The jejunum is the second part of the small intestine in and most higher vertebrates, including mammals, reptiles, and birds. Its lining is specialized for the absorption by enterocytes of small nutrient 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.
Structure
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
Microvillus. The transport of nutrients across epithelial cells through the jejunum and ileum includes the passive transport of sugar
fructose and the
active transport of
, small
,
, 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
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 that contain large numbers of and immune cells, like .
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)
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.
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.
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History
Etymology
Jejunum is derived from the
Latin word
(iēiūnus), meaning "
fasting." It was so called because this part of the
small intestine was frequently found to be void of food following
death,
due to its intensive
Peristalsis activity relative to the
duodenum and
ileum.
The Early Modern English adjective is derived from the same root.
External links
