Liver Physiology
Article by Dr Raghuram Y.S. MD (Ay) & Dr Manasa, B.A.M.S
Table of Contents
Functions of Liver
Liver carries out many functions in the body. They are mainly carried out by the hepatocytes (liver cells). It is said to be responsible for about 500 different functions. These functions are carried out by the liver in coordination with other systems and organs. About 20% of resting total body oxygen consumption is due to the liver.
Role of Liver in the metabolism
The liver plays a major role in the metabolism of carbohydrates, proteins, amino acids and lipids.
Carbohydrate metabolism
– Glycogenesis – The liver synthesizes and stores the glycogen via glycogenesis. Glycogenesis is a process in which glycogen is formed from glucose.
– Glycogenolysis – The liver breaks down the same glycogen into glucose and releases it into the blood by performing glycogenolysis as and when the body needs glucose.
– Gluconeogenesis – The liver takes part in the synthesis of glucose from some kinds of amino acids, lactate or glycerol. Fat is broken down by liver cells and adipose and glycerol is produced. This glycerol is used by the liver for gluconeogenesis.
– Gluconeogenesis – This is a process in which the liver synthesizes glycogen from lactic acid.
Protein metabolism
All – protein metabolism, synthesis and degradation are carried out by the liver. The liver is responsible for synthesizing all the plasma proteins except gamma-globulins and a large part of amino acid synthesis. It also produces clotting factors and red blood cells. Thrombopoietin which regulates the production of platelets by the bone marrow is produced majorly in the liver.
Lipid Metabolism
Liver is involved in the cholesterol synthesis, lipogenesis and triglyceride production. Major parts of the body’s lipoproteins are synthesized in the liver.
Digestion of food
Liver also plays a main role in the process of digestion of food. It produces and excretes bile. Bile is needed for emulsifying the fats and also helps in absorption of vitamin K from the diet. Some part of the bile is drained directly into the first part of the small intestine i.e. duodenum and some part of it is stored in the gallbladder.
Other functions of the liver
Growth factor
Insulin like growth factor 1 is a hormone which plays an important role in childhood growth. It is also produced in the liver. This factor continues to have its anabolic effects even in adults.
Liver is reservoir of blood
It is a storehouse / reservoir of blood. Liver stores large quantities of blood in its blood vessels since it is an expandable organ. Roughly 10% of the body’s total blood volume is found in the hepatic veins and hepatic sinuses. The liver expands due to the backpressure caused due to high pressure in the right atrium of the heart and half to one litre of extra blood is occasionally stored in the hepatic veins and sinuses. This happens in cardiac failure with peripheral congestion. According to Wikipedia the liver is a large and expandable venous organ. It acts as a blood reservoir in times of excess blood volume. It also supplies extra blood when the blood volume is diminished.
Drug metabolism
It is involved in drug metabolism wherein it breaks down or modifies toxic substances and most medicinal products.
Urea cycle
It converts ammonia into urea (urea cycle) and that urea is excreted in urine.
Lymph production
Roughly half of all the lymph which is formed in the body under resting conditions arises in the liver. This happens due to high permeability of the liver sinusoid epithelium.
Stores important components
Liver allows storage of vitamin A, D, B12, K and E and also copper, iron, zinc, cobalt and molybdenum.
Haemopoietic organ
Liver is a haemopoietic organ. It forms RBC and WBC in the embryonic stage. It forms RBC in the fetus in the first trimester.
Blood purifier
It purifies the blood. The Kupffer cells are involved in phagocytosis of dead blood cells and bacteria from the blood.
Immunological effects
The mononuclear phagocyte system present in the liver comprises many immunologically active cells. It acts as a sieve for antigens and hence is responsible for immunological effects.
Produces albumin
Albumin, the protein in the blood serum, is produced by the liver. It maintains oncotic pressure and enables transport of steroid hormones and fatty acids.
Liver helps in
– It breaks down insulin and other hormones.
– It breaks down bilirubin and facilitates its excretion into bile.
– It breaks down and excretes many waste products.
– Hydrogen peroxide is broken down into water and oxygen by the catalase enzyme produced by the liver. Hydrogen peroxide is a toxic oxidizing agent.
Synthesizes angiotensinogen
Angiotensinogen, a hormone which raises the blood pressure as a response to the activation by renin, is synthesized by the liver. Renin is an enzyme which is released by the kidney when it senses low blood pressure.
Liver Regeneration
The specialty of the liver is that it is the only human internal organ which has the capacity to regenerate its lost tissues. A complete liver can be regenerated from as little as 25% of liver. Bot being a true regeneration, it is only a compensatory growth in mammals. Whatever lobes have been removed will not regrow. The growth of liver is not that of its original form but is a restoration of function.
Physiology of Liver – Ayurveda Perspective
In Ayurveda, the liver is called Yakrit.
At the level of Raktadhara Kala i.e. the membranes or layers of liver (and spleen) which take part in the formation of blood cells in good quality and quantity.
Being a Raktashaya, along with spleen, the liver acts as a reservoir of blood.
Being a part of the roots of raktavaha srotas – the channels of conveyance of blood in the body, the liver along with spleen is responsible for distribution of blood to the blood conveying channels.
Being an abode / seat of Ranjaka Pitta, the liver, along with spleen takes part in the action of ‘rasa ranjana’ i.e. colouring the rasa tissue entering them into red colour and transforming it into rakta – blood tissue. It also imparts colour to the faeces and urine.
Related Reading – Yakrit Kriya Shareera – Physiology of Liver, Ayurveda Perspective
