Jejunum

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Jejunum

The jejunum is the middle section of the small intestine

Small intestine

In vertebrates, including mammals, reptiles, birds, and bony fish, the small intestine is the part of the gastrointestinal tract following the stomach, and is where the vast majority of digestion takes place....
in most higher vertebrates, including mammals, reptiles, and birds. In fish

Fish

A fish is any marine biology vertebrate animal that is typically ectothermic , covered with scale , and equipped with two sets of paired fins and several unpaired fins....
, 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

Duodenum

The duodenum is the first 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 anterior intestine or proximal intestine may be used instead of duodenum....
and the ileum

Ileum

The ileum is the final 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 posterior intestine or distal intestine may be used instead of ileum....
. The change from the duodenum to the jejunum is usually defined as the ligament of Treitz.

In adult human

Human

A human being, also human or man, is a member of a species of bipedalism primates in the family Hominidae . Mitochondrial DNA evidence indicates that modern humans originated in east Africa about 200,000 years ago....
s, the small intestine is usually between 5.5-6m long, 2.5m of which is the jejunum.

The pH

pH is a measure of the Acid or Base of a solution. It is defined as the cologarithm of the Activity of dissolved hydrogen ions . Hydrogen ion activity coefficients cannot be measured experimentally, so they are based on theoretical calculations....
in the jejunum is usually between 7 and 8 (neutral or slightly alkaline).

The jejunum and the ileum are suspended by mesentery

Mesentery

In anatomy, the mesentery is the double layer of peritoneum that suspends the jejunum and ileum from the posterior wall of the abdomen. Its meaning, however, is frequently extended to include double layers of peritoneum connecting various components of the abdominal cavity....
which gives the bowel great mobility within the abdomen

Abdomen

In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity....
.

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Internal structure

The inner surface of the jejunum, its mucous membrane

Mucous membrane

The mucous membranes are linings of mostly germ layer origin, covered in epithelium, which are involved in absorption and secretion. They line various body cavities that are exposed to the external environment and internal organ ....
, is covered in projections called villi

Villus

Intestinal villi are tiny, finger-like projections that come out from the wall of the small intestine and have additional extensions called microvilli which protrude from epithelial cells lining 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

Passive transport

Passive transport means Motion biochemistrys and atomic or molecular substances across the The four main kinds of passive transport are diffusion, facilitated diffusion, filtration and osmosis....
of sugar fructose and the active transport

Active transport

Active transport is the mediated process of moving particles across a biological membrane against a Concentration_gradient#In_biology . If the process uses chemical energy, such as from adenosine triphosphate , it is termed primary 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

Brunner's glands

Brunner glands are compound tubular Submucosa glands found in that portion of the duodenum which is above the sphincter of Oddi. The main function of these glands is to produce an alkaline secretion in order to:...
(found in the duodenum) or Peyer's patches

Peyer's patches

Peyer's patches are diffuse lymphoid tissue, named after the 17th-century Swiss anatomist Johann Conrad Peyer. They are aggregations of lymphoid tissue that are usually found in the lowest portion of the small intestine in humans; as such, they differentiate the ileum from the duodenum and jejunum in that the number of Peyer's patches increa...
(found in the ileum). Instead, it has many large circular folds

Circular folds

The circular folds are large valvular flaps projecting into the lumen of the bowel....
in its submucosa

Submucosa

In the gastrointestinal tract, the submucosa is the layer of dense irregular connective tissue that supports the mucosa, as well as joins the mucosa to the bulk of underlying smooth muscle ....
called plicae circulares which increase the surface area for nutrient absorption.

Etymology

Jejunum is derived from the adjective jejune, which means "fasting

Fasting

Fasting is primarily the act of willingly abstaining from some or all food, drink, or both, for a period of time. A fast may be total or partial concerning that from which one fasts, and may be prolonged or intermittent as to the period of fasting....
" or "hungry" in Early Modern English

Early Modern English

Early Modern English is the stage of the English language used from about the end of the Middle English period to 1650. Thus, the first edition of the King James Bible and the works of William Shakespeare both belong to the late phase of Early Modern English, although the King James Bible intentionally keeps some archaisms that were not comm...
. It was so called because this portion of the intestine was found "empty" following death. [Latin: jejunus, fasting; barren]

Jejunum

Internal structure

Etymology

Additional images

External links