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.
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