The
jejunum is the middle section of the
small intestineIn vertebrates, the small intestine is the part of the gastrointestinal tract following the stomach and followed by the large intestine, and is where the vast majority of digestion and absorption of food takes place. In invertebrates such as worms, the terms "gastrointestinal tract" and "large...
in most higher vertebrates, including mammals, reptiles, and birds. In
fishA fish is any aquatic vertebrate animal that is typically ectothermic , covered with scales, 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
duodenumThe 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
ileumThe 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 ligament of Treitz.
In adult
humanHumans are bipedal primates belonging to the species Homo sapiens in Hominidae, the great ape family. They are the only surviving member of the genus Homo. Humans have a highly developed brain, capable of abstract reasoning, language, introspection, and problem solving...
s, the small intestine is usually between 5.5-6m long, 2.5m of which is the jejunum.
The
pHpH is a measure of the acidity or basicity 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
mesenteryIn anatomy, the mesentary 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.-Mesentary :The...
which gives the bowel great mobility within the
abdomenIn 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...
.
The
jejunum is the middle section of the
small intestineIn vertebrates, the small intestine is the part of the gastrointestinal tract following the stomach and followed by the large intestine, and is where the vast majority of digestion and absorption of food takes place. In invertebrates such as worms, the terms "gastrointestinal tract" and "large...
in most higher vertebrates, including mammals, reptiles, and birds. In
fishA fish is any aquatic vertebrate animal that is typically ectothermic , covered with scales, 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
duodenumThe 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
ileumThe 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 ligament of Treitz.
In adult
humanHumans are bipedal primates belonging to the species Homo sapiens in Hominidae, the great ape family. They are the only surviving member of the genus Homo. Humans have a highly developed brain, capable of abstract reasoning, language, introspection, and problem solving...
s, the small intestine is usually between 5.5-6m long, 2.5m of which is the jejunum.
The
pHpH is a measure of the acidity or basicity 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
mesenteryIn anatomy, the mesentary 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.-Mesentary :The...
which gives the bowel great mobility within the
abdomenIn 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 is wrapped by smooth muscle that helps to move food along by a process known as
peristalsisPeristalsis is a radially symmetrical contraction of muscles which propagates in a wave down the muscular tube. In humans, peristalsis is found in the contraction of smooth muscles to propel contents through the digestive tract. Earthworms use a similar mechanism to drive their locomotion...
.
Internal structure
The inner surface of the jejunum, its
mucous membraneThe mucous membranes are linings of mostly endodermal 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 organs. It is at several places continuous with skin: at the nostrils, the...
, 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 transportPassive transport means moving biochemicals and atomic or molecular substances across the cell membrane. Unlike active transport, this process does not involve chemical energy....
of sugar fructose and the
active transportActive transport is the mediated process of moving particles across a biological membrane against a concentration gradient. If the process uses chemical energy, such as from adenosine triphosphate , it is termed primary active transport. Secondary active transport involves the use of an...
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 glandsBrunner glands are compound tubular submucosal glands found in that portion of the duodenum which is above the sphincter of Oddi...
(found in the duodenum) or
Peyer's patchesPeyer's patches are organized 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 ileum in humans; as such, they differentiate the ileum from the duodenum and...
(found in the ileum). Instead, it has many large
circular foldsThe circular folds are large valvular flaps projecting into the lumen of the bowel.-Composition:...
in its
submucosaIn 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 "
fastingFasting 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 EnglishEarly Modern English is the stage of the English language used from about the end of the Middle English period to 1650...
.
It was so called because this portion of the intestine was found "empty" following death. [Latin: jejunus, fasting; barren]
External links