Liver
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
glycogen storage,
plasma protein synthesis, and drug detoxification. It also produces
bile, which is important in
digestion. It performs and regulates a wide variety of high-volume biochemical reactions requiring specialized
tissues. Medical terms related to the liver often start in
hepato- or
hepatic from the
Greek word for liver,
hepar.
The adult human liver normally weighs between 1.3 - 3.0
kilograms, and is a soft, pinkish-brown "
boomerang shaped" organ. It is the second
largest organ (the largest organ being the
skin) and the largest gland within the human body. Its anatomical position in the body is immediately under the
diaphragm on the right side of the upper
abdomen. The liver lies on the right of the stomach and makes a kind of bed for the
gallbladder (which stores
bile).
The liver is supplied by two main blood vessels on its right lobe: the
hepatic artery and the
portal vein. The
hepatic artery normally comes off the
celiac trunk. The
portal vein brings venous blood from the
spleen,
pancreas, and
small intestines, so that the liver can process the
nutrients and byproducts of food digestion. The
hepatic veins drain directly into the
inferior vena cava.
The
bile produced in the liver is collected in
bile canaliculi, which merge to form bile ducts. These eventually drain into the right and left hepatic ducts, which in turn merge to form the common hepatic duct. The cystic duct (from the
gallbladder) joins with the
common hepatic duct to form the
common bile duct. Bile can either drain directly into the
duodenum via the
common bile duct or be temporarily stored in the
gallbladder via the
cystic duct. The
common bile duct and the
pancreatic duct enter the
duodenum together at the
ampulla of Vater. The branchings of the
bile ducts resemble those of a tree, and indeed the term "
biliary tree" is commonly used in this setting.
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. This is predominantly due to the
hepatocytes acting as unipotential
stem cells (i.e. a single
hepatocyte can divide into two
hepatocyte daughter cells). There is also some evidence of
bipotential stem cells, called
oval cells, which can differentiate into either
hepatocytes or
cholangiocytes (cells that line the
bile ducts).
Surface anatomy
|
The liver is the largest internal organ of the human body. |
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
peritoneum folds back on itself to form the
falciform ligament and the right and left
triangular ligaments. These "
ligaments" are in no way related to the true
anatomic ligaments in
joints, and have essentially no functional importance, but they are easily recognizable surface landmarks. Traditional gross anatomy divided the liver into four
lobes based on surface features.
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 anatomy
For purposes such as advanced liver surgery, it is crucial to understand the organization of liver based on blood supply and biliary drainage. 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 functional lobes are separated by a plane joining the gallbladder fossa to the inferior vena cava. In the widely used Couinaud or "French" system, the functional lobes are further divided into a total of eight segments based on secondary and tertiary branching of the blood supply. 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 |
The various functions of the liver are carried out by the liver cells or
hepatocytes.
* The liver produces and excretes
bile required for dissolving fats. Some of the bile drains directly into the
duodenum, and some is stored in the
gallbladder.
* 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)
**
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 also performs several roles in
lipid metabolism:
**
Cholesterol synthesis** The production of
triglycerides (fats).
* The liver produces
coagulation factors I (fibrinogen),
II (prothrombin),
V,
VII,
IX,
X and
XI, as well as
protein C,
protein S and
antithrombin.
* The liver breaks down
hemoglobin, creating
metabolites that are added to
bile as pigment.
* The liver breaks down
toxic substances and most medicinal products in a process called
drug metabolism. This sometimes results in
toxication, when the metabolite is more toxic than its precursor.
* The liver converts
ammonia to
urea.
* The liver stores a multitude of substances, including glucose in the form of glycogen,
vitamin B12,
iron, and
copper.
* In the first trimester
fetus, the liver is the main site of
red blood cell production. By the 32nd week of gestation, the
bone marrow has almost completely taken over that task.
* 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.Currently, there is no artificial organ or device capable of emulating all the functions of the liver. Some functions can be emulated by
liver dialysis, an experimental treatment for
liver failure.
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.
*
Hepatitis, inflammation of the liver, caused mainly by various
viruses but also by some poisons, autoimmunity or hereditary conditions.
*
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 viral hepatitis,
alcoholism or contact with other liver-toxic chemicals.
*
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 cancers, usually from other parts of the
gastrointestinal tract).
*
Wilson's disease, a
hereditary disease which causes the body to retain
copper.
*
Primary sclerosing cholangitis, an
inflammatory disease of the
bile duct, autoimmune in nature.
*
Primary biliary cirrhosis, autoimmune disease of small bile ducts
*
Budd-Chiari syndrome, obstruction of the hepatic vein.
*
Gilbert's syndrome, a genetic disorder of bilirubin metabolism, found in about 5% of the population.
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. These test for the presence of enzymes in blood that are normally most abundant in liver tissue, metabolites or products.
Human liver transplant was first performed by Tom 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. Most transplants are done for chronic liver diseases leading to
cirrhosis, such as chronic hepatitis C, alcoholism, autoimmune hepatitis, and many others. Less commonly, liver transplantation is done for
fulminant hepatic failure, in which liver failure occurs over days to weeks. Liver
allografts for
transplant usually come from non-living donors who have died from fatal brain injury.
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. This was first performed in 1989 for pediatric liver transplantation. 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. This procedure is more controversial as it entails performing a much larger operation on the donor, and indeed there have been at least two donor deaths out of the first several hundred cases.
The liver develops as an
endodermal outpocketing of the called the
hepatic diverticulum. Its initial blood supply is primarily from the
vitelline veins that drain blood from the
yolk sac. The superior part of the hepatic diverticulum gives rise to the hepatocytes and bile ducts, while the inferior part becomes the
gallbladder and its associated cystic duct.
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. There it joins with the left branch of the portal vein. 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 throught normal gestation, and does not perform the normal filtration of the infant liver. The liver does not perform digestive processes because the fetus does not consume meals directly, but reveives nourishment from the mother via the
placenta. 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. Once the fetus is delivered, the formation of blood stem cells in infants shifts to the red
bone marrow.
After birth, the umbilical vein and ductus venosus are completely obliterated two to five days postpartum; the former becomes the
ligamentum teres and the latter becomes the
ligamentum venosum. In the disease state of
cirrhosis and
portal hypertension, the umbilical vein can open up again.
Mammal and bird livers are commonly eaten as food: products include
liver paté,
Leberwurst,
Braunschweiger,
foie gras,
chopped liver and liver
sashimi.
Both animal and fish livers are rich in iron,
Vitamin A,
cod liver oil being commonly used as a supplement. Very high doses of Vitamin A can be
toxic, particularly in polar animals;
Antarctic explorers
Douglas Mawson and
Xavier Mertz were both poisoned, the latter fatally, from eating
husky liver. In the US, the USDA specifies 3000 μg per day as a tolerable upper limit, which amounts to about 50 kg of raw pork liver or 3 kg of polar-bear liver.
[A. Aggrawal, Death by Vitamin A], 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.
[Myhre et al., "Water-miscible, emulsified, and solid forms of retinol supplements are more toxic than oil-based preparations", Am. J. Clinical Nutrition'', 78, 1152 (2003)]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. Curiously, the liver is the only human internal organ that actually can regenerate itself to a significant extent, a characteristic which may have already been known to the Greeks.
The
Talmud (tractate Berakhot 61b) refers to the liver as the seat of
anger, with the
gallbladder counteracting this.
In both Arabic and Persian language, there exists figurative speeches regarding the liver, where the liver is used as a replacment for courage and strong feelings, or "their best". ie: "This
Mecca has thrown to you the pieces of its liver!"
[http://www.shawuniversitymosque.org/m/faq_qanda.php?id=94]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
[http://www.studentbmj.com/issues/02/05/life/158.php]:The following are standard medical textbooks:
* Eugene R. Schiff, Michael F. Sorrell, Willis C. Maddrey, eds.
Schiff's diseases of the liver, 9th ed. Philadelphia : Lippincott, Williams & Wilkins, 2003. ISBN 0781730074
* Sheila Sherlock, James Dooley.
Diseases of the liver and biliary system, 11th ed. Oxford, UK ; Malden, MA : Blackwell Science. 2002. ISBN 0632055820
* David Zakim, Thomas D. Boyer. eds.
Hepatology: a textbook of liver disease, 4th ed. Philadelphia: Saunders. 2003. ISBN 0721690513:These are for the lay reader or patient:
*Sanjiv Chopra.
The Liver Book: A Comprehensive Guide to Diagnosis, Treatment, and Recovery, Atria, 2002, ISBN 0743405854
*Melissa Palmer.
Dr. Melissa Palmer's Guide to Hepatitis and Liver Disease: What You Need to Know, Avery Publishing Group; Revised edition May 24, 2004, ISBN 1583331883.
her webpage.
*Howard J. Worman.
The Liver Disorders Sourcebook, McGraw-Hill, 1999, ISBN 0737300906.
his Columbia University web site, "Diseases of the liver" *
Liver function tests*
LIVER FAMILIES: An online support group for families whose lives have been touched by pediatric liver disease and transplant. *
American Association for the Study of Liver Diseases AASLD
*
American Liver Society ALS
*
WikiLiver: A Wiki dedicated to the liver*
The Liver and its Diseases at h2g2*
The position of the liver
