jejunum

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{{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 villi, which increase the surface area of tissue available to absorb nutrients from ingested foodstuffs. The epithelial cells which line these villi have 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 acids, small peptides, vitamins, 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.

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 nodes 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 nodules that contain large numbers of lymphocytes and immune cells, like microfold cells.

Image:Microvilli.jpg|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)

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.{{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}}

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.

{{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}}

Jejunum is derived from the Latin word 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,{{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}} due to its intensive peristaltic activity relative to the duodenum and ileum.

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

{{Reflist}}

• [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}} – "Abdominal Cavity: The Jejunum and the Ileum"
• {{SUNYAnatomyImage|7|8|56}}

{{Digestive tract}}

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Category:Small intestine