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Jejunum
	Small intestine
	Superior and inferior duodenal fossæ.
Gray's	subject #248 1170
Artery	jejunal arteries
Vein	jejunal veins
Nerve	celiac ganglia, vagus
Precursor	midgut
MeSH	Jejunum
Dorlands/Elsevier	Jejunum

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.^[2]

The jejunum lies between the duodenum and the ileum. The change from the duodenum to the jejunum is usually defined as the ligament of Treitz.

In adult humans, the small intestine is usually between 5.5-6m long, 2.5m of which is the jejunum.

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 is wrapped by smooth muscle that helps to move food along by a process known as peristalsis.

[edit] Internal structure

The inner surface of the jejunum, its mucous membrane, is covered in projections called villi, which increase the surface area of tissue available to absorb nutrients from the gut contents. The epithelial cells which line these villi possess even larger numbers of 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). Instead, it has many large circular folds in its submucosa called plicae circulares which increase the surface area for nutrient absorption.

[edit] Etymology

Jejunum is derived from the adjective jejune, which means "fasting" or "hungry" in Early Modern English. It was so called because this portion of the intestine was found "empty" following death.^{[citation needed]}[Latin: jejunus, fasting; barren]