Introduction:
The liver is one of the most vital organs of the human body. It performs numerous metabolic, synthetic, excretory, and protective functions essential for life. It acts as both a metabolic factory and an excretory organ. The bile secreted by the liver helps in fat digestion and absorption of fat-soluble vitamins (A, D, E, and K), while also serving as a route for the excretion of waste products and pigments. Complete loss of liver function is incompatible with life — death may occur within 24 hours.
Physiological Role of the Liver
- The liver is a large, reddish-brown organ located below the diaphragm on the right side of the body, weighing around 1500 grams.
- It receives a dual blood supply:
- Hepatic artery – brings oxygenated blood from the heart.
- Portal vein – brings nutrient-rich blood from the gastrointestinal tract, pancreas, and spleen.
- Blood from both these sources mixes in hepatic sinusoids, where exchange of nutrients, oxygen, and waste products takes place.
- The liver’s unique architecture — a network of hepatic cells (hepatocytes) arranged in plates — allows direct contact with both blood (via sinusoids) and bile canaliculi for efficient processing.
Structure and Function Overview
Each hepatocyte acts as a biochemical processing unit — receiving nutrients and oxygen from the blood and excreting waste materials and bile into the bile canaliculi. These canaliculi join to form bile ducts, which carry bile to the small intestine or gallbladder. The liver thus maintains a balance between metabolism, detoxification, and excretion.
Main Functions of the Liver
1. Metabolic Functions:
The liver plays a central role in the metabolism of carbohydrates, proteins, and lipids. It also regulates blood glucose levels and produces various important biochemical substances.
a) Carbohydrate Metabolism
- The liver stores glucose in the form of glycogen and breaks it down during fasting or exercise to maintain normal blood glucose levels.
- It performs gluconeogenesis – conversion of amino acids and fatty acids into glucose during energy deficiency.
- It also interconverts other hexoses (like fructose and galactose) into glucose for use by body tissues.
b) Protein Metabolism
- The liver synthesizes almost all plasma proteins (except gamma globulins), including:
- Albumin – maintains osmotic pressure.
- Lipoproteins – transport lipids in plasma.
- Carrier proteins – for metals and hormones.
- Clotting factors – such as prothrombin, fibrinogen, and Factors V, VII, IX, and X.
c) Lipid Metabolism
- The liver synthesizes plasma lipoproteins and key lipids such as cholesterol, phospholipids, and triglycerides.
- It converts cholesterol into bile acids — essential for fat digestion and absorption in the intestine.
- It also esterifies cholesterol and participates in fatty acid synthesis from acetyl-CoA.
d) Other Metabolic Conversions
- Performs transamination reactions – converting keto acids to amino acids.
- Produces ketone bodies (acetoacetic acid and β-hydroxybutyric acid) from acetyl-CoA.
- Converts lactic acid to glucose and vice versa.
- Synthesizes fatty acids and other intermediary compounds essential for metabolism.
2. Storage Functions
- The liver serves as a major storage site for several vital substances:
- Glycogen – carbohydrate storage.
- Vitamins – especially A, D, and B12.
- Iron – stored in the form of ferritin in liver, spleen, and bone marrow.
3. Excretory Functions
The liver secretes bile, which contains bile salts, bile pigments (mainly bilirubin esters), cholesterol, and other waste products. Bile plays two important roles:
- Digestive role: Bile salts act as detergents that emulsify fats, enabling their digestion and absorption in the small intestine.
- Excretory role: The liver removes metabolic waste, drugs, and toxins by excreting them into bile.
4. Protective Functions:
a) Phagocytic Action
The liver contains specialized macrophages called Kupffer cells that line the sinusoids. These cells form part of the reticuloendothelial system and act as scavengers to remove bacteria, worn-out cells, and other foreign particles from the blood.
b) Detoxification
- The liver detoxifies harmful substances and converts them into less toxic or more water-soluble forms for excretion via kidneys or bile.
- Common detoxification reactions include oxidation, reduction, methylation, and conjugation with glucuronic acid or sulfate.
- Ammonia (toxic by-product of amino acid metabolism) is converted into urea in the liver.
- Insoluble pigments such as bilirubin are conjugated with glucuronic acid to form bilirubin diglucuronide, which is excreted in bile.
- Excess steroid hormones are also esterified and excreted through the kidney, helping maintain hormonal balance.
5. Circulatory Functions
- The liver helps regulate blood volume by serving as a temporary blood reservoir.
- It mixes blood from the portal system and the systemic circulation, ensuring detoxification of materials absorbed from the gut before they reach the general circulation.
- The Kupffer cells contribute to the liver’s role in immune defense.
6. Function in Blood Coagulation
The liver synthesizes several essential proteins required for blood coagulation, including:
- Fibrinogen
- Prothrombin
- Factors V, VII, IX, and X
These clotting factors have short half-lives and depend on vitamin K for synthesis. Severe liver disease or vitamin K deficiency can therefore lead to bleeding disorders.
Detailed Notes:
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