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 and jejunum and is separated from the cecum by the ileocecal
valve.
In humans, the ileum is about 2–4 m long, and the pH is usually between 7 and 8.
Ileum is derived from the Greek word eilein, meaning "to twist up tightly."
Structure The ileum is part of the small intestine that
is a continuation from the jejunum.
The ileum surrounded by mesentery.
It connects with the cecum, the first part of the large intestine, at the ileocecal junction.
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 mesenteric and an anti-mesenteric vascular supply, while the jejunum has only
a mesenteric vasculature 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.
Histology
Embryology In the fetus the ileum is connected to the
navel by the vitelline duct.
In roughly 2% of humans, this duct fails to close during the first seven weeks after birth,
causing a condition called Meckel's diverticulum.
Function The function of the ileum is mainly to absorb
vitamin B12 and bile salts and whatever products of digestion were not absorbed by the jejunum.
The wall itself is made up of folds, each of which has many tiny finger-like projections
known as villi on its surface.
In turn, the epithelial cells that line these villi possess even larger numbers of microvilli.
Therefore the ileum has an extremely large surface area both for the adsorption of enzyme
molecules and for the absorption of products of digestion.
The DNES cells of the ileum secrete various hormones into the blood.
Cells in the lining of the ileum secrete the protease and carbohydrase enzymes responsible
for the final stages of protein and carbohydrate digestion into the lumen of the intestine.
These enzymes are present in the cytoplasm of the epithelial cells.
The villi contain large numbers of capillaries that take the amino acids and glucose produced
by digestion to the hepatic portal vein and the liver.
Lacteals are small lymph vessels, and are present in villi.
They absorb fatty acid and glycerol, the products of fat digestion.
Layers of circular and longitudinal smooth muscle enable the chyme to be pushed along
the ileum by waves of muscle contractions called peristalsis.
The remaining chyme is passed to the colon.
Other animals In veterinary anatomy, the ileum is distinguished
from the jejunum by being that portion of the jejunoileum that is connected to the caecum
by the ileocecal fold.
The ileum is the short terminal part of the small intestine and forms the connection to
the large intestine.
It is suspended by the caudal part of the mesentery and is attached, in addition, to
the cecum by the ileocecal fold.
The ileum terminates at the cecocolic junction of the large intestine forming the ileal orifice.
In the dog the ileal orifice is located at the level of the first or second lumbar vertebra,
in the ox in the level of the fourth lumbar vertebrae, in the sheep and goat at the level
of the caudal point of the costal arch.
By active muscular contraction of the ileum, and closure of the ileal opening as a result
of engorgement, the ileum prevents the backflow of ingesta and the equalization of pressure
between jejunum and the base of the cecum.
Disturbance of this sensitive balance is not uncommon and is one of the causes of colic
in horses.
During any intestinal surgery, for instance, during appendectomy, distal 2 feet of ileum
should be checked for the presence of Meckel's diverticulum.
References
External links Anatomy photo:37:11-0101 at the SUNY Downstate
Medical Center – "Abdominal Cavity: The Jejunum and the Ileum"
Anatomy image:7787 at the SUNY Downstate Medical Center
Anatomy image:8755 at the SUNY Downstate Medical Center
Histology image: 12001oca - Histology Learning System at Boston University
Ileal Villi at endoatlas.com Ileum Microscopic Cross Section at nhmccd.edu
Ileum 20x at deltagen.com