|Superior and inferior duodenal fossæ.|
|Gray's||subject #248 1170|
|Nerve||celiac ganglia, vagus |
The jejunum (//) is the middle 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 middle intestine or mid-gut may be used instead of jejunum.
The jejunum lies between the duodenum and the ileum. The change from the duodenum to the jejunum is usually defined as the Duodenojejunal flexure and is attached, and thus "hung up", to the ventricle (see stomach) by the ligament of Treitz. In adult humans, the small intestine is usually between 5.5 and 6m long, 2.5m of which is the jejunum.
If the jejunum is impacted by blunt force the emesis reflex will be initiated.
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 lining of the jejunum is specialized for the absorption of small nutrient particles by enterocytes, 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.
Internal structure 
The lumenal surface of the jejunum, 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 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 which increase the surface area for nutrient absorption. The plicae circulares are the best developed in the jejunum.
Differences between jejunum and ileum 
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.
Jejunum is derived from the Latin word jējūnus, meaning "fasting". It was so called because this part of the small intestine was frequently found to be void of food following death, thanks to intensive peristaltic activity compared to the duodenum and ileum.
Additional images 
- Physiology at MCG 6/6ch2/s6ch2_30
- OED 2nd edition, 1989.
- Entry "jejunum" in Merriam-Webster Online Dictionary.
- Guillaume, Jean; Praxis Publishing, Sadasivam Kaushik, Pierre Bergot, Robert Metailler (2001). Nutrition and Feeding of Fish and Crustaceans. Springer. p. 31. ISBN 1-85233-241-7, 9781852332419 Check
|isbn=value (help). Retrieved 2009-01-09.
- van Gijn J, Gijselhart JP (2011). "Treitz and his ligament.". Ned Tijdschr Geneeskd. 155 (8). PMID 21557825.
- The Gale Group, Inc. (2008). Gale Encyclopedia of Medicine
- Gastrolab.net:The Jejunum
- Peyer's patches
- SUNY Labs 37:11-0100 – "Abdominal Cavity: The Jejunum and the Ileum"
- SUNY Anatomy Image 7856