Homeostasis is the maintenance of a stable internal environment. Because the liver has a constant high metabolic rate it produces large amounts of heat and due to its excellent blood supply the heat is efficiently distributed around the body. In the role of homeostasis the liver regulates many aspects which are needed to maintain a healthy body. One is the regulation of glucose, the blood glucose levels increase after a digestion of a rich carbohydrate meal. The liver absorbs the glucose from the blood and converts it into a storage carbohydrate called glycogen; this is controlled by the hormone insulin which is secreted by the islets of Langerhans in the pancreas. Glucose is soluble so therefore is not suitable for storage but by converting it into glycogen which consists of large complex insoluble molecules it enables it to be stored. If the blood glucose levels fall, the liver cells hydrolyse glycogen back to glucose, this results in the pancreas secreting less insulin. If levels of glucose are too high it will be converted into lipids, these are transported to the fat depots, the subcutaneous adipose tissue and fat stores around the internal organs. To maintain the regulation of lipids the liver can absorb them, they can either be broken down to form glycogen or transported to the fat depots. Amino acids are proteins which are not stored in the body like carbohydrates and lipids therefore excess amino acids are destroyed by the liver cells in a process called deamination. This is where the amino group NH2 is removed from the amino acid by the formation of ammonia.
2 NH2RCHCOOH + O2 ROCOCOOH + NH3
Amino acid Oxygen Amino acid residue Ammonia
The amino acid residue then enters the Kreb’s triacarboxylic acid cycle and is oxidized to release energy. The ammonia must not be allowed to accumulate because it is highly toxic even in very small quantities. The ornithine cycle is a reaction when the ammonia reacts with carbon dioxide to form the less toxic urea which is transported in the bloodstream from the liver to the kidneys for elimination from the body in the urine.
2 NH3 + CO2 CO(NH2)2 + H2O
Ammonia Carbon dioxide urea water
Another function of the liver to maintain homeostasis is the synthesis of proteins, this is when the liver synthesizes some important proteins, and the serum protein fibrinogen is a protein involved in the clotting of blood after injury. Amino acids which are not essential can be produced from other amino acids by the process called transmination, this is where transaminase enzymes can transfer amino groups from amino acids to carboxylic acids, and this produces as new amino acid.
GOT
Glutamic acid + Oxaloacetic acid aspartic acid + Oxalogutaric acid
This is the way the liver can make a variety of amino acids which may not be in the diet of the person.
The production of bile occurs when the liver cells synthesize bile which is temporarily stored in the gall bladder, from here it passes along the bile duct to enter the duodenum where it is used in digestive functions. The liver also makes cholesterol, when the dietary intake is sufficient, this is sterol. Cholesterol is an important component of cell membranes and a precursor in the formation of steroid hormones. Any excess bile is excreted in the bile, a considerable excess may be precipitated in the gall bladder or bile duct as gall stones. If gall stones block the bile duct, obstructive jaundice will occur when the skin becomes yellow due to the retention of bilirubin.
The liver also stores vitamins and minerals, fat soluble vitamins such as A for retinal production, Vitamin D for calcium absorption, Vitamin B12 for the manufacture of haemoglobin and coenzymes and also vitamin E. The minerals which it stores are K+, Cu2+, and Fe2+ all of which are important for enzyme function.
After the old red blood cells have been engulfed by the phagocytic cells, the haemoglobin is broken down by the liver cells into two parts, haem containing no iron and iron globin complex. Haem is converted to a green pigment biliverdin which is reduced to the brown pigment, bilirubin which is excreted by bile, around 8g of haemoglobin are removed each day this way. Iron globin complex is broken down into amino acids which are used for protein synthesis by the liver, the iron is used for the synthesis of new haemoglobin by the liver cells. The formation of red blood cells are produced by the liver in a foetus but this function is done by the bone marrow after birth. The adult liver plays a very important function in red blood cells formation, it synthesizes the haematinic principle from vitamin B12 which is stored in the liver. If there is a deficiency of this vitamin pernicious anaemia develops where there will be a drastic reduction in the red blood cells.
The liver makes many chemicals which are toxic less harmful this is called Detoxification. Deamination converts ammonia to urea. Hydrogen peroxide, a toxic by product of certain metabolic pathways, is converted into oxygen and water by catalyse, an enzyme in the liver. Excess insulin from the pancreas is broken down by enzymes in the liver. After they have performed their functions, sex hormones are modified in the liver and then excreted either in the bile or via the kidneys. The food preservative benzencarboxylic is joined to glycine, an amino acid, to form N-benzolyglycine which is excreted in the urine. Ethanol is processed by the liver cells.
