically metabolized.

Metabolic function

Glucose metabolism

Glucose from the intestine through the portal vein to the liver is mainly glucose, the rest is galactose and fructose. Fructose and galactose will be converted into glucose by the liver before use. In addition, the liver can make glucose from the sugar-producing amino acids, fatty acids, glycerol, and lactic acid. These substances will be converted to pyruvic acid or phosphopyruvic acid and then to glucose-6-phosphate before converting to glucose.

Lipid metabolism

The liver synthesizes fatty acids from carbohydrates, proteins, and lipid degradation products. Fatty acids are metabolized in a cycle (Knoop's oxidation for energy (accounting for 60% of the body's fatty acid metabolism).

In addition, the liver also synthesizes cholesterol, cholesterol esters, phospholipids, triglycerides, and lipoproteins (HDL, LDL and VLDL).

Phospholipids and lipoproteins are the major forms of lipid transport in the body. Cholesterol esters are the transporter of fatty acids.

Protein metabolism

The liver is the organ that metabolizes and stores proteins. Protein metabolism in the liver occurs very strongly, including 2 processes: amino acid metabolism and protein synthesis.

Amino acid metabolism:

Amino acid metabolism in the liver occurs very strongly through 3 processes of decarboxylation, deamination, and amine exchange.

 Carboxyl eliminate:

Thanks to decarboxylase enzymes, however, this process in the liver is not important.

Kh amin:

Thanks to specific enzymes deaminases produce ketonic acid and NH3. This process is closely related to amine metabolism.

Amino exchange:

As the most important process for the liver to synthesize endogenous amino acids specific to the body from ingested amino acids by an important enzyme, transaminase. In it, there are 2 very important enzymes, GPT and GOT:

GOT (glutamate oxaloacetate transaminase) hay ASAT (aspartate transaminase).

GPT (glutamate aspiring transaminase) hay ALT (Alan transaminase).

They catalyse the following reactions:

Aspartate + α ketoglutarate ß GOT to Oxaloacetate + Glutamate.

Alarmin + α ketoglutarate ß GPT à Pyruvate + Glutamate.

Protein synthesis:

Liver cells produce nearly 50% of the protein in the body. Therefore, the liver has a very strong regenerative capacity. After partial removal, the liver can regenerate again.

Synthesis of plasma proteins:

The liver synthesizes all plasma albumin, apart ( and globulin. Therefore, in liver failure, blood protein decreases, reducing oncotic pressure, fluid from the blood vessels escapes into the tissues, causing oedema.

Synthesis of clotting factors:

The liver synthesizes fibrinogen and clotting factors II, VII, IX and X from vitamin K. When liver failure, the blood clotting process is disturbed, the patient is very prone to bleeding.

Reserve function

The liver stores many important substances for the body: blood, glucide, iron and some vitamins such as A, D, B12 of which vitamin B12 is important.

Blood Reserve

The amount of blood contained in the liver is normally quite large (about 600 - 700 ml). When the blood pressure in the hepatic vein increases (infusion, after meals, drinking a lot of water...), the liver can swell to accommodate about 200-400 ml more.

Conversely, when the body is active or when the blood volume decreases, the liver contracts, sending some blood into the circulatory system.

Glucose storage

The liver stores glucose in the form of glycogen, which is enough to provide energy for the body to function within a few hours.

Through glycogen storage, the liver is involved in blood sugar regulation. When blood sugar rises, glycogen synthesis increases for storage. Conversely, when blood sugar is low, the breakdown of glycogen increases to bring glucose into the blood to keep blood sugar stable.

Thus, the liver plays a very important role in blood sugar regulation. The endocrine and nervous systems that regulate blood sugar are all through the liver. When liver failure, blood sugar regulation will be disturbed even though the endocrine and nervous systems are still good.

Iron reserves

The liver is one of the three organs of the body that stores iron (liver, spleen and bone marrow, which store 20% of the body's iron, about 1 g). This storage of iron comes from food or from Hb degradation. The liver stores iron in the form of ferritin. When needed, the liver sends iron to the blood-forming organs through an iron-transporting protein, transferrin, produced by the liver.

Store vitamin B12

The liver can store a few milligrams, while the body's needs are about 3 (g in a day. So, the body is very rarely deficient in B12, it takes 3-5 years to stop providing symptoms of vitamin B12 deficiency. .

Vitamin B12 deficiency causes megaloblastic pernicious anaemia. 

Password generator function

Bile is an excretory product of liver cells. After excretion, bile follows the microscopic bile ducts into the bile duct at the portal. From here, bile flows through the right and left hepatic ducts into the common bile duct and then through the cystic duct to the gallbladder. Here, the bile is condensed and under the influence of some stimuli, the gallbladder will contract to bring bile into the duodenum through the sphincter of Oddi. Before entering the duodenum, bile is mixed with pancreatic juice in the main pancreatic duct.

Bile is a liquid, green or yellow, with a pH of around 7 - 7.7.

The amount of excretion is about 0.5 liters/day.

Bile juice consists of many components (table 1). In it, there are some important components such as bile salts, bile pigments, cholesterol...

Table: Components of bile.

Ingredient

Ratio %

Country

bile salt

Bile pigment

Cholesterol

Inorganic salt

Fatty Acids

Lecithin

Fat

Phosphatase kiềm

97,0%

0,7%

0,2%

0,06%

0,7%

0,15%

0,1%

0,1%

...

 Bile salt

Bile salts are the potassium or sodium salts of cholesterol-derived conjugated bile acids with glycine or taurine. There are two types of bile salts: sodium glycocholate (potassium) and sodium taurocholate (potassium).

Bile salts have an important function in the digestion and absorption of lipids in the small intestine, leading to the absorption of lipid-soluble vitamins: A, D, E and K. 

Upon reaching the ileum, 95% of the bile salts are reabsorbed and then returned to the liver by the portal vein and re-excreted called the enterohepatic cycle (Figure 1).

The remaining 5% of bile salts are excreted in the stool, which has the effect of retaining water in the stool and maintaining peristalsis of the large intestine.

Bile pigment

Bile pigment (also known as direct bilirubin, conjugated bilirubin) is a substance formed in the liver from the breakdown product of Hb in the body and then excreted in bile.

Cholesterol

Hepatocytes synthesize cholesterol to produce bile salts, a part of cholesterol is excreted in bile to keep blood cholesterol constant.

Once it reaches the intestines, some cholesterol is reabsorbed.

Cholesterol is insoluble in bile, to be soluble it must be in the form of micelles with bile salts and lecithin and is called cholesterol saturation of bile. When bile loses this saturation (either from increased cholesterol or from a decrease in bile salts and lecithin), cholesterol precipitates to form stones.

Figure: Enterohepatic cycle of bile salts.

Anti-poison function

The liver is considered a protective barrier of the body against harmful factors that enter through the digestive tract. At the same time, it reduces toxicity and eliminates some substances produced during the body's metabolism. The antitoxic mechanism of the liver is performed by both Kupffer cells and hepatocytes.

Kupffer cells

Phagocytosis of bacteria entering the body through the gastrointestinal tract, and phagocytosis of both old red blood cells and broken red blood cells.

Liver cells

Anti-poison by 2 mechanisms:

Retain some heavy metals such as copper, lead, mercury...and some colourants like Bromo-Sulfo-Phtalein (BSP). Then it will be discharged.

By chemical reactions to turn toxic substances into non-toxic or less toxic substances and then excreted through the bile or kidney.

The reaction to generate urea from NH3

NH3 is made in the body through deamination or absorbed from the large intestine into the bloodstream. This is a poison to the body, especially to the nervous system. The liver converts NH3 to urea through the liver-only Ocnitin cycle. Then urea is excreted in the urine.

In liver failure, blood NH3 increases, causing hepatic coma.

Detoxification by redox, methylation reactions. acetylation

Oxidize alcohol to acetic acid.

Reduce aldehydes to alcohols.

Acetylation of Sulphanilamide to a less toxic substance.

Detoxification by conjugation reactions

Sulfonic conjugates:

Toxins created by rotten yeast in the intestine and partially absorbed into the blood such as indol, phenol, scatole... will combine with sulfuric acid in the liver into less toxic sulphates and be excreted in the urine.

Conjugation with glycine:

For example, benzoic acid is a toxin that is conjugated with glycine to form hippuric acid and excreted in the urine.

Conjugation with glucuronic acid:

This is the main antitoxic mechanism of the liver. Many substances such as bilirubin, alkaloids, phenols, steroid hormones, some drugs such as aspirin, antibiotics, barbiturates... will be conjugated with glucuronic acid. These substances are then excreted in the urine or in bile.