Jejunum

The jejunum is the second part of the small intestine in humans and most higher vertebrates, including mammals, reptiles, and birds. Its lining is specialized for the absorption by enterocytes of small nutrient molecules which have been previously digested by enzymes in the duodenum.

Jejunum
Small intestine
Superior and inferior duodenal fossæ.
Details
Pronunciation/dʒɪˈdʒuːnəm/[1][2]
Precursormidgut
Part ofSmall intestine
SystemDigestive system
Arteryjejunal arteries
Veinjejunal veins
Nerveceliac ganglia, vagus[3]
Identifiers
Latinjejunum
MeSHD007583
TA98A05.6.03.001
TA22958
FMA7207
Anatomical terminology
Major parts of the
Gastrointestinal tract
Upper gastrointestinal tract
Lower gastrointestinal tract
See also

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.[4] The division between the jejunum and ileum is not anatomically distinct.[5] In adult humans, the small intestine is usually 6–7 m (20–23 ft) long (post mortem), about two-fifths of which (about 2.5 m (8.2 ft)) is the jejunum.[4]

Structure
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.

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 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.

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.[6]

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.[7]

History

Etymology

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,[8] due to its intensive peristaltic activity relative to the duodenum and ileum.

The Early Modern English adjective jejune is derived from the same root.[9]

References
  1. OED 2nd edition, 1989.
  2. Entry "jejunum" in Merriam-Webster Online Dictionary.
  3. Nosek, Thomas M. "Section 6/6ch2/s6ch2_30". Essentials of Human Physiology. Archived from the original on 2016-03-24.
  4. Drake, Richard L.; Vogl, Wayne; Tibbitts, Adam W. M. Mitchell; illustrations by Richard; Richardson, Paul (2005). Gray's anatomy for students. Philadelphia: Elsevier/Churchill Livingstone. pp. 273–275. ISBN 978-0-8089-2306-0.
  5. Deakin, Barbara Young; et al. (2006). Wheater's functional histology : a text and colour atlas (5th ed.). Churchill Livingstone/Elsevier. p. 263. ISBN 978-0-443-068-508.
  6. CRANE, RK (Oct 1960). "Intestinal absorption of sugars". Physiological Reviews. 40 (4): 789–825. doi:10.1152/physrev.1960.40.4.789. PMID 13696269.
  7. Guillaume, Jean; Praxis Publishing; Sadasivam Kaushik; Pierre Bergot; Robert Metailler (2001). Nutrition and Feeding of Fish and Crustaceans. Springer. p. 31. ISBN 9781852332419. Retrieved 2009-01-09.
  8. Harper, Douglas. "jejunum". Etymology Online. Retrieved 15 November 2014.
  9. "Oxford English Dictionary: jejune, adj".
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