Jejunum

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.

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^{[citation needed]}.

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

The pH in the jejunum is usually between 7 and 9 (neutral or slightly alkaline).

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.

Function

The jejunum is the next portion of the small intestine, and it has a lining which is specialized in the absorption of carbohydrates and proteins. The proteins have been broken down in the stomach by enzymes called pepsin and acid into amino acids. The carbohydrates are broken down in the duodenum by enzymes from the pancreas and liver into sugars. Fats are broken down in the duodenum by "lipase" from the pancreas into fatty acids. Amino acid, sugar, fatty acid particles, vitamins, minerals, electrolytes and water are small enough to soak into the villi of the jejunum and drop into the blood stream. The blood takes all these nutrients to all the other parts of the body to provide fuel to do their jobs.^{[citation needed]}

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). 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.

Differences between jejunum and ileum

There is no line of demarcation between the jejunum and the ileum. There are, however, subtle differences between the two.

• The ileum has more fat inside the mesentery than the jejunum.
• The ileum has a paler color, and tends to be of a smaller caliber as well.
• While the length of the intestinal tract contains lymphoid tissue, only the ileum has abundant Peyer's patches, unencapsulated lymphoid nodules that contain large numbers of lymphocytes and other cells of the immune system.

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]

Additional images

• 

  Duodenojejunal fossa.

• 

  Transmission electron microscope (TEM) of Human Jejunum

• 

  TEM of Mouse Jejunum x14000

External links

• Gastrolab.net:The Jejunum
• Peyer's patches
• SUNY Labs 37:11-0100 – "Abdominal Cavity: The Jejunum and the Ileum"
• SUNY Anatomy Image 7856