liver - Anatomy, Physiology, Diseases of the liver, Liver transplantation, Development, Liver as food, Cultural allusions, References
In vertebrates, a large, unpaired gland with digestive functions, situated in the upper part of the right-hand side of the abdominal cavity under cover of the ribs, separated from the thoracic contents by the diaphragm. It is attached by the peritoneum to the abdominal wall and the stomach, and divided into four lobes. The liver performs many important functions. It secretes bile, which is emptied into the duodenum (via the common bile duct), and facilitates the digestion and absorption of fats. It deals with the newly absorbed products of digestion (eg the formation of glycogen or fats from monosaccharides, the release of glucose into the blood stream). It manufactures the anticoagulant heparin and other plasma proteins. It stores glycogen, fat, iron, copper, and the vitamins A, D, E, and K. It detoxifies harmful substances such as drugs and toxins, and consumes and destroys red blood cells.
Anatomy
The adult human liver normally weighs between 1.3 - 3.0 kilograms, and is a soft, pinkish-brown "boomerang shaped" organ.
The liver is supplied by two main blood vessels on its right lobe: the hepatic artery and the portal vein.
The bile produced in the liver is collected in bile canaliculi, which merge to form bile ducts.
The liver is among the few internal human organs capable of natural regeneration of lost tissue; as little as 25% of remaining liver can regenerate into a whole liver again.
Surface anatomy
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.
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.
Functional anatomy
For purposes such as advanced liver surgery, it is crucial to understand the fundamental importance of the liver on the blood supply and biliary drainage system. The central area where the common bile duct, portal vein, and hepatic artery enter the liver is the hilum or "porta hepatis". The duct, vein, and artery divide into left and right branches, and the portions of the liver supplied by these branches constitute the functional left and right lobes. The segments corresponding to the surface anatomical lobes are as follows:
| Lobe | Couinaud segments |
|---|---|
| Caudate | 1 |
| Left | 2, 3 |
| Quadrate | 4 |
| Right | 5, 6, 7, 8 |
Physiology
The various functions of the liver are carried out by the liver cells or hepatocytes. The liver performs several roles in carbohydrate metabolism: Gluconeogenesis (the formation of glucose from certain amino acids, lactate or glycerol) Glycogenolysis (the formation of glucose from glycogen) (muscle tissues can also do this) Glycogenesis (the formation of glycogen from glucose) The breakdown of insulin and other hormones The liver is responsible for the mainstay of protein metabolism. The liver is responsible for immunological effects- the reticuloendothelial system of the liver contains many immunologically active cells, acting as a 'sieve' for antigens carried to it via the portal system. Some functions can be emulated by liver dialysis, an experimental treatment for liver failure.
Diseases of the liver
Many diseases of the liver are accompanied by jaundice caused by increased levels of bilirubin in the system. Cirrhosis is the formation of fibrous tissue in the liver, replacing dead liver cells.
There are also many pediatric liver disease, including biliary atresia, alpha-1 antitrypsin deficiency, alagille syndrome, and progressive familial intrahepatic cholestasis, to name but a few.
A number of liver function tests are available to test the proper function of the liver.
Liver transplantation
Human liver transplant was first performed by Thomas Starzl in USA and Roy Calne in England in 1963 and 1965 respectively. Liver transplantation is the only option for those with irreversible liver failure. Less commonly, liver transplantation is done for fulminant hepatic failure, in which liver failure occurs over days to weeks. Living donor liver transplantation is a technique in which a portion of a living person's liver is removed and used to replace the entire liver of the recipient. Only 20% of an adult's liver (Couinaud segments 2 and 3) is needed to serve as a liver allograft for an infant or small child. More recently, adult-to-adult liver transplantation has been done using the donor's right hepatic lobe which amounts to 60% of the liver. Due to the ability of the liver to regenerate, both the donor and recipient end up with normal liver function if all goes well. () The risk of postoperative complications (and death) is far greater in right sided hepatectomy than left sided operations
Development
The liver develops as an endodermal outpocketing of the foregut called the hepatic diverticulum.
Fetal blood supply
In the growing fetus, a major source of blood to the liver is the umbilical vein which supplies nutrients to the growing fetus. The umbilical vein enters the abdomen at the umbilicus, and passes upward along the free margin of the falciform ligament of the liver to the inferior surface of the liver. The ductus venosus carries blood from the left portal vein to the left hepatic vein and thence to the inferior vena cava, allowing placental blood to bypass the liver.
In the fetus, the liver is developing throughout normal gestation, and does not perform the normal filtration of the infant liver. The fetal liver releases some blood stem cells that migrate to the fetal thymus, so initially the lymphocytes, called T-cells, are created from fetal liver stem cells.
Liver as food
|
Pork liver Nutritional value per 100 g |
|
|---|---|
| Energy 130 kcal | 2.5 g |
| Fat | 3.7 g |
| Protein | 21 g |
| Vitamin A 6500 μg | 722% |
| Riboflavin (Vit. | 100% |
| Vitamin B6 0.7 mg | 54% |
| Folate (Vit. | 1083% |
| Iron 23 mg | 184% |
| Sodium 87 mg | 6% |
|
Beef and chicken liver are comparable. Percentages are relative to US RDI values for adults. Source: USDA Nutrient database |
Mammal and bird livers are commonly eaten as food: products include liver paté, Liverwurst, Braunschweiger, foie gras, chopped liver, leverpostej and liver sashimi.
Both animal and fish livers are rich in iron and Vitamin A and cod liver oil is commonly used as a supplement. In the US, the USDA specifies 3000 μg per day as a tolerable upper limit, which amounts to about 50 g of raw pork liver or 3 g of polar-bear liver., However, acute vitamin A poisoning is not likely to result from liver consumption, since it is present in a less toxic form than in many dietary supplements.
Cultural allusions
In Greek mythology, Prometheus was punished by the gods for revealing fire to humans by being chained to a rock where a vulture (or an eagle, Ethon) would peck out his liver, which would regenerate overnight.
The Talmud (tractate Berakhot 61b) refers to the liver as the seat of anger, with the gallbladder counteracting this.
In Arabic and Persian language, the liver is used in figurative speech to refer to courage and strong feelings, or "their best," e.g.
The legend of Liver-Eating Johnson says that he would cut out and eat the liver of each man killed.
In the motion picture The Message, Hind bint Utbah is implyed or portrayed eating the liver of Hamza ibn Abd-al-Muttalib during the Battle of Uhud.
It is well known that Inuit will not eat the liver of polar bears or seals 8lo5iuwzauiuayhjrtrutyruthuryturytuyrutyruigh4yt8bg85utosuhyb4ui5hyu4yhbui4bui456obyi
References
^ A.Additional images
|
The liver and the veins in connection with it, of a human embryo, twenty-four or twenty-five days old, as seen from the ventral surface. |
Front of abdomen, showing surface markings for liver, stomach, and great intestine |
Topography of thoracic and abdominal viscera. |
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