Jejunum and ileum

The jejunum and ileum (Latin: jejunum et ileum) are the middle and final parts of the small intestine providing further digestion and nutrient breakdown from the duodenum and absorption of various nutrients needed for a normal human body functioning. 

Both parts are located in the abdominal cavity between the first part of the small intestine (duodenumand the beginning of the large intestine (caecum with a vermiform appendix). The total length of the small intestine is around 5-6 meters (197-236 inches), including about 2,5 meters (98 inches) of the jejunum and 3,5 meters (138 inches) of the ileum. The ileum is the longest part.

Anatomy of jejunum and ileum

The jejunum is located in the umbilical and left lateral (lumbar) regions, while the ileum takes place in the right lateral (lumbar), right inguinal (iliac), and pubic (hypogastric) areas. The ileum part also lies down in the pelvis.

The narrowest parts of the intestines are at the terminal end of the ileum.

The loops of the jejunum are located more in the middle region of the abdomen and on the left side from the midline. The ileum is situated in the lower area and more on the right side. 

The jejunum is located between the duodenum and the ileum. But the ileum is situated between the jejunum and the caecum.

The jejunum makes around two-fives, while the ileum three-fives of the total length of the small intestine.

The jejunum starts at the second lumbar vertebra (L2) level at the duodenojejunal flexure. The transition zone between the ileum and jejunum is not very well demarcated. Ileum ends with ileocaecal junction, ileocecal orifice, and valve in the right iliac fossa

The ileocecal valve demarcates the transition zone between small and large intestines. It is more a functional valve, preventing the intestinal content backflow from the large intestine into the small intestine. But it can't control the movement from the ileum to the caecum. 

The valve is made up of the circular muscle layer fibers of the caecum and ileum.

The mesentery with its root provides the attachment of the ileum and jejunum to the posterior wall of the abdominal cavity along an oblique line starting from the left edge of the second lumbar vertebra (L2) level, crossing the spinal cord and ending in the right iliac fossa. 

Jejunum vs. ileum

As mentioned before, the transition zone is not very well-demarcated and expressed, but both parts have slightly different appearances. 

The jejunum is broader and better perfused with thicker intestinal walls and more red than pink in color.

The diameter of the ileum is smaller, the walls are thinner, and the color of the intestine is much paler and pinker. 

The jejunum contains fewer arterial loops than the ileum has, and they are more significant than in ileum

The mesentery of the jejunum contains less fat, and the arterial arcades are simpler. 

The mesentery of the jejunum is semitranslucent or transparent between the vasa recta, and that's why it is called the peritoneal window. The ileum doesn't contain mentioned window as its mesentery is opaque. 

The vasa recta are longer and less in number in the jejunum than in ileum.

Anatomical relations

The jejunum and ileum are surrounded by the large intestine from almost all sides.

Posterior to jejunum and ileum are located organs of retroperitoneal space.

The greater omentum covers the anterior surface of the intestines.

Intestine loops of the right side lie next to ascending colon and caecum, but the loops of the left side- to descending and sigmoid colon.

Superior to the intestine is located transverse colon and mesocolon

Inferiorly intestine slips into the lesser pelvis.

Jejunum and ileum microanatomy

The wall of the jejunum and ileum consists of four parts:

  • the mucosa,
  • submucosa,
  • muscular layer,
  • serosa.

Mucosa

The mucosa is covered by simple columnar epithelium. Underneath it, locate the lamina propria made of the connective tissue and muscular lamina made of two smooth muscle layers (inner- circular and outer- longitudinal). 

Intestinal epithelial cells are called enterocytes. Besides enterocytes, the epithelium also contains the goblet cells producing the mucus and the endocrine cells producing hormones. Goblet cell number increases in the direction of the ileum.

The mucosa, together with the submucosa, creates circular mucosal folds (Kerckring folds). The jejunum contains relatively high folds in height, but towards the ileum and in the ileum, the folds become lower and at the terminal end almost disappear.

The typical characteristic of the small intestines is the villi. Villi are mucosal growths or projections covered by mucosal epithelium. In the jejunum, they are finger-like, but they are more conical in shape and shorter in the ileum.

Villi contain:

  • epithelium,
  • connective tissue and smooth muscle fibers,
  • many blood vessels and lymphatic vessels. 

The apical end of enterocytes is covered by the microvilli, finger-like or hair-like tiny and thin protrusions or projections.

The circular folds, villi, and microvilli significantly increase the absorption area of the small intestine.

Through the enterocytes go all the absorbed substances- monosaccharides, fats, and amino acids. 

Between two villi are located crypts with Paneth cells helping in the protection from various antigenes. 

The crypt base contains simple tubular glands, also known as the intestinal crypts (Lieberkuhn). Glands sometimes can have branches.

The crypts contain various cells:

  • epithelium stem cells providing the regeneration of epithelium,
  • enterocytes which secrete and absorb multiple substances,
  • goblet cells producing the mucus,
  • Paneth cells producing antibacterial substances,
  • endocrine cells providing the secretion of hormones.

The mucosal connective tissue contains many cells- fibroblasts, macrophages, plasmocytes, lymphocytes, mast cells, and many more. Mucosa contains solitary lymphoreticular intestinal tissue. The ileum part has large lymphoreticular tissue aggregates in the mucosa. They are called Peyer's patches, and they have a continuation in the submucosa. Peyer's patches are the typical characteristic of the ileum. Sites containing mentioned patches usually have reduced crypts, villi, and muscle layers. The Payer's patches are a part of the gut-associated lymphoid tissue.

Submucosa

The submucosa contains loose, unformed connective tissue. 

The tissue contains individual adipocytes, lymphatic and blood vessels, and nerve fibers. 

The submucosa contains two submucosal nerve plexuses- the Meissner's and Henle's plexuses. 

Muscular layer

The muscular part has two layers- the inner is the circular layerbut the outer layer is made of the longitudinal direction going muscle fibers. 

The inner layer is thicker than the outer. 

Between both layers is located the myenteric nerve plexus- the Auerbach's plexus.

Serosa

The serosa is made of the visceral layer of the peritoneum. It covers the jejunum and ileum from all sides.

Jejunum and ileum functions

The jejunum and ileum primary provide the absorption of digested products into the lymph and bloodstream and transportation of unabsorbed and waste products to the large intestine.

  • Amino acids, small peptides, monosaccharides are absorbed in the bloodstream, but fats and fatty acids into the lymph and only partially into the bloodstream. 
  • Jejunum absorbs the amino acids, fatty acids and sugars, fat-soluble vitamins (K, E, D, A), cholesterol, various microelements, and other vitamins.
  • The jejunum absorbs significant amounts of water.
  • The ileum absorbs the remaining and unabsorbed nutrients. Ileum absorbs vitamin B12 with the help of the stomach produced intrinsic factor. 
  • The ileum also absorbs bile salts and acids.
  • Lymphatic tissue aggregates and Paneth cells protect against antigenes.
  • Mainly the ileum provides the immunological function as it contains Peyer's plate.

Neurovascular supply

Arterial blood supply

The arterial blood supply for the jejunum and ileum primary provides the jejunal and ileal branches from the superior mesenteric artery, abdominal aorta's branch. 

The ileocolic artery supplies the end part of the ileum. The mentioned artery is also a branch of the superior mesenteric artery.

Jejunal and ileal arteries anastomose and form many loops that are called the arcades. Arcades contain straight arteries that arise from them and go in the mesentery. Mentioned arteries are called vasa recta.

Venous drainage

The venous drainage of the jejunum and ileum is provided by small jejunal and ileal veins carrying blood to the superior mesenteric vein. 

Superior mesenteric vein drains into hepatic portal vein's system.

Lymphatic drainage

The superior mesenteric nodes provide lymphatic drainage for both small intestine parts.

Innervation

Parasympathetic innervation for the jejunum and ileum is provided by the vagus nerve (CN X). But the sympathetic innervation from the celiac plexus and superior mesenteric plexus.

Enteritis

Enteritis is an inflammation of the small intestines. It is a pretty common condition and most commonly caused by various infections

The inflammation can also be present in the stomach, and then it is called gastroenteritis. Together with the large intestine's inflammation, it is known as enterocolitis.

Causes

The most common causes include:

  • viruses, parasites, bacteria,
  • various medications (mostly nonsteroidal anti-inflammatory drugs-ibuprofen),
  • radiation therapy (usually happens weeks after the radiation treatment),
  • drugs (cocaine),
  • a large amount of alcohol consumption,
  • poor blood circulation in the small intestine can cause enteritis,
  • an inflammatory disorder caused enteritis (Crohn's disease, celiac disease).

Bacterial-caused enteritis is food poisoning via contaminated water, food, or bad hygiene. 

Eating raw meat or drinking unpasteurized milk, or eating food that has not been kept in stable conditions can lead to infections. The most common bacteria are E.coliS. aureus, also Salmonella, and C.jejuni.

Symptoms

The condition appears as diarrhea and vomiting, nausea, weakness and fatigue, appetite changes, or lossAbdominal cramps, pain, and low-grade fever can be present.

Treatment

Usually, enteritis is a self-limiting disease. It takes only a few days for a full recovery

Still, there can be a need for antibiotic treatment in bacterial etiology or elimination of the medications if that is the causing agent.

The best treatment is drinking lots of water frequently in small sips as the fluid loss can lead to dehydration. Also, taking some rest and eating small portions of easily digestible food can promote recovery.

Prevention

To prevent enteritis, practice good hygiene, wash your hands frequently, prepare and cook your food correctlyStore food under the right conditions, avoid usage of alcohol and nonsteroidal anti-inflammatory drugs.

Summary on jejunum and ileum

Which part of the intestine is the jejunum?

The jejunum is the middle part of the small intestine between the duodenum and ileum.

Where does jejunum begin?

The jejunum starts at the second lumbar vertebra (L2) level at the duodenojejunal flexure.

How long is jejunum?

The jejunum is around 2,5 meters (98 inches) long.

What is the primary function of the jejunum?

The primary function of the jejunum is absorption. Jejunum absorbs the amino acids, fatty acids and sugars, fat-soluble vitamins (K, E, D, A), cholesterol, various microelements, other vitamins, and water.

What part of the intestines is the ileum?

The ileum is the final or the third part of the small intestine between the jejunum and the large intestine's first part- the caecum.

What comes after the ileum?

The large intestine with caecum follows after the ileum.

Where is the distal ileum located?

The distal ileum is located in the right inguinal region and ends in the right iliac fossa.

Is ileum a part of the small intestine?

Yes, the ileum is a part of the small intestine, and it is the final part of it.

What is the difference between the jejunum and ileum?

The jejunum is broader and better perfused with a thicker intestinal wall and more red than pink in color. The ileum diameter is smaller, the wall is thinner, and the intestine is much paler and pinker. The jejunum contains fewer arterial loops, and they are more significant than in ileum. The mesentery of the jejunum has less fat, and the arterial arcades are simple. The vasa recta are longer than in ileum. The jejunum contains relatively high folds in height, but towards the ileum and in the ileum, the folds become lower and at the terminal end almost disappear. In the jejunum, villi are finger-like, but they are more conical in shape and shorter in the ileum.

What is terminal ileum?

The terminal ileum is the final or distal part of the ileum.

What is the ileocecal valve?

The ileocecal valve is located in the right iliac fossa, and it is the connection site between the ileum and caecum. The valve connects the small intestine with the large.

What is the function of the ileum?

The primary function of the ileum is the absorption of nutrients. The ileum absorbs the remaining and unabsorbed nutrients. Ileum absorbs vitamin B12 with the help of the stomach produced intrinsic factor. The ileum also absorbs bile salts and acids.

What happens to food in the ileum?

The ileum absorbs remaining and unabsorbed nutrients before transporting the intestinal content further to the large intestine.

What are Payer patches in the ileum?

The Payer patches are relatively big lymphoreticular tissue aggregates. They lie in the mucosal part with a continuation in the submucosa. 

What is exclusively absorbed in the ileum?

Ileum absorbs vitamin B12 with the help of the stomach produced intrinsic factor.

What is enteritis?

Enteritis is an inflammation of the small intestines.

What causes human enteritis?

The most common causes include viruses, parasites, bacteria, various medications (primarily nonsteroidal anti-inflammatory drugs (ibuprofen)), radiation therapy, drugs (cocaine), a large amount of alcohol consumption. Poor blood circulation in the small intestine and inflammatory disorders (Crohn's disease, celiac disease) can cause enteritis.

Does enteritis go away on its own?

Yes, enteritis can go away on its own as it usually is a self-limiting disease.

What is the difference between gastritis and enteritis?

Enteritis is an inflammation of the small intestines, while gastritis is an inflammation and irritation of the stomach wall's inner lining.

How do you treat enteritis?

Usually, enteritis is a self-limiting disease, and it takes only a few days for a full recovery. But there can be a need for antibiotic treatment in bacterial etiology or elimination of the medications if that is the causing agent. The best treatment is drinking lots of water frequently in small sips as the fluid loss can lead to dehydration. Also, taking some rest and eating small portions of easily digestible food can promote recovery.