In human beings, the duodenum is a hollow jointed tube about 25–38 centimetres (10–15 inches) long connecting thestomach to themiddle part of the small intestine.[4][5] It begins with theduodenal bulb and ends at thesuspensory muscle of duodenum.[6] The duodenum can be divided into four parts: the first (superior), the second (descending), the third (transverse) and the fourth (ascending) parts.[5]
The duodenum is the first section of thesmall intestine in mosthigher vertebrates, includingmammals,reptiles, andbirds. Infish, the divisions of the small intestine are not as clear, and the termsanterior intestine orproximal intestine may be used instead of duodenum.[7] In mammals the duodenum may be the principal site foriron absorption.[8]
In humans, the duodenum is a C-shaped hollow jointed tube, 25–38 centimetres (10–15 inches) in length, lying adjacent to thestomach (and connecting it to the small intestine). It is divided anatomically into four sections. The first part lies within theperitoneum but its other parts areretroperitoneal.[9]: 273
Thefirst orsuperior part of the duodenum is a continuation from thepylorus to the transpyloric plane. It is superior (above) to the rest of the segments, at thevertebral level ofL1. Theduodenal bulb, about 2 cm (3⁄4 in) long, is the first part of the duodenum and is slightly dilated. The duodenal bulb is a remnant of the mesoduodenum, amesentery that suspends the organ from the posterior abdominal wall in fetal life.[10] The first part of the duodenum is mobile, and connected to the liver by thehepatoduodenal ligament of thelesser omentum. The first part of the duodenum ends at the corner, thesuperior duodenal flexure.[9]: 273
Thesecond ordescending part of the duodenum begins at the superior duodenal flexure. It goesinferior to the lower border of vertebral body L3, before making a sharp turnmedially into theinferior duodenal flexure, the end of the descending part.[9]: 274
Thefourth orascending part of the duodenum passes upward, joining with thejejunum at theduodenojejunal flexure. The fourth part of the duodenum is at the vertebral level L3, and may pass directly on top, or slightly to the left, of theaorta.[9]: 274
The first (superior) part of the duodenum, right after thepylorus of the stomach, is not supplied by the arcades. Instead, it is supplied by thesupraduodenal artery and posterior superior pancreaticoduodenal artery, along with some branches of theright gastroepiploic artery and the anterior superior pancreaticoduodenal artery. In many people part of the first centimeter of the duodenum is also supplied by branches of theright gastric artery.[11]
The remaining three parts (descending, horizontal, and ascending) of the duodenum are supplied by two arcades (rings) of arteries, one anterior (in front) of the duodenum and pancreas and one posterior to (behind) them. Each arcade is made of twoanastomosed (connected) arteries. The superior artery of each arcade comes from thesuperior pancreaticoduodenal artery, which arises from theceliac artery via thegastroduodenal artery. The inferior artery of each arcade comes from theinferior pancreaticoduodenal artery, a branch of thesuperior mesenteric artery. The anterior arcade is formed by the anterior superior pancreaticoduodenal artery and the anterior inferior pancreaticoduodenal artery; the posterior arcade is formed by the posterior superior pancreaticoduodenal artery and the posterior inferior pancreaticoduodenal artery.[11][12]
Vessels from the arcades supply themuscularis externa (muscular layer) before forming aplexus (network of blood vessels) in thesubmucosa (a layer of connective tissue) called thesubmucosal plexus. Vessels continue from the submucosal plexus through themuscularis mucosae (another thin muscular layer) before forming another plexus under theepithelium of thevilli, the layer where nutrients are absorbed. These vessels entering the duodenum from the arcades are sometimes calledvasae rectae or arteriae rectae.[11]
The venous drainage of the duodenum mainly follows the arteries, ultimately draining into theportal system. The venous arcades are usually superficial to the arterial arcades. The anterior superior pancreaticoduodenal vein drains into theright gastroepiploic vein, as do the veins of the lower first part of the duodenum and the pylorus (subpyloric veins). The upper first part of the duodenum is drained by suprapyloric veins, which can drain into theportal vein or the posterior superior pancreaticoduodenal vein, which drains into the portal vein. The inferior veins of the arcades drain into thesuperior mesenteric,inferior mesenteric,splenic, or firstjejunal vein.[11]
Embryologically, the duodenum arises from both theforegut andmidgut, constituting the boundary between the two. However, the "midgut" is defined surgically as the parts of the intestine supplied by the superior mesenteric artery. Since the duodenum is supplied both by theceliac artery and the superior mesenteric artery, these two definitions are similar but not exactly the same.[11]
Thelymphatic vessels follow the arteries in a retrograde fashion. The anterior lymphatic vessels drain into the pancreatoduodenallymph nodes located along the superior and inferior pancreatoduodenal arteries and then into the pyloric lymph nodes (along the gastroduodenal artery). The posterior lymphatic vessels pass posterior to the head of the pancreas and drain into the superior mesenteric lymph nodes. Efferent lymphatic vessels from the duodenal lymph nodes ultimately pass into the celiac lymph nodes.
The duodenum's close anatomical association with the pancreas creates differences in function based on the position and orientation of the organs. The congenital abnormality, annular pancreas, causes a portion of the pancreas to encircle the duodenum. In an extramural annular pancreas, the pancreatic duct encircles the duodenum which results in gastrointestinal obstruction. An intramural annular pancreas is characterized by pancreatic tissue that is fused with the duodenal wall, causing duodenal ulceration.[14]
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About 20,000 protein coding genes are expressed in human cells and 70% of these genes are expressed in the normal duodenum.[15][16] Some 300 of these genes are more specifically expressed in the duodenum with very few genes expressed only in the duodenum. The corresponding specific proteins are expressed in the duodenal mucosa, and many of these are also expressed in the small intestine, such asalanine aminopeptidase, a digestive enzyme,angiotensin-converting enzyme, involved in controllingblood pressure, andRBP2, a protein involved in the uptake ofvitamin A.[17]
The duodenum is largely responsible for the breakdown of food in the small intestine, usingenzymes. The duodenum also regulates the rate of emptying of the stomach via hormonal pathways.Secretin andcholecystokinin are released from cells in the duodenalepithelium in response to acidic and fatty stimuli present there when thepylorus opens and emits gastricchyme into the duodenum for further digestion. These cause theliver andgallbladder to releasebile, and thepancreas to release bicarbonate and digestive enzymes such astrypsin,lipase andamylase into the duodenum as they are needed.[18]
The duodenum is a critical contributor to the regulation of food intake[19] and glycemic control.[20] As the first part of the small intestine, the duodenum is the initial site of nutrient absorption in the gastrointestinal tract. The duodenum senses nutrient intake and composition, and signals to the liver, pancreas, adipose tissue and brain[21] through the direct and indirect[22] release of several key hormones and signaling molecules, including theincretin peptidesGlucose-dependent insulinotropic polypeptide (GIP) andGlucagon-like peptide-1 (GLP-1),[22] as well asCholecystokinin (CCK) andSecretin. The duodenum also signals to the brain directly via vagal afferents enabling neural control over food intake and glycemia.[23] Intestinal secretion of GIP and GLP-1 stimulates glucose-dependent insulin secretion from pancreatic beta-cells, known as the incretin effect.[24] Incretin peptides, principally GLP-1 and GIP, regulate islet hormone secretion, glucose concentrations, lipid metabolism, gut motility, appetite and body weight, and immune function.[25]
The villi of the duodenum have a leafy-looking appearance, which is a histologically identifiable structure.Brunner's glands, which secretemucus, are only found in the duodenum. The duodenum wall consists of a very thin layer of cells that form themuscularis mucosae.
Ulcers of the duodenum commonly occur because of infection by the bacteriaHelicobacter pylori. These bacteria, through a number of mechanisms, erode the protective mucosa of the duodenum, predisposing it to damage from gastric acids. The first part of the duodenum is the most common location of ulcers since it is where the acidic chyme meets the duodenal mucosa before mixing with the alkaline secretions of the duodenum.[26] Duodenal ulcers may cause recurrent abdominal pain anddyspepsia, and are often investigated using aurea breath test to test for the bacteria, andendoscopy to confirm ulceration and take abiopsy. If managed, these are often managed throughantibiotics that aim to eradicate the bacteria, andproton-pump inhibitors andantacids to reduce the gastric acidity.[27]
TheBritish Society of Gastroenterology guidelines specify that a duodenal biopsy is required for the diagnosis of adultceliac disease. The biopsy is ideally performed at a moment when the patient is on a gluten-containing diet.[28]
Duodenal cancer is a cancer in the first section of the small intestine. Cancer of the duodenum is relatively rare compared tostomach cancer andcolorectal cancer; malignant tumors in the duodenum constitute only around 0.3% of all the gastrointestinal tract tumors but around half of cancerous tissues that develop in the small intestine.[29] Its histology is often observed to beadenocarcinoma, meaning that the cancerous tissue arises from glandular cells in theepithelial tissue lining the duodenum.[30]
Inflammation of the duodenum is referred to asduodenitis. There are multiple known causes.[45] Celiac disease and inflammatory bowel disease are two of the known causes.[46]
The nameduodenum isMedieval Latin, short forintestīnum duodēnum digitōrum, meaning "intestine of twelve finger-widths (in length)", genitive[citation needed] ofduodēnī, "twelve each", (related toduodecim "twelve"). Coined by Gerard of Cremona (d. 1187) in his Latin translation of "Canon Avicennae", "اثنا عشر" itself a loan-translation of Greek dodekadaktylon (δωδεκάδάκτυλον), literally "twelve fingers long". The intestine part was so called by the Greek physicianHerophilus (c. 335–280 BCE) for its length, about equal to the breadth of 12 fingers.[47]
Many languages usecalques for this word. For example,GermanZwölffingerdarm,DutchTwaalfvingerige darm andTurkishOniki parmak bağırsağı.
^"NCI Dictionary of Cancer Terms".National Cancer Institute. Retrieved2022-06-07.The first part of the small intestine. It connects to the stomach. The duodenum helps to further digest food coming from the stomach. It absorbs nutrients (vitamins, minerals, carbohydrates, fats, proteins) and water from food so they can be used by the body.
^"Duodenum: MedlinePlus Medical Encyclopedia".MedlinePlus. Retrieved2022-06-07.It is located between the stomach and the middle part of the small intestine. After foods mix with stomach acid, they move into the duodenum, where they mix with bile from the gallbladder and digestive juices from the pancreas.
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^Deakin, Barbara Young; et al. (2006).Wheater's functional histology : a text and colour atlas (5th ed.). [Edinburgh?]: Churchill Livingstone/Elsevier.ISBN978-0-443-06850-8.
^Uhlén, Mathias; Fagerberg, Linn; Hallström, Björn M.; Lindskog, Cecilia; Oksvold, Per; Mardinoglu, Adil; Sivertsson, Åsa; Kampf, Caroline; Sjöstedt, Evelina (2015-01-23). "Tissue-based map of the human proteome".Science.347 (6220): 1260419.doi:10.1126/science.1260419.ISSN0036-8075.PMID25613900.S2CID802377.
^Gremel, Gabriela; Wanders, Alkwin; Cedernaes, Jonathan; Fagerberg, Linn; Hallström, Björn; Edlund, Karolina; Sjöstedt, Evelina; Uhlén, Mathias; Pontén, Fredrik (2015-01-01). "The human gastrointestinal tract-specific transcriptome and proteome as defined by RNA sequencing and antibody-based profiling".Journal of Gastroenterology.50 (1):46–57.doi:10.1007/s00535-014-0958-7.ISSN0944-1174.PMID24789573.S2CID21302849.
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