Functions of the Liver
Physiology textbooks state that the liver has more than 200 different functions. The most important functions are discussed here. The liver is arranged in such a way that it can screen the blood going into the systemic circulation and adjust the levels of various substances in the blood.
Effect on carbohydrates: The liver helps main-tain the blood glucose level. It stores glucose ob-tained from the gut as glycogen, if the blood level of glucose is too high. If the glucose level drops, the glycogen is converted to glucose to return the blood glucose level to normal. Lipids and proteins are also used to manufacture glucose.
Effect on lipids: Similar to carbohydrates, theliver adjusts lipid levels in blood by mobilizing or storing lipids.
Effect on proteins: Amino acids absorbed fromthe gut may be stored in the liver for conversion into lipids, carbohydrates, or proteins. When needed, they are broken down. Ammonia is formed when amino acids are broken down. The liver converts the ammo-nia into urea, which is later excreted by the kidneys.
The liver manufactures most of the plasma pro-teins such as albumin, proteins required for the clot-ting process, and proteins used as transport vehicles in the blood. The liver also removes antibodies from the blood.
Removal of waste products: The liver detoxifiestoxins and drugs. Many drugs are rapidly converted to ineffective forms by the liver. That is why certain drugs must be given in larger quantities and in fre-quent doses. Conversely, dosage must be reduced in liver failure.
Removal of pathogens: The Kupffer cells, whichare fixed macrophages, remove pathogens and old and damaged blood cells. The breakdown product of hemoglobin (from red blood cells) —bilirubin—is re- moved from the blood and secreted into the bile for disposal.
Removal of circulating hormones: The liver is im-portant role for removing hormones that circulate in the blood, such as epinephrine, norepinephrine, thy-roid hormones, corticosteroids, and sex hormones.
Formation of vitamin D: One important functionof the liver is to convert a precursor of vitamin D that is manufactured in the skin or absorbed in the gut into an intermediary product that can be acted upon by the kidney. The kidney is the organ that finally forms vitamin D—one of the hormones that regulates calcium levels in the blood .
Storage of vitamins: The liver stores many vita-mins, especially vitamin B12 and the fat-soluble vita-mins, A, D, E, and K.
Mineral storage: An important mineral stored bythe liver is iron.
Bile synthesis: The liver manufactures bile, thesecretion vital for fat digestion and absorption. Bile is a yellow liquid that is mostly water. The most impor-tant component of bile is bile salts.
Because fat in the diet is not water-soluble, it coa-lesces to form large drops of fat in the gut. This makes it difficult for the enzymes secreted by the pancreas to act on the fat located deep inside the drop. The bile salts break the large drops into smaller ones, making it easier for the enzymes to act. In ad-dition, the bile salts facilitate the action of the en-zymes and help with the absorption of lipids through the mucosa into the body. Most bile salts that enter the gut via the bile duct are reabsorbed into the cir-culation and recycled by the liver.
Other than bile salts, bile also contains the bilepigment bilirubin, which is a breakdown product ofhemoglobin.