Editing Small intestine (section)

==Structure==

===Size===
The length of the small intestine can vary greatly, from as short as {{convert|3|m|ft|abbr=off|frac=2}} to as long as {{convert|10.5|m|ft|abbr=on|frac=2}}, also depending on the measuring technique used.<ref name=Di2016/> The typical length in a living person is {{Convert|3–5|m|abbr=on|frac=2}}.<ref>{{Cite book|title=Principles of Anatomy & Physiology|last=Tortora|first=Gerard|publisher=Wiley|year=2014|isbn=978-1-118-34500-9|location=USA|pages=[https://archive.org/details/principlesofanat0000tort/page/913 913]|quote=..its length is about 3m in a living person and about 6.5m in a cadaver due to loss of smooth muscle tone after death.|url=https://archive.org/details/principlesofanat0000tort/page/913}}</ref><ref>{{Cite book|title=Gray's Anatomy|last=Standring|first=Susan|publisher=Elsevier|year=2016|isbn=978-0-7020-5230-9|location=UK|pages=1124|quote=..and has a mean length of 5 metres (3 - 8.5 metres) when measured intraoperatively in the living adult (Tietelbaum et al 2013).}}</ref> The length depends both on how tall the person is and how the length is measured.<ref name=Di2016/> Taller people generally have a longer small intestine and measurements are generally longer after death and when the bowel is empty.<ref name=Di2016>{{cite book|last1=DiBaise|first1=John K.|last2=Parrish|first2=Carol Rees|last3=Thompson|first3=Jon S.|title=Short Bowel Syndrome: Practical Approach to Management|date=2016|publisher=CRC Press|isbn=9781498720809|page=31|url=https://books.google.com/books?id=GBhjDAAAQBAJ&pg=PA31|language=en}}</ref>
{|class="wikitable" align="right"
|+ Small bowel dilation on [[CT scan]] in adults<ref name="JacobsRozenblit2007">{{cite journal|last1=Jacobs|first1=S.L.|last2=Rozenblit|first2=A.|last3=Ricci|first3=Z.|last4=Roberts|first4=J.|last5=Milikow|first5=D.|last6=Chernyak|first6=V.|last7=Wolf|first7=E.|title=Small bowel faeces sign in patients without small bowel obstruction|journal=Clinical Radiology|volume=62|issue=4|year=2007|pages=353–357|issn=0009-9260|doi=10.1016/j.crad.2006.11.007|pmid=17331829}}</ref>
|-
|  <2.5&nbsp;cm || Non-dilated
|-
| 2.5-2.9&nbsp;cm || Mildly dilated
|-
| 3–4&nbsp;cm || Moderately dilated
|-
| >4&nbsp;cm || Severely dilated
|}
It is approximately {{Convert|1.5|cm|abbr=off|frac=8}} in diameter in [[neonate|newborns]] after 35 weeks of [[Gestational age (obstetrics)|gestational age]],<ref>{{cite journal|title=Overview of short bowel syndrome and intestinal transplantation|author=Debora Duro, Daniel Kamin|journal=Colombia Médica|volume=38|issue=1|year=2007}}</ref> and {{Convert|2.5–3|cm|abbr=on|frac=8}} in diameter in adults. On [[abdominal X-ray]]s, the small intestine is considered to be abnormally dilated when the diameter exceeds 3&nbsp;cm.<ref name=medscape>{{cite web|url=http://emedicine.medscape.com/article/374962-overview|title=Small-Bowel Obstruction Imaging|website=[[Medscape]]|date=2016-09-22|author=Ali Nawaz Khan|access-date=2017-02-07}}</ref><ref>{{cite web|url=http://www.radiologymasterclass.co.uk/tutorials/abdo/abdomen_x-ray_abnormalities/pathology_small_bowel_obstruction|title=Abdominal X-ray - Abnormal bowel gas pattern|website=radiologymasterclass.co.uk|access-date=2017-02-07}}</ref> On [[CT scan]]s, a diameter of over 2.5&nbsp;cm is considered abnormally dilated.<ref name=medscape/><ref name="GazelleGoldberg1994">{{cite journal|last1=Gazelle|first1=G S|last2=Goldberg|first2=M A|last3=Wittenberg|first3=J|last4=Halpern|first4=E F|last5=Pinkney|first5=L|last6=Mueller|first6=P R|title=Efficacy of CT in distinguishing small-bowel obstruction from other causes of small-bowel dilatation.|journal=American Journal of Roentgenology|volume=162|issue=1|year=1994|pages=43–47|issn=0361-803X|doi=10.2214/ajr.162.1.8273687|pmid=8273687|doi-access=free}}</ref> The surface area of the human [[Human gastrointestinal tract#Mucosa|small intestinal mucosa]], due to enlargement caused by folds, villi and microvilli, averages {{Convert|30|m2}}.<ref>{{cite journal|last1=Helander|first1=Herbert F|last2=Fändriks|first2=Lars|s2cid=11094705|title=Surface area of the digestive tract – revisited|journal=Scandinavian Journal of Gastroenterology|volume=49|issue=6|year=2015|pages=681–689|issn=0036-5521|doi=10.3109/00365521.2014.898326|pmid= 24694282 }}</ref>

===Parts===
[[File:Blausen 0817 SmallIntestine Anatomy.png|thumb|Labeled diagram of the small intestine and its surrounding structures]]

The small intestine is divided into three structural parts.
* The '''[[duodenum]]''' is a short structure ranging from {{Convert|20-25|cm|abbr=on|frac=2}} in length, and shaped like a "C".<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 |page=273}}</ref> It surrounds the head of the pancreas. It receives gastric [[chyme]] from the stomach, together with digestive juices from the [[pancreas]] ([[digestive enzymes]]) and the [[liver]] ([[bile]]). The digestive enzymes break down proteins and bile [[emulsify|emulsifies]] fats into [[micelles]]. The [[duodenum]] contains [[Brunner's glands]], which produce a mucus-rich alkaline secretion containing [[bicarbonate]]. These secretions, in combination with bicarbonate from the pancreas, neutralize the stomach acids contained in gastric chyme.
* The '''[[jejunum]]''' is the midsection of the small intestine, connecting the duodenum to the ileum. It is about {{Convert|2.5|m|ft|abbr=on|frac=2}} long, and contains the [[circular folds]], and [[intestinal villus|intestinal villi]] that increase its surface area. Products of digestion (sugars, amino acids, and fatty acids) are absorbed into the bloodstream here. The [[suspensory muscle of duodenum]] marks the division between the duodenum and the jejunum.
* The '''[[ileum]]''': The final section of the small intestine. It is about 3 m (9.8 feet) long, and contains villi similar to the jejunum. It absorbs mainly [[vitamin B12]] and [[bile acids]], as well as any other remaining nutrients. The ileum joins to the [[cecum]] of the [[large intestine]] at the [[ileocecal junction]].{{cn|date=May 2022}}

The jejunum and ileum are suspended in the [[abdominal cavity]] by [[mesentery]]. The mesentery is part of the [[peritoneum]]. Arteries, veins, lymph vessels and nerves travel within the mesentery.<ref name=GRAYS2005C>{{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 |page=271}}</ref>

===Blood supply===
The small intestine receives a blood supply from the [[celiac trunk]] and the [[superior mesenteric artery]]. These are both branches of the [[aorta]]. The duodenum receives blood from the celiac trunk via the [[superior pancreaticoduodenal artery]] and from the superior mesenteric artery via the [[inferior pancreaticoduodenal artery]]. These two arteries both have [[anterior and posterior]] branches that meet in the midline and [[Anastomosis|anastomose]].  The jejunum and ileum receive blood from the superior mesenteric artery.<ref name=GRAYS2005B>{{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 |pages=295–299}}</ref> Branches of the superior mesenteric artery form a series of arches within the mesentery known as [[arterial arcades]], which may be several layers deep. Straight blood vessels known as [[Vasa recta (intestines)|vasa recta]] travel from the arcades closest to the ileum and jejunum to the organs themselves.<ref name=GRAYS2005B />

===Microanatomy===
{{Main|Gastrointestinal wall}}
[[File:Small intestine low mag.jpg|thumb|right|200px|[[Micrograph]] of the small intestine [[mucosa]] showing the [[intestinal villi]] and [[crypts of Lieberkühn]].]]
The three sections of the small intestine look similar to each other at a microscopic level, but there are some important differences. The parts of the intestine are as follows:
[[File:Cross Section of a Small Intestine.png|thumb|This cross section diagram shows the 4 layers of the small intestine wall.]]
{| class="wikitable"
! Layer || [[Duodenum]] || [[Jejunum]] || [[Ileum]]
|-
| [[Serosa]] || 1st part serosa, 2nd–4th adventitia || Normal || Normal
|-
| [[Muscularis externa]] || Longitudinal and circular layers, with [[Auerbach's plexus|Auerbach's (myenteric) plexus]] in between || Same as duodenum || Same as duodenum
|-
| [[Submucosa]] || [[Brunner's glands]] and [[Meissner's plexus|Meissner's (submucosal) plexus]] || No BG || No BG
|-
| [[Mucosa]]: [[muscularis mucosae]] || Normal || Normal  || Normal
|-
| Mucosa: [[lamina propria]] || No PP || No PP || [[Peyer's patches]]
|-
| Mucosa: [[intestinal epithelium]] || [[Simple columnar epithelium|Simple columnar]]. Contains [[goblet cells]], [[Paneth cell]]s || Similar to [[duodenum]], but the [[intestinal]] villus is long || Similar to [[duodenum]], but the [[intestinal villus]] is short
|}

=== Gene and protein expression ===
About 20,000 protein coding genes are [[Gene expression|expressed]] in human cells and 70% of these genes are expressed in the normal duodenum.<ref>{{Cite web|url=https://www.proteinatlas.org/humanproteome/small+intestine|title=The human proteome in small intestine - 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|s2cid=802377|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}}</ref> Some 300 of these genes are more specifically expressed in the duodenum with very few genes expressed only in the small intestine. The corresponding specific proteins are expressed in glandular cells of the mucosa, such as [[Fatty acid-binding protein|fatty acid binding protein]] [[FABP6]].  Most of the more specifically expressed genes in the small intestine are also expressed in the duodenum, for example [[FABP2]] and the [[DEFA6]] protein expressed in secretory granules of [[Paneth cell]]s.<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|s2cid=21302849|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|issn=0944-1174}}</ref>

===Development===
{{See also|Development of the digestive system}}
The small intestine develops from the [[midgut]] of the [[primitive gut tube]].<ref name=LARSENS2009>{{cite book|first1=Gary C. |last1=Schoenwolf |first2=Steven B. |last2=Bleyl |first3=Philip R. |last3=Brauer |first4=Philippa H. |last4=Francis-West |title=Larsen's human embryology|date=2009|publisher=Churchill Livingstone/Elsevier|location=Philadelphia |isbn=9780443068119 |chapter=Development of the Urogenital system |edition=4th |page=237}}</ref> By the fifth week of [[embryo]]logical life, the [[ileum]] begins to grow longer at a very fast rate, forming a U-shaped fold called the [[primary intestinal loop]]. The loop grows so fast in length that it outgrows the abdomen and protrudes through the [[Navel|umbilicus]]. By week 10, the loop retracts back into the abdomen. Between weeks six and ten the small intestine rotates anticlockwise, as viewed from the front of the embryo. It rotates a further 180 degrees after it has moved back into the abdomen. This process creates the twisted shape of the [[large intestine]].<ref name=LARSENS2009 />

<gallery>
File:Sobo 1906 399.png|First stage of the development of the intestinal canal and the peritoneum, seen from the side (diagrammatic). From colon 1 the ascending and transverse colon will be formed and from colon 2 the descending and sigmoid colons and the rectum.
File:Sobo 1906 400.png|Second stage of development of the intestinal canal and peritoneum, seen from in front (diagrammatic). The liver has been removed and the two layers of the ventral mesogastrium (lesser omentum) have been cut. The vessels are represented in black and the peritoneum in the reddish tint.
File:Sobo 1906 401.png|Third state of the development of the intestinal canal and peritoneum, seen from in front (diagrammatic). The mode of preparation is the same as in [[:Image:Sobo 1906 400.png|Fig 400]]
</gallery>