Functions
The Liver is involved in a wide variety of functions with a range of substances. A few of the essential ones include:
Carbohydrate Metabolism
The liver is essential in maintaining a normal blood glucose level of about 90mg/dl. When blood glucose is low the liver breaks down glycogen reserves to glucose which is then released into the blood. The liver can also convert other sugars like fructose into glucose if needed. After eating a meal blood levels of glucose rise and the liver converts glucose to glycogen which it then stores. These metabolic activities are regulated by circulating hormones such as insulin and glucagon.
Lipid Metabolism
The liver also regulates circulating levels of triglycerides, fatty acids, and cholesterol. When levels are low the liver breaks down its lipid stores and releases them into the blood stream. When lipid levels are high the liver removes them from the blood and stores them. Aswell as its function in regulating lipid levels the liver breaks down fatty acids to synthesize lipoproteins which transport fatty acids and cholesterol to and from the cells. They can also generate ATP from fatty acids, and synthesize cholesterol to make bile salts
Protein metabolism
Hepatocytes deaminate amino acids so they can be used for ATP production or converted to other products such as fats or carbohydrates. Leftover ammonia, which is toxic, is neutralized into the less toxic urea which is excreted in the urine.
Processing of drugs
The liver absorbs and removes circulating drugs, limiting their effects in the body. This must be considered when taking into account a drugs effect. Doctors must compensate this removal when prescribing drugs possibly by administering it every few hours to keep the levels high enough to be therapeutic. The liver detoxifies substances such as alcohol and can excrete drugs into the bile.
Lipid soluble toxins are also absorbed by the liver and stored in lipid deposits. This prevents them from interfering in cellular functions.
Processing of Hormones
The liver is the primary site of absorption and recyling of hormones including adrenaline, noradrenaline, insulin, thyroid hormones, and steroid hormones (such as estrogens and androgens) and corticosteroids. It is able to chemically alter thyroid and steroid hormones like the estrogens.
Bile Secretion
Hepatocytes make and secrete bile which is needed for the absorption of dietary fats and leads to the metabolism of bilirubin which is then excreted in the faeces.
Storage
As well as storing glycogen the liver also stores fat soluble vitamins such as A, B12, D, E and K. These reserves are used when the diet does not provide enough of them. Minerals such as iron and copper are also absorbed from the bloodstream and stored in the liver. Iron is converted into ferritin which allows it to be stored in a protein-iron complex. Along with the skin and kidneys, the liver is involved in the synthesis of the active form of vitamin D. It absorbs cholecalciferol (Viteman D3) from the blood and converts it into the intermediary protein 25-hydroxy-D3. It releases this into the bloodstream, and it is then picked up and used by the Kidneys.
Protection from Infection
Phagocytic cells help protect from disease by engulfing and destroying old red blood cells, bacteria and worn out white blood cells. These cells, called Kupffer cells are also antigen presenting cells that can stimulate an immune response.
Synthesis of plasma proteins
Hepatocytes synthesize and release plasma proteins into the blood. These include albumins which contribute to the osmotic concentration of blood, clotting proteins, and other transport plasma proteins.
Figure : A Sheeps Liver
Image courtesy of https://en.wikipedia.org/wiki/File:Leber_Schaf.jpg. This image is in the public domain and thus free of any copyright restrictions.
