Jejunum
Overview
 
The jejunum is the middle section of the small intestine
Small intestine
The small intestine is the part of the gastrointestinal tract following the stomach and followed by the large intestine, and is where much of the digestion and absorption of food takes place. In invertebrates such as worms, the terms "gastrointestinal tract" and "large intestine" are often used to...

 in most higher vertebrates, including mammals, reptiles, and birds. In fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

, 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 ileum follows the duodenum...

. The change from the duodenum to the jejunum is usually defined as the Duodenojejunal flexure
Duodenojejunal flexure
The duodenojejunal flexure or duodenojejunal junction is the border between the duodenum and the jejunum.The ascending portion of the duodenum ascends on the left side of the aorta, as far as the level of the upper border of the second lumbar vertebra, where it turns abruptly forward to become the...

 and is attached, and thus "hung up", to the ventricle (see stomach
Stomach
The stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...

) by the ligament of Treitz.

In adult human
Human
Humans are the only living species in the Homo genus...

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

The pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

 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
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.-Mesentery :The...

 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...

.
Encyclopedia
The jejunum is the middle section of the small intestine
Small intestine
The small intestine is the part of the gastrointestinal tract following the stomach and followed by the large intestine, and is where much of the digestion and absorption of food takes place. In invertebrates such as worms, the terms "gastrointestinal tract" and "large intestine" are often used to...

 in most higher vertebrates, including mammals, reptiles, and birds. In fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

, 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 ileum follows the duodenum...

. The change from the duodenum to the jejunum is usually defined as the Duodenojejunal flexure
Duodenojejunal flexure
The duodenojejunal flexure or duodenojejunal junction is the border between the duodenum and the jejunum.The ascending portion of the duodenum ascends on the left side of the aorta, as far as the level of the upper border of the second lumbar vertebra, where it turns abruptly forward to become the...

 and is attached, and thus "hung up", to the ventricle (see stomach
Stomach
The stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...

) by the ligament of Treitz.

In adult human
Human
Humans are the only living species in the Homo genus...

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

The pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

 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
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.-Mesentery :The...

 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...

. It also contains circular and longitudinal smooth muscle
Smooth muscle
Smooth muscle is an involuntary non-striated muscle. It is divided into two sub-groups; the single-unit and multiunit smooth muscle. Within single-unit smooth muscle tissues, the autonomic nervous system innervates a single cell within a sheet or bundle and the action potential is propagated by...

 which helps to move food along by a process known as peristalsis
Peristalsis
Peristalsis is a radially symmetrical contraction and relaxation of muscles which propagates in a wave down the muscular tube, in an anterograde fashion. In humans, peristalsis is found in the contraction of smooth muscles to propel contents through the digestive tract. Earthworms use a similar...

.

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.

Internal structure

The inner surface of the jejunum, its mucous membrane
Mucous membrane
The mucous membranes are linings of mostly endodermal origin, covered in epithelium, which are involved in absorption and secretion. They line cavities that are exposed to the external environment and internal organs...

, 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
Passive transport
Passive transport means moving biochemicals and other atomic or molecular substances across membranes. Unlike active transport, this process does not involve chemical energy, because, unlike in an active transport, the transport across membrane is always coupled with the growth of entropy of the...

 of sugar fructose and the active transport
Active transport
Active transport is the movement of a substance against its concentration gradient . In all cells, this is usually concerned with accumulating high concentrations of molecules that the cell needs, such as ions, glucose, and amino acids. If the process uses chemical energy, such as from adenosine...

 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's glands are compound tubular submucosal glands found in that portion of the duodenum which is above the hepatopancreatic sphincter...

 (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
Circular folds
The circular folds are large valvular flaps projecting into the lumen of the bowel.-Composition:...

 in its submucosa
Submucosa
In the gastrointestinal tract, the submucosa is the layer of dense irregular connective tissue or loose connective tissue that supports the mucosa, as well as joins the mucosa to the bulk of underlying smooth muscle .-Contents:Blood vessels, lymphatic vessels, and nerves will run through...

 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 lymphocyte
    Lymphocyte
    A lymphocyte is a type of white blood cell in the vertebrate immune system.Under the microscope, lymphocytes can be divided into large lymphocytes and small lymphocytes. Large granular lymphocytes include natural killer cells...

    s and other cells of the immune system
    Immune system
    An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...

    .

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. An absolute fast is normally defined as abstinence from all food and liquid for a defined period, usually a single day , or several days. Other fasts may be only partially restrictive,...

" 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...

.
It was so called because this portion of the intestine was found "empty" following death. [Latin: jejunus, fasting; barren]

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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