Key Liver Functions: Metabolism, Storage, Synthesis & More
Functions of the Liver
The liver is the largest gland and one of the vital organs of the body. It performs numerous metabolic and homeostatic functions, which are summarized below.
1. Metabolic Function
The liver is the organ where maximum metabolic reactions, such as the metabolism of carbohydrates, proteins, fats, vitamins, and many hormones, are carried out.
2. Storage Function
Many substances, such as glycogen, amino acids, iron, folic acid, and vitamins A, B, and D, are stored in the liver.
3. Synthetic Function
The liver produces glucose through gluconeogenesis. It synthesizes all plasma proteins and other proteins (except immunoglobulins), such as clotting factors, complement factors, and hormone-binding proteins. It also synthesizes steroids, somatomedin, and heparin.
4. Secretion of Bile
The liver secretes bile, which contains bile salts, bile pigments, cholesterol, fatty acids, and lecithin. The functions of bile are mainly due to bile salts. Bile salts are required for the digestion and absorption of fats in the intestine. Bile helps to carry away waste products and break down fats, which are excreted through feces or urine.
5. Excretory Function
The liver excretes cholesterol, bile pigments, heavy metals (such as lead, arsenic, and bismuth), toxins, bacteria, and viruses (like that of yellow fever) through bile.
6. Heat Production
An enormous amount of heat is produced in the liver due to its metabolic reactions. The liver is the organ where maximum heat is produced.
7. Hematopoietic Function
In the fetus (hepatic stage), the liver produces blood cells (Chapter 9). It stores vitamin B12, necessary for erythropoiesis, and iron, necessary for the synthesis of hemoglobin. The liver also produces thrombopoietin, which promotes the production of thrombocytes.
8. Hemolytic Function
Senile red blood cells (RBCs), after their lifespan of 120 days, are destroyed by the reticuloendothelial cells (Kupffer cells) of the liver.
9. Inactivation of Hormones and Drugs
The liver catabolizes hormones such as growth hormone, parathormone, cortisol, insulin, glucagon, and estrogen. It also inactivates drugs, particularly fat-soluble drugs. These fat-soluble drugs are converted into water-soluble substances, which are then excreted through bile or urine.
10. Defensive and Detoxification Functions
The reticuloendothelial cells (Kupffer cells) of the liver play an important role in the body’s defense. The liver is also involved in the detoxification of foreign bodies. Foreign bodies, such as bacteria or antigens, are engulfed and digested by the reticuloendothelial cells of the liver by means of phagocytosis. The reticuloendothelial cells of the liver also produce substances such as interleukins and tumor necrosis factors, which activate the body’s immune system (Chapter 15).
Liver cells are involved in removing the toxic properties of various harmful substances. The removal of the toxic property of a harmful agent is known as detoxification.
Detoxification in the liver occurs in two ways:
- Total destruction of the substances by means of metabolic degradation.
- Conversion of toxic substances into non-toxic materials by means of conjugation with glucuronic acid or sulfates.