Liver (original) (raw)
The liver is an organ in vertebrates including humans. It plays a major role in metabolism and has a number of functions in the body including detoxification, glycogen storage and plasma protein synthesis. It also produces bile which is important for digestion. Medical terms related to the liver often start in hepato- or hepatic from the Greek word hepar for "liver".
Anatomy of the liver
| Human liver, view from above |
|---|
| View from below |
The adult human liver normally weighs between 1.0 - 2.5
kilograms, and is a soft, reddish-brown "wedge-shaped" organ. It is the largest organ in the abdomen and sits immediately under the diaphragm on the right side of the upper abdomen. The gallbladder lies beneath the liver. The right kidney lies below the liver.
The liver is supplied by two blood supplies - by the hepatic artery and by the portal vein. The hepatic artery normally comes off the celiac trunk. The portal vein brings venous blood from the digestive tract, so that the liver can process the nutrients and toxins extracted from food. The hepatic veins drain directly into the inferior vena cava.
The bile produced in the liver is collected in bile capillaries which merge to form bile ducts. These eventually drain into two large bile ducts which in turn merge to form the common bile duct. The bile then either drains directly into the duodenum via the common bile duct or is temporarily stored in the gallbladder via the cystic duct.
Apart from a patch where it connects to the diaphragm, the liver is covered entirely by visceral peritoneum, a thin, double-layered membrane that reduces friction against other organs. This peritoneum thickens into so-called ligaments, which hold the liver in place.
Anatomical lobes
One of these ligaments, the falciform ligament, is visible on the front (anterior side) of the liver. This divides the liver into a left anatomical lobe, and a right anatomical lobe.
If the liver is flipped over, to look at it from behind (the visceral surface), there are two additional lobes between the right and left. These are the caudate lobe (the more superior), and below this the quadrate lobe.
From behind, the lobes are divided up by the ligamentum venosum and ligamentum teres (anything left of these is the left lobe), the transverse fissure (or porta hepatis) divides the caudate from the quadrate lobe, and the right sagittal fossa, which the inferior vena cava runs over, separates these two lobes from the right lobe.
Functional lobes
The blood supply does not follow the same pattern as the anatomical lobes. The left and right branches of the portal vein and hepatic artery, as well as the left and right hepatic ducts, are divided slightly differently.
The functional lobes are divided along the line of the inferior vena cava. Each half is supplied by a different division of an artery. These functional lobes are further divided into segments, four on each, giving a total of eight numbered segments of the liver.
Ligaments and impressions
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Functions of the liver
The various functions of the liver are carried out by the liver cells or hepatocytes.
- produces and excretes bile required for food digestion. Some of the bile drains directly into the duodenum, some is stored in the gallbladder.
- roles in carbohydrate metabolism:
- 'gluconeogenesis' (the formation of glucose from certain amino acids, lactate or glycerol)
- 'glycogenolysis' (the formation of glucose from glycogen)
- 'glycogen synthesis' (the formation of glycogen from glucose)
- break down insulin and other hormones
- roles in lipid metabolism:
- cholesterol synthesis
- production of triglycerides
- production of coagulation factors (I (fibrinogen) III V VII IX and XI)
- neutralization of toxins, most medicinal products and hemoglobin
- conversion of ammonia into urea
- storage of a multitude of substances, including glucose in the form of glycogen, vitamin B12, iron, copper
- In the first trimester fetus, the liver is the main site of red blood cell production. By the 42nd week of gestation, the bone marrow has almost completely taken over that task.
Disorders of the liver
Many diseases of the liver are accompanied by
jaundice caused by increased levels of bilirubin in the system. The bilirubin results from the breakup of the hemoglobin of dead red blood cells; normally, the liver removes bilirubin from the blood and excretes it through bile.
- Viral hepatitis, infections of the liver by various viruses.
- Cirrhosis is the formation of fibrous tissue in the liver, replacing dead liver cells. The death of the liver cells can for example be caused by alcoholism or other toxins, or hepatitis
- Primary biliary cirrhosis
- Hemochromatosis, a hereditary disease causing the accumulation of iron in the body, eventually leading to liver damage
- Cancer of the liver (primary hepatocellular carcinoma or cholangiocarcinoma and metastatic)
- Wilson's disease, a hereditary disease which causes the body to retain copper
A number of liver function tests are available to test the proper function of the liver.
Liver transplantation
Damaged livers often regenerate on their own. When they do not, the nonfunctioning liver can be surgically replaced. Livers for transplant can be taken either from new cadavers or from live donors. In the latter case, the donor has surgery to remove part of his or her liver, which is transplanted into the recipient. Normally, each half regenerates into a complete, functional liver. (There have been rare cases of the donors dying shortly after the surgery.)
Liver-like organs in other animals
Arthropods have a digestive gland that functions like a combination of the liver and the pancreas. In insects this organ is known as the fat body.