Editing Duodenum (section)

==Overview==
The duodenum is the first section of the [[small intestine]] in most [[higher vertebrates]], including [[mammals]], [[reptiles]], and [[birds]]. In [[fish]], the divisions of the small intestine are not as clear, and the terms ''anterior intestine'' or ''proximal intestine'' may be used instead of duodenum.<ref name="fish_feeding_book">
{{cite book |last=Guillaume |first=Jean |url=https://books.google.com/books?id=As0flTZo_EAC&q=fish+cytology+jejunum+duodenum&pg=PA31 |title=Nutrition and Feeding of Fish and Crustaceans |author2=Praxis Publishing |author3=Sadasivam Kaushik |author4=Pierre Bergot |author5=Robert Metailler |publisher=Springer |year=2001 |isbn=978-1-85233-241-9 |page=31 |access-date=2009-01-09}}
</ref> In mammals the duodenum may be the principal site for [[iron]] absorption.<ref>{{cite journal |author=Latunde-Dada GO |author2=Van der Westhuizen J |author3=Vulpe CD |last4=Anderson |first4=G.J. |last5=Simpson |first5=R.J. |last6=McKie |first6=A.T. |display-authors=3 |year=2002 |title=Molecular and functional roles of duodenal cytochrome B (Dcytb) in iron metabolism |journal=Blood Cells Mol. Dis. |volume=29 |issue=3 |pages=356–60 |doi=10.1006/bcmd.2002.0574 |pmid=12547225}}</ref>

In humans, the duodenum is a C-shaped hollow jointed tube, {{convert|25–38|cm|4=0|abbr=off}} in length, lying adjacent to the [[stomach]] (and connecting it to the small intestine). It is divided anatomically into four sections. The first part lies within the [[peritoneum]] but its other parts are [[retroperitoneal]].<ref name="GRAYS2005" />{{rp|273}}

===Parts===
The ''first'' or ''superior part'' of the duodenum is a continuation from the [[pylorus]] to the transpyloric plane. It is superior (above) to the rest of the segments, at the [[vertebral]] level of [[lumbar vertebrae|L1]]. The [[duodenal bulb]], about {{Convert|2|cm|in|abbr=on|frac=4}} long, is the first part of the duodenum and is slightly dilated. The duodenal bulb is a remnant of the mesoduodenum, a [[mesentery]] that suspends the organ from the posterior abdominal wall in fetal life.<ref>{{cite book|last=Singh|first=Inderbir|author2=GP Pal|title=Human Embryology|publisher=Macmillan Publishers India|location=Delhi|date=2012|edition=9|page=163|chapter=13|isbn=978-93-5059-122-2}}</ref> The first part of the duodenum is mobile, and connected to the liver by the [[hepatoduodenal ligament]] of the [[lesser omentum]]. The first part of the duodenum ends at the corner, the ''superior duodenal flexure''.<ref name="GRAYS2005">{{cite book|last=Drake|first=Richard L.|title=Gray's anatomy for students|year=2005|publisher=Elsevier/Churchill Livingstone|location=Philadelphia|isbn=978-0-8089-2306-0|author2=Vogl, Wayne |author3=Tibbitts, Adam W.M. Mitchell  |author4=illustrations by Richard |author5= Richardson, Paul }}</ref>{{rp|273}}

<!-- Only the first 2 cm of the superior part is mobile (covered by [[peritoneum]]) – the [[Anatomical terms of location#Proximal and distal|distal]] 3&nbsp;cm of the first part along with the rest of the duodenum is retroperitoneal (immobile). unsourced-->Relations:{{citation needed|date=December 2013}}
* Anterior
** [[Gallbladder]]
** [[Quadrate lobe of liver]]
* Posterior
** [[Bile duct]]
** [[Gastroduodenal artery]]
** [[Portal vein]]
** [[Inferior vena cava]]
** [[Head of pancreas]]
* Superior
** [[Neck of gallbladder]]
** [[Hepatoduodenal ligament]] ([[lesser omentum]])
* Inferior
** [[Neck of pancreas]]
** [[Greater omentum]]
** [[Head of pancreas]]
The ''second'' or ''descending part'' of the duodenum begins at the superior duodenal flexure. It goes [[Anatomical terms of location#Superior and inferior|inferior]] to the lower border of vertebral body L3, before making a sharp turn [[Lateral and medial|medial]]ly into the ''inferior duodenal flexure'', the end of the descending part.<ref name="GRAYS2005" />{{rp|274}}

The [[pancreatic duct]] and [[common bile duct]] enter the descending duodenum, through the [[major duodenal papilla]]. The second part of the duodenum also contains the minor duodenal papilla, the entrance for the [[accessory pancreatic duct]]. The junction between the embryological [[foregut]] and [[midgut]] lies just below the major duodenal papilla.<ref name="GRAYS2005" />{{rp|274}}

The ''third'', ''horizontal'' or ''inferior part'' of the duodenum is 10~12&nbsp;cm in length. It begins at the ''inferior duodenal flexure'' and passes transversely to the left, passing in front of the [[inferior vena cava]], [[abdominal aorta]] and the [[vertebral column]]. The [[superior mesenteric artery]] and [[superior mesenteric vein|vein]] are [[anterior]] to the third part of the duodenum.<ref name="GRAYS2005" />{{rp|274}} This part may be compressed between the aorta and SMA causing [[superior mesenteric artery syndrome]].

The ''fourth'' or ''ascending part'' of the duodenum passes upward, joining with the [[jejunum]] at the [[duodenojejunal 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 the [[aorta]].<ref name="GRAYS2005" />{{rp|274}}

===Blood supply===
The first (superior) part of the duodenum, right after the [[pylorus]] of the stomach, is not supplied by the arcades. Instead, it is supplied by the [[supraduodenal artery]] and posterior superior pancreaticoduodenal artery, along with some branches of the [[right 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 the [[right gastric artery]].<ref name=":4">{{Cite journal |last=Androulakis |first=John |last2=Colborn |first2=Gene L. |last3=Skandalakis |first3=Panagiotis N. |last4=Skandalakis |first4=Lee J. |last5=Skandalakis |first5=John E. |date=2000-02-01 |title=EMBRYOLOGIC AND ANATOMIC BASIS OF DUODENAL SURGERY |url=https://linkinghub.elsevier.com/retrieve/pii/S0039610905704011 |journal=Surgical Clinics of North America |volume=80 |issue=1 |pages=171–199 |doi=10.1016/S0039-6109(05)70401-1 |issn=0039-6109|url-access=subscription }}</ref>

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 two [[Anastomosis|anastomosed]] (connected) arteries. The superior artery of each arcade comes from the [[superior pancreaticoduodenal artery]], which arises from the [[celiac artery]] via the [[gastroduodenal artery]]. The inferior artery of each arcade comes from the [[inferior pancreaticoduodenal artery]], a branch of the [[superior 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.<ref name=":4" /><ref>{{Citation |last=Aasen |first=S. |title=Stomach Duodenum Normal Anatomy, Function and Congenital Anomalies |date=2013 |work=Abdominal Imaging |pages=367–382 |editor-last=Hamm |editor-first=Bernd |url=https://link.springer.com/referenceworkentry/10.1007/978-3-642-13327-5_17 |access-date=2025-03-09 |place=Berlin, Heidelberg |publisher=Springer |language=en |doi=10.1007/978-3-642-13327-5_17 |isbn=978-3-642-13327-5 |last2=Lundin |first2=K. E. A. |editor2-last=Ros |editor2-first=Pablo R.|url-access=subscription }}</ref> 

Vessels from the arcades supply the [[Muscular layer|muscularis externa]] (muscular layer) before forming a [[plexus]] (network of blood vessels) in the [[submucosa]] (a layer of connective tissue) called the [[submucosal plexus]]. Vessels continue from the submucosal plexus through the [[muscularis mucosae]] (another thin muscular layer) before forming another plexus under the [[epithelium]] of the [[Intestinal villus|villi]], the layer where nutrients are absorbed. These vessels entering the duodenum from the arcades are sometimes called [[Vasa recta (intestines)|vasae rectae]] or arteriae rectae.<ref name=":4" /> 

The venous drainage of the duodenum mainly follows the arteries, ultimately draining into the [[Portal venous system|portal system]]. The venous arcades are usually superficial to the arterial arcades. The anterior superior pancreaticoduodenal vein drains into the [[right 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 the [[portal vein]] or the posterior superior pancreaticoduodenal vein, which drains into the portal vein. The inferior veins of the arcades drain into the [[Superior mesenteric vein|superior mesenteric]], [[Inferior mesenteric vein|inferior mesenteric]], [[Splenic vein|splenic]], or first [[Jejunal veins|jejunal vein]].<ref name=":4" />

Embryologically, the duodenum arises from both the [[foregut]] and [[midgut]], 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 the [[celiac artery]] and the superior mesenteric artery, these two definitions are similar but not exactly the same.<ref name=":4" />

===Lymphatic drainage===
The [[Lymph vessel|lymphatic vessels]] follow the arteries in a retrograde fashion. The anterior lymphatic vessels drain into the pancreatoduodenal [[lymph node]]s 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.

===Histology===
Under [[microscopy]], the duodenum has a [[Intestinal villus|villous]] [[mucosa]]. This is distinct from the mucosa of the [[pylorus]], which directly joins the duodenum. Like other structures of the [[Gastrointestinal wall|gastrointestinal tract]], the duodenum has a [[mucosa]], [[submucosa]], [[muscularis externa]], and [[adventitia]]. Glands line the duodenum, known as [[Brunner's gland]]s, which secrete [[mucus]] and [[bicarbonate]] in order to neutralise stomach acids. These are distinct glands not found in the ileum or jejunum, the other parts of the small intestine.<ref name="WHEATERS2006">{{cite book|last=Deakin|first=Barbara Young  |display-authors=etal |title=Wheater's functional histology : a text and colour atlas|year=2006|publisher=Churchill Livingstone/Elsevier|location=[Edinburgh?]|isbn=978-0-443-06850-8|edition=5th}}</ref>{{rp|274–275}}
<gallery>
 File:Dogduodenum100x3.jpg|Dog duodenum 100X
 Image:Small bowel duodenum with amyloid deposition congo red 10X.jpg|Duodenum with amyloid deposition in lamina propria
 Image:Gray1058.png|Section of duodenum of cat. X 60
 Image:Giardiasis duodenum low.jpg |[[Micrograph]] showing [[giardiasis]] on a duodenal biopsy ([[H&E stain]])
 Image:Microvilli-Duodenum.JPG|Duodenum with brush border ([[microvilli|microvillus]])
</gallery>

===Variation===
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.<ref>{{Cite journal |last1=Borghei |first1=Peyman |last2=Sokhandon |first2=Farnoosh |last3=Shirkhoda |first3=Ali |last4=Morgan |first4=Desiree E. |date=January 2013 |title=Anomalies, Anatomic Variants, and Sources of Diagnostic Pitfalls in Pancreatic Imaging |url=http://pubs.rsna.org/doi/10.1148/radiol.12112469 |journal=Radiology |language=en |volume=266 |issue=1 |pages=28–36 |doi=10.1148/radiol.12112469 |pmid=23264525 |issn=0033-8419|url-access=subscription }}</ref>{{expand section|date=December 2013}}

===Gene and protein expression===
{{Further |Bioinformatics#Gene and protein expression}}
About 20,000 protein coding genes are expressed in human cells and 70% of these genes are expressed in the normal duodenum.<ref>{{Cite web|url=https://www.proteinatlas.org/humanproteome/duodenum|title=The human proteome in duodenum - The Human Protein Atlas|website=www.proteinatlas.org|access-date=2017-09-26}}</ref><ref>{{Cite journal|last1=Uhlén|first1=Mathias|last2=Fagerberg|first2=Linn|last3=Hallström|first3=Björn M.|last4=Lindskog|first4=Cecilia|last5=Oksvold|first5=Per|last6=Mardinoglu|first6=Adil|last7=Sivertsson|first7=Åsa|last8=Kampf|first8=Caroline|last9=Sjöstedt|first9=Evelina|date=2015-01-23|title=Tissue-based map of the human proteome|journal=Science|language=en|volume=347|issue=6220|pages=1260419|doi=10.1126/science.1260419|issn=0036-8075|pmid=25613900|s2cid=802377}}</ref> 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 as [[alanine aminopeptidase]], a digestive enzyme, [[angiotensin-converting enzyme]], involved in controlling [[blood pressure]], and [[RBP2]], a protein involved in the uptake of [[vitamin A]].<ref>{{Cite journal|last1=Gremel|first1=Gabriela|last2=Wanders|first2=Alkwin|last3=Cedernaes|first3=Jonathan|last4=Fagerberg|first4=Linn|last5=Hallström|first5=Björn|last6=Edlund|first6=Karolina|last7=Sjöstedt|first7=Evelina|last8=Uhlén|first8=Mathias|last9=Pontén|first9=Fredrik|date=2015-01-01|title=The human gastrointestinal tract-specific transcriptome and proteome as defined by RNA sequencing and antibody-based profiling|journal=Journal of Gastroenterology|language=en|volume=50|issue=1|pages=46–57|doi=10.1007/s00535-014-0958-7|pmid=24789573|s2cid=21302849|issn=0944-1174}}</ref>