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The ileum is the final 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 posterior intestine or distal intestine may be used instead of ileum.

The ileum follows 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 jejunum
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....
, and is separated from the cecum
Cecum

The cecum or caecum is a pouch connected to the ascending colon of the large intestine and the ileum. It is separated from the ileum by the ileocecal valve or Bauhin's valve, and is considered to be the beginning of the large intestine....
 by the ileocecal valve
Ileocecal valve

The ileocecal valve is a sphincter muscle situated at the junction of the small intestine and the large intestine. Its critical function is to limit the reflux of colonic contents into the ileum....
 (ICV). In humans, the ileum is about 2-4 m long, and the pH
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....
 is usually between 7 and 8 (neutral or slightly alkaline).

function of the ileum is mainly to absorb vitamin B12
Vitamin B12

Vitamin B12 is a water soluble vitamin with a key role in the normal functioning of the brain and nervous system, and for the formation of blood....
 and bile salts and whatever products of digestion were not absorbed by the jejunum.






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The ileum is the final 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 posterior intestine or distal intestine may be used instead of ileum.

The ileum follows 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 jejunum
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....
, and is separated from the cecum
Cecum

The cecum or caecum is a pouch connected to the ascending colon of the large intestine and the ileum. It is separated from the ileum by the ileocecal valve or Bauhin's valve, and is considered to be the beginning of the large intestine....
 by the ileocecal valve
Ileocecal valve

The ileocecal valve is a sphincter muscle situated at the junction of the small intestine and the large intestine. Its critical function is to limit the reflux of colonic contents into the ileum....
 (ICV). In humans, the ileum is about 2-4 m long, and the pH
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....
 is usually between 7 and 8 (neutral or slightly alkaline).

Function

The function of the ileum is mainly to absorb vitamin B12
Vitamin B12

Vitamin B12 is a water soluble vitamin with a key role in the normal functioning of the brain and nervous system, and for the formation of blood....
 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 which line these villi possess even larger numbers of microvilli. Therefore the ileum has an extremely large surface area both for the adsorption
Adsorption

Adsorption is a process that occurs when a gas or liquid solute accumulates on the surface of a solid or a liquid , forming a film of molecules or atoms ....
 (attachment) of enzyme
Enzyme

Enzymes are biomolecules that catalysis chemical reactions. Almost all enzymes are proteins. In enzymatic reactions, the molecules at the beginning of the process are called Substrate , and the enzyme converts them into different molecules, the products....
 molecules and for the absorption
Absorption (chemistry)

File:Absorber.svgAbsorption, in chemistry, is a physical or chemical phenomenon or a Process in which atoms, molecules, or ions enter some bulk phase - gas, liquid or solid material....
 of products of digestion
Digestion

Digestion is the mechanical and chemical breaking down of food into smaller components, to a form that can be Absorption, for instance, by a blood stream....
. The DNES (diffuse neuroendocrine system) cells that line the ileum contain the protease and carbohydrase enzymes (gastrin, secretin, cholecystokinin) responsible for the final stages of protein
Protein

Proteins are organic compounds made of amino acids arranged in a linear chain and joined together by peptide bonds between the carboxyl and amino groups of adjacent amino acid Residue ....
 and carbohydrate
Carbohydrate

Carbohydrates or saccharides are the most abundant of the four major classes of biomolecules. They fill numerous roles in living things, such as the storage and transport of energy and structural components ....
 digestion. These enzymes are present in the cytoplasm
Cytoplasm

The cytoplasm is the part of a Cell that is enclosed within the plasma membrane. In eukaryote cells the cytoplasm contains organelles, such as mitochondrion, that are filled with liquid kept separate from the rest of the cytoplasm by biological membranes....
 of the epithelial cells. The villi contain large numbers of capillaries which take the amino acids and glucose produced by digestion to the hepatic portal vein
Hepatic portal vein

The hepatic portal vein is a vein in the abdominal cavity that drains blood from the gastrointestinal tract and spleen. It is usually formed by the confluence of the superior mesenteric vein and splenic veins, and also receives blood from the inferior mesenteric vein, gastric vein, and cystic veins....
 and the liver.

Lacteals are small lymph vessels, and are present in villi. They absorb fatty acid
Fatty acid

In chemistry, especially biochemistry, a fatty acid is a carboxylic acid often with a long unbranched aliphatic tail , which is either saturation or Unsaturated compound....
 and glycerol
Glycerol

Glycerol is a chemical compound also commonly called glycerin or glycerine. It is a colorless, odorless, Viscosity liquid that is widely used in pharmaceutical formulations....
, the products of fat digestion. Layers of circular and longitudinal smooth muscle
Smooth muscle

Smooth muscle is a type of non-striated muscle, found within the tunica media layer of large and small arteries and veins, the urinary bladder, uterus, male and female reproductive tracts, gastrointestinal tract, respiratory tract, the ciliary muscle, and iris of the eye....
 enable the digested food to be pushed along the ileum by waves of muscle contractions called peristalsis
Peristalsis

Peristalsis is the rhythmic contraction of smooth muscles to propel contents through the digestive tract. The word is derived from New Latin and comes from the Greek language peristaltikos, peristaltic, from peristellein, "to wrap around," and stellein, "to place."...
..

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 is 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
    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...
    .


These unencapsulated lymphoid nodules contain large amounts of lymphocyte
Lymphocyte

A lymphocyte is a type of white blood cell in the vertebrate immune system.By their appearance under the light microscope, there are two broad categories of lymphocytes, namely the large granular lymphocytes and the small lymphocytes....
s and other cells of the immune system
Immune system

An immune system is a collection of biological processes within an organism that protects against disease by identifying and killing pathogens and tumour cells....
.

Embryology

In the fetus
Fetus

A fetus is a developing mammal or other viviparous vertebrate, after the embryonic stage and before childbirth. The plural is fetuses, or sometimes feti....
 the ileum is connected to the navel
Navel

The navel is a scar on the abdomen, caused when the umbilical cord is removed from a newborn baby. All Placentalia mammals have a navel. It is fairly conspicuous in humans....
 by the vitelline duct
Vitelline duct

At the end of the fourth week the yolk-sac presents the appearance of a small pear-shaped vesicle opening into the digestive tube by a long narrow tube, the vitelline duct, also known as the omphalomesenteric duct....
. In roughly 3% of humans, this duct fails to close during the first seven weeks after birth, causing a condition called Meckel's diverticulum
Meckel's diverticulum

A Meckel's diverticulum, a true congenital diverticulum, is a small bulge in the small intestine present at birth. It is a vestigial remnant of the omphalomesenteric duct , and is the most frequent malformation of the gastrointestinal tract....
.

Veterinary anatomy

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
Ileocecal fold

The inferior ileocecal fossa is situated behind the angle of junction of the ileum and cecum. It is formed by the ileocecal fold of peritoneum , the upper border of which is fixed to the ileum, opposite its mesenteric attachment, while the lower border, passing over the ileocecal junction, joins the mesenteriole of the vermiform process, and...
.

Additional images


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