View source for Jejunum

{{Short description|Part of the small intestine}}
{{Distinguish|duodenum}}
{{Infobox anatomy
| Name        = Jejunum
| Latin       = jejunum
| Image       = Illu small intestine.jpg
| Caption     = Small intestine
| Width       = 
| Image2      = Gray1041.png
| Caption2    = Superior and inferior duodenal fossæ.
| Precursor   = [[laude]]
| Artery      = [[Jejunal arteries]]
| Vein        = [[Jejunal veins]]
| Nerve       = [[Celiac ganglia]], [[vagus]]<ref>{{cite book| title= Essentials of Human Physiology| first= Thomas M. |last= Nosek| chapter=Section 6/6ch2/s6ch2_30 |chapter-url=http://humanphysiology.tuars.com/program/section6/6ch2/s6ch2_30.htm |archive-url=https://web.archive.org/web/20160324124828/http://humanphysiology.tuars.com/program/section6/6ch2/s6ch2_30.htm|archive-date=2016-03-24}}</ref>
| Lymph       = 
| PartOf      = [[Small intestine]]
| System      = [[Digestion|Digestive system]]
| Pronunciation = {{IPAc-en|dʒ|ᵻ|ˈ|dʒ|uː|n|əm}} {{respell|jij|OO|nəm}}<ref>''[[OED]]'' 2nd edition, 1989.</ref><ref>[http://www.merriam-webster.com/dictionary/jejunum Entry "jejunum"] in ''[http://www.merriam-webster.com/ Merriam-Webster Online Dictionary]''.</ref>
}}
{{Gastrointestinal tract sidebar}}
The '''jejunum''' is the second part of the [[small intestine]] in [[human]]s and most [[higher vertebrates]], including [[mammals]], [[reptiles]], and [[birds]].  Its lining is specialized for the absorption by [[enterocytes]] of small nutrient [[molecule]]s which have been previously digested by [[enzymes]] in the [[duodenum]].

The jejunum lies between the [[duodenum]] and the [[ileum]] and is considered to start at the [[suspensory muscle of the duodenum]], a location called the [[duodenojejunal flexure]].<ref name=GRAYS2005 /> The division between the jejunum and ileum is not anatomically distinct.<ref name=WHEATERS2006>{{cite book|last=Deakin|first=Barbara Young |display-authors=etal |title=Wheater's functional histology : a text and colour atlas|date=2006|publisher=Churchill Livingstone/Elsevier<!--|location=[Edinburgh?]-->|isbn=978-0-443-068-508|page=263|edition=5th}}</ref> In adult [[human]]s, the small intestine is usually {{convert|6|-|7|m|ft|abbr=on}} long (post mortem), about two-fifths of which (about {{convert|2.5|m|ft|abbr=on}}) is the jejunum.<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 |pages=273–275}}</ref>

==Structure==
{{uncited section|date=December 2024}}

{{See also|Small intestine}}

The interior surface of the jejunum—which is exposed to ingested food—is covered in finger–like projections of mucosa, called [[Intestinal villus|villi]], which increase the surface area of tissue available to absorb nutrients from ingested foodstuffs. The epithelial cells which line these villi have [[Microvillus|microvilli]]. The transport of nutrients across epithelial cells through the jejunum and ileum includes the [[passive transport]] of sugar [[fructose]] and the [[active transport]] of [[amino acid]]s, small [[peptide]]s, [[vitamin]]s, and most [[glucose]]. The villi in the jejunum are much longer than in the duodenum or ileum.

The [[pH]] in the jejunum is usually between 7 and 8 (neutral or slightly [[alkaline]]).

The jejunum and the ileum are suspended by [[mesentery]] which gives the bowel great mobility within the [[abdomen]]. It also contains circular and longitudinal [[smooth muscle]] which helps to move food along by a process known as [[peristalsis]].

===Histology===

The jejunum contains very few [[Brunner's glands]] (found in the duodenum) or [[Peyer's patches]] (found in the ileum). However, there are a few jejunal [[lymph node]]s suspended in its mesentery. The jejunum has many large [[circular folds]] in its [[submucosa]] called [[plicae circulares]] that increase the surface area for nutrient absorption. The plicae circulares are best developed in the jejunum.

There is no line of demarcation between the jejunum and the ileum. However, there are subtle [[histological]] differences:
* The jejunum has less fat inside its mesentery than the ileum. 
* The jejunum is typically of larger diameter than the ileum.
* The villi of the jejunum look like long, finger-like projections, and are a histologically identifiable structure. 
* While the length of the entire intestinal tract contains [[lymphoid tissue]], only the ileum has abundant [[Peyer's patches]], which are unencapsulated [[lymphoid nodule]]s that contain large numbers of [[lymphocyte]]s and immune cells, like [[microfold cell]]s.

<gallery mode="packed">
 Image:Microvilli.jpg|[[Transmission electron microscopy|Transmission electron microscope]] (TEM) image of human jejunum
 Image:Nudemousejejunum EM.jpg|TEM image of mouse jejunum (14,000-fold magnification)
 Image:Dogjejunum100x.jpg|Dog jejunum (magnified 100-fold)
</gallery>

==Function==
The lining of the jejunum is specialized for the absorption by [[enterocytes]] of small nutrient particles which have been previously digested by [[enzymes]] in the [[duodenum]]. Once absorbed, nutrients (with the exception of fat, which goes to the lymph) pass from the enterocytes into the [[enterohepatic circulation]] and enter the [[liver]] via the hepatic portal vein, where the blood is processed.<ref>{{cite journal|last1=CRANE|first1=RK|title=Intestinal absorption of sugars.|journal=Physiological Reviews|date=Oct 1960|volume=40|issue=4|pages=789–825|pmid=13696269|doi=10.1152/physrev.1960.40.4.789}}</ref>

==Other animals==
In [[fish]], the divisions of the small intestine are not as clear and the terms '''middle intestine''' or '''mid-gut''' may be used instead of jejunum.<ref name=fish_feeding_book>
{{cite book
 | last=Guillaume
 | first=Jean
 |author2=Praxis Publishing |author3=Sadasivam Kaushik |author4=Pierre Bergot |author5=Robert Metailler
  | title=Nutrition and Feeding of Fish and Crustaceans
 | url=https://books.google.com/books?id=As0flTZo_EAC&pg=PA31 
 | page=31
 | year=2001
 | access-date=2009-01-09
 | publisher=Springer
 | isbn=9781852332419}}
</ref>

==History==

===Etymology===
''Jejunum'' is derived from the [[Latin]] word ''[[wikt:ieiunus#Latin|jējūnus]] (iēiūnus)'', meaning "[[fasting]]." It was so called because this part of the [[small intestine]] was frequently found to be void of food following [[death]],<ref>{{cite web|last1=Harper|first1=Douglas|title=jejunum|url=http://www.etymonline.com/index.php?term=jejunum|website=Etymology Online|access-date=15 November 2014}}</ref> due to its intensive [[Peristalsis|peristaltic]] activity relative to the [[duodenum]] and [[ileum]].
<!-- Editors: The contributor intentionally used this variant ("jējūnus") so that the origin of the word is clearer, and many sources cite this variant. Do not change to "iēiūnus". -->

The [[Early Modern English]] adjective ''[[wikt:jejune|jejune]]'' is derived from the same root.<ref>{{cite web|url=http://www.oed.com/view/Entry/101004?redirectedFrom=jejune#eid|title=Oxford English Dictionary: jejune, adj.}}</ref>

==References==
{{Reflist}}

==External links==
*[https://web.archive.org/web/20050620080036/http://www.gastrolab.net/bb12je.htm Gastrolab.net:The Jejunum]
*[http://www.thehealthnews.org/news/06/08/01/Peyers.Patches.html Peyer's patches] {{Webarchive|url=https://web.archive.org/web/20160304064745/http://www.thehealthnews.org/news/06/08/01/Peyers.Patches.html |date=2016-03-04 }}
* {{SUNYAnatomyLabs|37|11|01|00}} &ndash; "Abdominal Cavity: The Jejunum and the Ileum"
* {{SUNYAnatomyImage|7|8|56}}

{{Digestive tract}}
{{Authority control}}

[[Category:Small intestine]]