The
ileum is the final 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
posterior intestine or
distal intestine may be used instead of ileum.
The ileum follows 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
jejunumThe 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.The jejunum lies between the duodenum...
, and is separated from the
cecumThe cecum or caecum is a pouch, connecting the ileum with the ascending colon of the large intestine. 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 valveThe 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
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...
is usually between 7 and 8 (neutral or slightly alkaline).
Function
The function of the ileum is mainly to absorb
vitamin B12Vitamin 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. It is one of the eight B vitamins...
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
adsorptionAdsorption is the accumulation of atoms or molecules on the surface of a material. This process creates a film of the adsorbate on the adsorbent's surface. It is different from absorption, in which a substance diffuses into a liquid or solid to form a solution...
(attachment) of
enzymeEnzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process are called substrates, and the enzyme converts them into different molecules, called the products. Almost all processes in a biological cell need enzymes to occur at...
molecules and for the
absorptionAbsorption, 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. This is a different process from adsorption, since molecules undergoing absorption are taken up by the volume, not by the surface...
of products of
digestionDigestion is the mechanical and chemical breaking down of food into smaller components, to a form that can be absorbed, for instance, by a blood stream. Digestion is a form of catabolism.that means break down of macro food molecules to smaller one....
. The DNES (diffuse neuroendocrine system) cells of the ileum secrete various hormones (gastrin, secretin, cholecystokinin) into the blood. Cells in the lining of the ileum secrete the protease and carbohydrase enzymes responsible for the final stages of
proteinProteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer chain are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues...
and
carbohydrateCarbohydrates
[Means "hydrates of carbon"] or saccharides
[The word comes from the Greek σάκχαρον, sákcharon, meaning "sugar").] are the most abundant of the four major classes of biomolecules...
digestion into the lumen of the intestine. These enzymes are present in the
cytoplasmThe cytoplasm is the part of a cell that is enclosed within the cell membrane. In eukaryotic cells, the cytoplasm contains organelles, such as mitochondria, which are filled with liquid that is kept separate from the rest of the cytoplasm by biological membranes. The contents of the cell nucleus...
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 veinThe 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 and splenic veins, and also receives blood from the inferior mesenteric, gastric, and cystic veins...
and the liver. Lacteals are small lymph vessels, and are present in villi. They absorb
fatty acidIn chemistry, especially biochemistry, a fatty acid is a carboxylic acid often with a long unbranched aliphatic tail , which is either saturated or unsaturated...
and
glycerolGlycerol is an organic compound, also commonly called glycerin or glycerine. It is a colorless, odorless, viscous liquid that is widely used in pharmaceutical formulations. Glycerol has three hydrophilic hydroxyl groups that are responsible for its solubility in water and its hygroscopic nature. ...
, the products of fat digestion. Layers of circular and longitudinal
smooth muscleSmooth muscle is an involuntary non-striated muscle, found within the tunica media layer of large and small arteries and veins, the bladder, uterus, male and female reproductive tracts, gastrointestinal tract, respiratory tract, arrector pili of skin, the ciliary muscle, and iris of...
enable the digested food to be pushed along the ileum by waves of muscle contractions called
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...
.
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 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...
.
These unencapsulated lymphoid nodules contain large amounts of
lymphocyteA 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. Functionally distinct subsets of lymphocytes correlate with...
s and other cells of the
immune systemAn immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumour cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...
.
Embryology
In the
fetusA fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth. The plural is fetuses....
the ileum is connected to the
navelThe navel is a scar on the abdomen, caused when the umbilical cord is removed from a newborn baby. All placental mammals have a navel...
by the
vitelline ductIn the human embryo, the vitelline duct, also known as the omphalomesenteric duct, is a long narrow tube that joins the yolk-sac to the digestive tube...
. In roughly 3% of humans, this duct fails to close during the first seven weeks after birth, causing a condition called
Meckel's diverticulumA 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 foldThe ileocecal fold is an anatomical structure in the human abdomen. It is formed by a layer of peritoneum. The upper border is fixed to the ileum, opposite its mesenteric attachment, while the lower border passes over the ileocecal junction to join the mesenteriole of the vermiform process, and...
.
External links
- "Abdominal Cavity: The Jejunum and the Ileum"