Thursday, 4 August 2011

STOMACH

In some animals, including vertebrates, echinoderms, insects (mid-gut) and molluscs, the stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract. It is involved in the second phase of digestion, following mastication (chewing). The stomach is located between the oesophagus and the small intestine. It secretes protein-digesting enzymes and strong acids to aid in food digestion, (sent to it via oesophageal peristalsis) through smooth muscular contortions (called segmentation) before sending partially digested food (chyme) to the small intestines.
The word stomach is derived from the Latin stomachus which is derived from the Greek word stomachos, ultimately from stoma (στόμα), "mouth". The words gastro- and gastric (meaning related to the stomach) are both derived from the Greek word gaster (γαστήρ).

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[edit] Role in digestion

Bolus (masticated food) enters the stomach through the oesophagus via the oesophageal sphincter. The stomach releases proteases (protein-digesting enzymes such as pepsin) and hydrochloric acid, which kills or inhibits bacteria and provides the acidic pH of 2 for the proteases to work. Food is churned by the stomach through muscular contractions of the wall - reducing the volume of the fundus, before looping around the fundus[3] and the body of stomach as the boluses are converted into chyme (partially digested food). Chyme slowly passes through the pyloric sphincter and into the duodenum, where the extraction of nutrients begins. Depending on the quantity and contents of the meal, the stomach will digest the food into chyme anywhere between 40 minutes and a few hours.

[edit] Anatomy of the stomach

The stomach lies between the oesophagus and the duodenum (the first part of the small intestine). It is on the left upper part of the abdominal cavity. The top of the stomach lies against the diaphragm. Lying behind the stomach is the pancreas. The greater omentum hangs down from the greater curvature.
Two sphincters keep the contents of the stomach contained. They are the esophageal sphincter (found in the cardiac region, not an anatomical sphincter) dividing the tract above, and the Pyloric sphincter dividing the stomach from the small intestine.
The stomach is surrounded by parasympathetic (stimulant) and orthosympathetic (inhibitor) plexuses (networks of blood vessels and nerves in the anterior gastric, posterior, superior and inferior, celiac and myenteric), which regulate both the secretions activity and the motor (motion) activity of its muscles.
In adult humans, the stomach has a relaxed, near empty volume of about 45 ml. Because it is a distensible organ, it normally expands to hold about 1 litre of food,[4] but can hold as much as 2-3 litres. The stomach of a newborn human baby will only be able to retain about 30ml.

[edit] Sections

The stomach is divided into 4 sections, each of which has different cells and functions. The sections are:
Cardia Where the contents of the oesophagus empty into the stomach.
Fundus Formed by the upper curvature of the organ.
Body or Corpus The main, central region.
Pylorus The lower section of the organ that facilitates emptying the contents into the small intestine.

Sections of the stomach

[edit] Blood supply


Schematic image of the blood supply to the stomach: left and right gastric artery, left and right gastro-omental artery and short gastric artery.[5]

A more realistic image, showing the celiac artery and its branches; the liver has been raised, and the lesser omentum and anterior layer of the greater omentum removed.
The lesser curvature of the stomach is supplied by the right gastric artery inferiorly, and the left gastric artery superiorly, which also supplies the cardiac region. The greater curvature is supplied by the right gastroepiploic artery inferiorly and the left gastroepiploic artery superiorly. The fundus of the stomach, and also the upper portion of the greater curvature, are supplied by the short gastric artery.
Like the other parts of the gastrointestinal tract, the stomach walls are made of the following layers, from inside to outside:
mucosa The first main layer. This consists of the epithelium and the lamina propria (composed of loose connective tissue), with a thin layer of smooth muscle called the muscularis mucosae separating it from the submucosa beneath.
submucosa This layer lies over the mucosa and consists of fibrous connective tissue, separating the mucosa from the next layer. The Meissner's plexus is in this layer.
muscularis externa Over the submucosa, the muscularis externa in the stomach differs from that of other GI organs in that it has three layers of smooth muscle instead of two.
  • inner oblique layer: This layer is responsible for creating the motion that churns and physically breaks down the food. It is the only layer of the three which is not seen in other parts of the digestive system. The antrum has thicker skin cells in its walls and performs more forceful contractions than the fundus.
  • middle circular layer: At this layer, the pylorus is surrounded by a thick circular muscular wall which is normally tonically constricted forming a functional (if not anatomically discrete) pyloric sphincter, which controls the movement of chyme into the duodenum. This layer is concentric to the longitudinal axis of the stomach.
  • outer longitudinal layer: Auerbach's plexus is found between this layer and the middle circular layer.
serosa This layer is over the muscularis externa, consisting of layers of connective tissue continuous with the peritoneum.

Micrograph showing a cross section of the stomach wall, in the body portion of the stomach. H&E stain.

Microscopic cross section of the pyloric part of the stomach wall.

[edit] Glands

The epithelium of the stomach forms deep pits. The glands at these locations are named for the corresponding part of the stomach:
Cardiac glands
(at cardia)		 Pyloric glands
(at pylorus)		 Fundic glands
(at fundus)
Gray1053.png Gray1054.png Gray1055.png
Different types of cells are found at the different layers of these glands:
Layer of stomach Name Secretion Region of stomach Staining
Isthmus of gland Mucous neck cells mucus gel layer Fundic, cardiac, pyloric Clear
Body of gland parietal (oxyntic) cells gastric acid and intrinsic factor Fundic only Acidophilic
Base of gland chief (zymogenic) cells pepsinogen Fundic only Basophilic
Base of gland enteroendocrine (APUD) cells hormones gastrin, histamine, endorphins, serotonin, cholecystokinin and somatostatin Fundic, cardiac, pyloric -

[edit] Control of secretion and motility

The movement and the flow of chemicals into the stomach are controlled by both the autonomic nervous system and by the various digestive system hormones:
Gastrin The hormone gastrin causes an increase in the secretion of HCl from the parietal cells, and pepsinogen from chief cells in the stomach. It also causes increased motility in the stomach. Gastrin is released by G-cells in the stomach in response to distenstion of the antrum, and digestive products(especially large quantities of incompletely digested proteins). It is inhibited by a pH normally less than 4 (high acid), as well as the hormone somatostatin.
Cholecystokinin Cholecystokinin (CCK) has most effect on the gall bladder, causing gall bladder contractions, but it also decreases gastric emptying and increases release of pancreatic juice which is alkaline and neutralizes the chyme.
Secretin In a different and rare manner, secretin, produced in the small intestine, has most effects on the pancreas, but will also diminish acid secretion in the stomach.
Gastric inhibitory peptide Gastric inhibitory peptide (GIP) decreases both gastric acid release and motility.
Enteroglucagon enteroglucagon decreases both gastric acid and motility.
Other than gastrin, these hormones all act to turn off the stomach action. This is in response to food products in the liver and gall bladder, which have not yet been absorbed. The stomach needs only to push food into the small intestine when the intestine is not busy. While the intestine is full and still digesting food, the stomach acts as storage for food.

[edit] EGF in gastric defense

Epidermal growth factor or EGF results in cellular proliferation, differentiation, and survival.[6] EGF is a low-molecular-weight polypeptide first purified from the mouse submandibular gland, but since then found in many human tissues including submandibular gland, parotid gland. Salivary EGF, which seems also regulated by dietary inorganic iodine, plays also an important physiological role in the maintenance of oro-oesophageal and gastric tissue integrity. The biological effects of salivary EGF include healing of oral and gastroesophageal ulcers, inhibition of gastric acid secretion, stimulation of DNA synthesis as well as mucosal protection from intraluminal injurious factors such as gastric acid, bile acids, pepsin, and trypsin and to physical, chemical and bacterial agents.[7]

[edit] Stomach as nutrition sensor

The stomach can "taste" sodium glutamate using glutamate receptors[8] and this information is passed to the lateral hypothalamus and limbic system in the brain as a palatability signal through the vagus nerve.[9] The stomach can also sense independently to tongue and oral taste receptors glucose,[10] carbohydrates[11] proteins,[11] and fats.[12] This allows the brain to link nutritional value of foods to their tastes.[10]

[edit] Diseases of the stomach

Main article: Stomach disease
Historically, it was widely believed that the highly acidic environment of the stomach would keep the stomach immune from infection. However, a large number of studies have indicated that most cases of peptic ulcers, gastritis, and stomach cancer are caused by Helicobacter pylori infection.

[edit] In other animals


An endoscopy of a normal stomach of a healthy 65-year-old woman.
Although the precise shape and size of the stomach varies widely among different vertebrates, the relative positions of the oesophageal and duodenal openings remain relatively constant. As a result, the organ always curves somewhat to the left before curving back to meet the pyloric sphincter. However, lampreys, hagfishes, chimaeras, lungfishes, and some teleost fish have no stomach at all, with the oesophagus opening directly into the intestine. These animals all consume diets that either require little storage of food, or no pre-digestion with gastric juices, or both.[13]
The gastric lining is usually divided into two regions, an anterior portion lined by fundic glands, and a posterior with pyloric glands. Cardiac glands are unique to mammals, and even then are absent in a number of species. The distributions of these glands vary between species, and do not always correspond with the same regions as in man. Furthermore, in many non-human mammals, a portion of the stomach anterior to the cardiac glands is lined with epithelium essentially identical to that of the oesophagus. Ruminants, in particular, have a complex stomach, the first three chambers of which are all lined with oesophageal mucosa.[13]
In birds and crocodilians, the stomach is divided into two regions. Anteriorly is a narrow tubular region, the proventriculus, lined by fundic glands, and connecting the true stomach to the crop. Beyond lies the powerful muscular gizzard, lined by pyloric glands, and, in some species, containing stones that the animal swallows to help grind up food.[13]

Comparison of stomach glandular regions from several mammalian species. Yellow: oesophagus; green: aglandular epithelium; purple: cardiac glands; red: gastric glands; blue: pyloric glands; dark blue: duodenum. Frequency of glands may vary more smoothly between regions than is diagrammed here. Asterisk (ruminant) represents the omasum, which is absent in Tylopoda (Tylopoda also has some cardiac glands opening onto ventral reticulum and rumen[14]) Many other variations exist among the mammals.[15][16]

[edit] See also

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PANCREAS

For other uses, see Pancreas (disambiguation).
Pancreas
Illu pancrease.svg
the pancreas
Illu pancreas duodenum.jpg
1: Head of pancreas
2: Uncinate process of pancreas
3: Pancreatic notch
4: Body of pancreas
5: Anterior surface of pancreas
6: Inferior surface of pancreas
7: Superior margin of pancreas
8: Anterior margin of pancreas
9: Inferior margin of pancreas
10: Omental tuber
11: Tail of pancreas
12: Duodenum
Gray's subject #251 1199
Artery inferior pancreaticoduodenal artery, superior pancreaticoduodenal artery, splenic artery
Vein pancreaticoduodenal veins, pancreatic veins
Nerve pancreatic plexus, celiac ganglia, vagus[1]
Precursor pancreatic buds
MeSH Pancreas
Dorlands/Elsevier Pancreas
The pancreas (English pronunciation: /ˈpæŋkrɪəs/) is a gland organ in the digestive and endocrine system of vertebrates. It is both an endocrine gland producing several important hormones, including insulin, glucagon, and somatostatin, as well as an exocrine gland, secreting pancreatic juice containing digestive enzymes that pass to the small intestine. These enzymes help to further break down the carbohydrates, proteins, and lipids in the chyme.

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[edit] Histology

Under a microscope, stained sections of the pancreas reveal two different types of parenchymal tissue.[2] Lightly staining clusters of cells are called islets of Langerhans, which produce hormones that underlie the endocrine functions of the pancreas. Darker staining cells form acini connected to ducts. Acinar cells belong to the exocrine pancreas and secrete digestive enzymes into the gut via a system of ducts.
Structure Appearance Function
Islets of Langerhans Lightly staining, large, spherical clusters Hormone production and secretion (endocrine pancreas)
Pancreatic acini Darker staining, small, berry-like clusters Digestive enzyme production and secretion (exocrine pancreas)

[edit] Function

See also: endocrine pancreas and exocrine pancreas.
The pancreas is a dual-function gland, having features of both endocrine and exocrine glands.
The part of the pancreas with endocrine function is made up of approximately a million[3] cell clusters called islets of Langerhans. Four main cell types exist in the islets. They are relatively difficult to distinguish using standard staining techniques, but they can be classified by their secretion: α cells secrete glucagon (increase glucose in blood), β cells secrete insulin (decrease glucose in blood), δ cells secrete somatostatin (regulates/stops α and β cells), and PP cells secrete pancreatic polypeptide.[4]
The islets are a compact collection of endocrine cells arranged in clusters and cords and are crisscrossed by a dense network of capillaries. The capillaries of the islets are lined by layers of endocrine cells in direct contact with vessels, and most endocrine cells are in direct contact with blood vessels, by either cytoplasmic processes or by direct apposition. According to the volume The Body, by Alan E. Nourse,[5] the islets are "busily manufacturing their hormone and generally disregarding the pancreatic cells all around them, as though they were located in some completely different part of the body." The islet of Langerhans plays an imperative role in glucose metabolism and regulation of blood glucose concentration.
The pancreas as an exocrine gland helps out the digestive system. It secretes pancreatic juice that contains digestive enzymes that pass to the small intestine. These enzymes help to further break down the carbohydrates, proteins, and lipids (fats) in the chyme.
In humans, the secretory activity of the pancreas is regulated directly via the effect of hormones in the blood on the islets of Langerhans and indirectly through the effect of the autonomic nervous system on the blood flow.[6]
Sympathetic (adrenergic)
α2: decreases secretion from beta cells, increases secretion from alpha cells, β2: increases secretion from beta cells
Parasympathetic (muscarinic)
M3: increases stimulation of alpha cells and beta cells[7]

[edit] Anatomy

Surface projections of the organs of the trunk, showing pancreas at the transpyloric plane
1. Bile ducts: 2. Intrahepatic bile ducts, 3. Left and right hepatic ducts, 4. Common hepatic duct, 5. Cystic duct, 6. Common bile duct, 7. Ampulla of Vater, 8. Major duodenal papilla
9. Gallbladder, 10-11. Right and left lobes of liver. 12. Spleen.
13. Esophagus. 14. Stomach. Small intestine: 15. Duodenum, 16. Jejunum
17. Pancreas: 18: Accessory pancreatic duct, 19: Pancreatic duct.
20-21: Right and left kidneys (silhouette).
The anterior border of the liver is lifted upwards (brown arrow). Gallbladder with Longitudinal section, pancreas and duodenum with frontal one. Intrahepatic ducts and stomach in transparency.

The pancreas lies in the epigastrium and left hypochondrium areas of the abdomen
It is composed of the following parts:
  • The head lies within the concavity of the duodenum.
  • The uncinate process emerges from the lower part of head, and lies deep to superior mesenteric vessels.
  • The neck is the constricted part between the head and the body.
  • The body lies behind the stomach.
  • The tail is the left end of the pancreas. It lies in contact with the spleen and runs in the lienorenal ligament.
The superior pancreaticoduodenal artery from gastroduodenal artery and the inferior pancreaticoduodenal artery from superior mesenteric artery run in the groove between the pancreas and duodenum and supply the head of pancreas. The pancreatic branches of splenic artery also supply the neck, body and tail of the pancreas. The largest of those branches is called the arteria pancreatica magna; its occlusion, although rare, is fatal.
The body and neck of the pancreas drain into splenic vein; the head drains into the superior mesenteric and portal veins.
Lymph is drained via the splenic, celiac and superior mesenteric lymph nodes.

[edit] Diseases

Main article: Pancreatic disease
Because the pancreas is a storage depot for digestive enzymes, injury to the pancreas is potentially very dangerous. A puncture of the pancreas generally requires prompt and experienced medical intervention.
Pancreatic cancers, particularly cancer of the exocrine pancreas, remain one of the most deadly cancers, and the mortality rate is very high.
Diabetes mellitus type 1 is a chronic autoimmune disorder in which the immune system attacks the insulin-secreting cells in the pancreas.

[edit] History

The pancreas was first identified for western civilization by Herophilus (335–280 BC), a Greek anatomist and surgeon. Only a few hundred years later, Rufus of Ephesus, another Greek anatomist, gave the pancreas its name. The term "pancreas" is derived from the Greek πᾶν ("all", "whole"), and κρέας ("flesh").[8] – presumably because of its fleshy consistency.

[edit] Embryological development

Schematic illustrating the development of the pancreas from a dorsal and a ventral bud. During maturation the ventral bud flips to the other side of the gut tube (arrow) where it typically fuses with the dorsal lobe. An additional ventral lobe which usually regress during development is omitted.
The pancreas forms from the embryonic foregut and is therefore of endodermal origin. Pancreatic development begins [with] the formation of a ventral and dorsal anlage (or buds). Each structure communicates with the foregut through a duct. The ventral pancreatic bud becomes the head and uncinate process, and comes from the hepatic diverticulum.
Differential rotation and fusion of the ventral and dorsal pancreatic buds results in the formation of the definitive pancreas.[9] As the duodenum rotates to the right, it carries with it the ventral pancreatic bud and common bile duct. Upon reaching its final destination, the ventral pancreatic bud fuses with the much larger dorsal pancreatic bud. At this point of fusion, the main ducts of the ventral and dorsal pancreatic buds fuse, forming the duct of Wirsung, the main pancreatic duct.
Differentiation of cells of the pancreas proceeds through two different pathways, corresponding to the dual endocrine and exocrine functions of the pancreas. In progenitor cells of the exocrine pancreas, important molecules that induce differentiation include follistatin, fibroblast growth factors, and activation of the Notch receptor system.[9] Development of the exocrine acini progresses through three successive stages. These include the predifferentiated, protodifferentiated, and differentiated stages, which correspond to undetectable, low, and high levels of digestive enzyme activity, respectively.
Progenitor cells of the endocrine pancreas arise from cells of the protodifferentiated stage of the exocrine pancreas.[9] Under the influence of neurogenin-3 and Isl-1, but in the absence of notch receptor signaling, these cells differentiate to form two lines of committed endocrine precursor cells. The first line, under the direction of Pax-0, forms α- and γ- cells, which produce glucagon and pancreatic polypeptides, respectively. The second line, influenced by Pax-6, produces β- and δ-cells, which secrete insulin and somatostatin, respectively.
Insulin and glucagon can be detected in the human fetal circulation by the fourth or fifth month of fetal development.[9]

[edit] In animals

Pancreatic tissue is present in all vertebrate species, but its precise form and arrangement varies widely. There may be up to three separate pancreases, two of which arise from ventral buds, and the other dorsally. In most species (including humans), these fuse in the adult, but there are several exceptions. Even when a single pancreas is present, two or three pancreatic ducts may persist, each draining separately into the duodenum (or equivalent part of the foregut). Birds, for example, typically have three such ducts.[10]
In teleosts, and a few other species (such as rabbits), there is no discrete pancreas at all, with pancreatic tissue being distributed diffusely across the mesentery and even within other nearby organs, such as the liver or spleen. In a few teleost species, the endocrine tissue has fused to form a distinct gland within the abdominal cavity, but otherwise it is distributed amongst the exocrine components. The most primitive arrangement, however, appears to be that of lampreys and lungfish, in which pancreatic tissue is found as a number of discrete nodules within the wall of the gut itself, with the exocrine portions being little different from other glandular structures of the intestine.[10]

[edit] Gallery

  • Accessory digestive system
  • Digestive organs
  • The celiac artery and its branches; the stomach has been raised and the peritoneum removed.
  • Lymphatics of stomach, etc., the stomach has been turned upward.
  • Transverse section through the middle of the first lumbar vertebra, showing the relations of the pancreas
  • The duodenum and pancreas
  • The pancreatic duct
  • Pancreas of a human embryo of five weeks
  • Pancreas of a human embryo at end of sixth week
  • Front of abdomen, showing surface markings for duodenum, pancreas, and kidneys
  • Dudenumandpancreas.jpg
  • Dog pancreas magnified 100 times
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LIVER

The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion. The liver is necessary for survival; there is currently no way to compensate for the absence of liver function long term, although liver dialysis can be used short term.
This organ plays a major role in metabolism and has a number of functions in the body, including glycogen storage, decomposition of red blood cells, plasma protein synthesis, hormone production, and detoxification. It lies below the diaphragm in the abdominal-pelvic region of the abdomen. It produces bile, an alkaline compound which aids in digestion via the emulsification of lipids. The liver's highly specialized tissues regulate a wide variety of high-volume biochemical reactions, including the synthesis and breakdown of small and complex molecules, many of which are necessary for normal vital functions.[2]
Medical terms related to the liver often start in hepato- or hepatic from the Greek word for liver, hēpar (ἡπαρ).

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[edit] Anatomy

The liver is a reddish brown organ with four lobes of unequal size and shape. A human liver normally weighs 1.44–1.66 kg (3.2–3.7 lb),[3] and is a soft, pinkish-brown, triangular organ. It is both the largest internal organ (the skin being the largest organ overall) and the largest gland in the human body. It is located in the right upper quadrant of the abdominal cavity, resting just below the diaphragm. The liver lies to the right of the stomach and overlies the gallbladder. It is connected to two large blood vessels, one called the hepatic artery and one called the portal vein. The hepatic artery carries blood from the aorta, whereas the portal vein carries blood containing digested nutrients from the entire gastrointestinal tract and also from the spleen and pancreas. These blood vessels subdivide into capillaries, which then lead to a lobule. Each lobule is made up of millions of hepatic cells which are the basic metabolic cells.

[edit] Blood flow

The liver receives a dual blood supply from the hepatic portal vein and hepatic arteries. Supplying approximately 75% of the liver's blood supply, the hepatic portal vein carries venous blood drained from the spleen, gastrointestinal tract, and its associated organs. The hepatic arteries supply arterial blood to the liver, accounting for the remainder of its blood flow. Oxygen is provided from both sources; approximately half of the liver's oxygen demand is met by the hepatic portal vein, and half is met by the hepatic arteries.[4]
Blood flows through the sinusoids and empties into the central vein of each lobule. The central veins coalesce into hepatic veins, which leave the liver.

[edit] Biliary flow

The biliary tree
The term biliary tree is derived from the arboreal branches of the bile ducts. The bile produced in the liver is collected in bile canaliculi, which merge to form bile ducts. Within the liver, these ducts are called intrahepatic (within the liver) bile ducts, and once they exit the liver they are considered extrahepatic (outside the liver). The intrahepatic ducts eventually drain into the right and left hepatic ducts, which 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 second part of the duodenum together at the ampulla of Vater.

[edit] Surface anatomy

[edit] Peritoneal ligaments

Apart from a patch where it connects to the diaphragm (the so-called "bare area"), 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 "lits" 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. An exception to this is the falciform ligament, which attaches the liver to the posterior portion of the anterior body wall.

[edit] Lobes

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 the quadrate lobe (the more inferior).
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.
Each of the lobes is made up of lobules; a vein goes from the centre, which then joins to the hepatic vein to carry blood out from the liver.
On the surface of the lobules, there are ducts, veins and arteries that carry fluids to and from them.

[edit] Functional anatomy

Correspondence between anatomic lobes and Couinaud segments
Segment* Couinaud segments
Caudate 1
Lateral 2, 3
Medial 4a, 4b
Right 5, 6, 7, 8
* or lobe, in the case of the caudate lobe
Each number in the list corresponds to one in the table. 1. Caudate
2. Superior subsegment of the lateral segment
3. Inferior subsegment of the lateral segment
4a. Superior subsegment of the medial segment
4b. Inferior subsegment of the medial segment
5. Inferior subsegment of the anterior segment
6. Inferior subsegment of the posterior segment
7. Superior subsegment of the posterior segment
8. Superior subsegment of the anterior segment
The central area where the common bile duct, hepatic portal vein, and hepatic artery proper enter 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 an imaginary plane joining the gallbladder fossa to the inferior vena cava. The plane separates the liver into the true right and left lobes. The middle hepatic vein also demarcates the true right and left lobes. The right lobe is further divided into an anterior and posterior segment by the right hepatic vein. The left lobe is divided into the medial and lateral segments by the left hepatic vein. The fissure for the ligamentum teres also separates the medial and lateral segments. The medial segment is also called the quadrate lobe. In the widely used Couinaud (or "French") system, the functional lobes are further divided into a total of eight subsegments based on a transverse plane through the bifurcation of the main portal vein. The caudate lobe is a separate structure which receives blood flow from both the right- and left-sided vascular branches.[5][6]

[edit] In other animals

The liver is found in all vertebrates, and is typically the largest visceral organ. Its form varies considerably in different species, and is largely determined by the shape and arrangement of the surrounding organs. Nonetheless, in most species it is divided into right and left lobes; exceptions to this general rule include snakes, where the shape of the body necessitates a simple cigar-like form. The internal structure of the liver is broadly similar in all vertebrates.[7]
An organ sometimes referred to as a liver is found associated with the digestive tract of the primitive chordate Amphioxus. However, this is an enzyme secreting gland, not a metabolic organ, and it is unclear how truly homologous it is to the vertebrate liver.[7]

[edit] Physiology

The various functions of the liver are carried out by the liver cells or hepatocytes. 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. The liver is thought to be responsible for up to 500 separate functions, usually in combination with other systems and organs.

[edit] Synthesis

Further information: Proteins produced and secreted by the liver

[edit] Breakdown

[edit] Other functions

[edit] Diseases of the liver

Main article: Liver disease
Left lobe liver tumor
The liver supports almost every organ in the body and is vital for survival. Because of its strategic location and multidimensional functions, the liver is also prone to many diseases.[8]
The most common include: Infections such as hepatitis A, B, C, E, alcohol damage, fatty liver, cirrhosis, cancer, drug damage (especially acetaminophen (also known as paracetamol) and cancer drugs)
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.
There are also many pediatric liver diseases including biliary atresia, alpha-1 antitrypsin deficiency, alagille syndrome, progressive familial intrahepatic cholestasis, and Langerhans cell histiocytosis, to name but a few.
Diseases that interfere with liver function will lead to derangement of these processes. However, the liver has a great capacity to regenerate and has a large reserve capacity. In most cases, the liver only produces symptoms after extensive damage.
Liver diseases may be diagnosed by liver function tests, for example, by production of acute phase proteins.

[edit] Disease symptoms

The classic symptoms of liver damage include the following:
  • Pale stools occur when stercobilin, a brown pigment, is absent from the stool. Stercobilin is derived from bilirubin metabolites produced in the liver.
  • Dark urine occurs when bilirubin mixes with urine
  • Jaundice (yellow skin and/or whites of the eyes) This is where bilirubin deposits in skin, causing an intense itch. Itching is the most common complaint by people who have liver failure. Often this itch cannot be relieved by drugs.
  • Swelling of the abdomen, ankles and feet occurs because the liver fails to make albumin.
  • Excessive fatigue occurs from a generalized loss of nutrients, minerals and vitamins.
  • Bruising and easy bleeding are other features of liver disease. The liver makes substances which help prevent bleeding. When liver damage occurs, these substances are no longer present and severe bleeding can occur.[9]

[edit] Diagnosis

The diagnosis of liver function is made by blood tests. Liver function tests can readily pinpoint the extent of liver damage. If infection is suspected, then other serological tests are done. Sometimes, one may require an ultrasound or a CT scan to produce an image of the liver.
Physical examination of the liver is not accurate in determining the extent of liver damage. It can only reveal presence of tenderness or the size of liver, but in all cases, some type of radiological study is required to examine it.[10]

[edit] Biopsy

The ideal way to determine damage to the liver is with a biopsy. A biopsy is not required in all cases, but may be necessary when the cause is unknown. A needle is inserted into the skin just below the rib cage and a biopsy is obtained. The tissue is sent to the laboratory, where it is analyzed under a microscope. Sometimes, a radiologist may assist the physician performing a liver biopsy by providing ultrasound guidance.[11]

[edit] Regeneration

The liver is the only internal human organ capable of natural regeneration of lost tissue; as little as 25% of a liver can regenerate into a whole liver. This is, however, not true regeneration but rather compensatory growth.[12] The lobes that are removed do not regrow and the growth of the liver is a restoration of function and not original form. This contrasts with true regeneration where both original function and form are restored.
This is predominantly due to the hepatocytes re-entering the cell cycle. That is, the hepatocytes go from the quiescent G0 phase to the G1 phase and undergo mitosis. This process is activated by the p75 receptors.[13] There is also some evidence of bipotential stem cells, called ovalocytes or hepatic oval cells, which are thought to reside in the canals of Hering. These cells can differentiate into either hepatocytes or cholangiocytes, the latter being the cells that line the bile ducts.
Scientific and medical works about liver regeneration often refer to the Greek Titan Prometheus who was chained to a rock in the Caucasus where, each day, his liver was devoured by an eagle, only to grow back each night. Some think the myth indicates the ancient Greeks knew about the liver’s remarkable capacity for self-repair, though this claim has been challenged.[14]

[edit] Liver transplantation

Main article: Liver transplantation
Human liver transplants were first performed by Thomas Starzl in the United States and Roy Calne in Cambridge, England in 1963 and 1965, respectively.
After resection of left lobe liver tumor
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 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 percent 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 percent 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. A recent publication has addressed the problem of donor mortality, and at least 14 cases have been found.[15] The risk of postoperative complications (and death) is far greater in right-sided operations than that in left-sided operations.
With the recent advances of noninvasive imaging, living liver donors usually have to undergo imaging examinations for liver anatomy to decide if the anatomy is feasible for donation. The evaluation is usually performed by multidetector row computed tomography (MDCT) and magnetic resonance imaging (MRI). MDCT is good in vascular anatomy and volumetry. MRI is used for biliary tree anatomy. Donors with very unusual vascular anatomy, which makes them unsuitable for donation, could be screened out to avoid unnecessary operations.
  • MDCT image. Arterial anatomy contraindicated for liver donation
  • MDCT image. Portal venous anatomy contraindicated for liver donation
  • MDCT image. 3D image created by MDCT can clearly visualize the liver, measure the liver volume, and plan the dissection plane to facilitate the liver transplantation procedure.
  • Phase contrast CT image. Contrast is perfusing the right liver but not the left due to a left portal vein thrombus.

[edit] Development

[edit] 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 then to the inferior vena cava, allowing placental blood to bypass the liver.
In the fetus, the liver develops throughout 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 receives 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 in two to five days; 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.

[edit] As food

Main article: Liver (food)

[edit] 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) would peck out his liver, which would regenerate overnight. (The liver is the only human internal organ that actually can regenerate itself to a significant extent.) Many ancient peoples of the Near East and Mediterranean areas practiced a type of divination called haruspicy, where they tried to obtain information by examining the livers of sheep and other animals.
In Plato, and in later physiology, the liver was thought to be the seat of the darkest emotions (specifically wrath, jealousy and greed) which drive men to action.[16] The Talmud (tractate Berakhot 61b) refers to the liver as the seat of anger, with the gallbladder counteracting this.
The Persian, Urdu, and Hindi languages (جگر or जिगर or jigar) refer to the liver in figurative speech to indicate courage and strong feelings, or "their best"; e.g., "This Mecca has thrown to you the pieces of its liver!".[17] The term jan e jigar, literally "the strength (power) of my liver", is a term of endearment in Urdu. In Persian slang, jigar is used as an adjective for any object which is desirable, especially women. In the Zulu language, the word for liver (isibindi) is the same as the word for courage.
The legend of Liver-Eating Johnson says that he would cut out and eat the liver of each man killed after dinner.
In the motion picture The Message, Hind bint Utbah is implied or portrayed eating the liver of Hamza ibn ‘Abd al-Muttalib during the Battle of Uhud. Although there are narrations that suggest that Hind did "taste", rather than eat, the liver of Hamza, the authenticity of these narrations has to be questioned.

[edit] See also

Look up liver in Wiktionary, the free dictionary.
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