Jejunum and ileum

The jejunum and ileum (Latin: jejunum et ileum) are the middle and final portions of the small intestine, and they are parts of the digestive system. The jejunum and ileum are located in the abdominal cavity between the first portion of the small intestine called the duodenum and the proximal part of the large intestine formed by the cecum and the vermiform appendix. The jejunum and ileum provide further digestion and nutrient breakdown of the digested food coming from the duodenum. And they are also responsible for the absorption of various nutrients needed for the normal functioning of the human body.

 

Jejunum and ileum anatomy

The loops of the jejunum are situated more in the middle area of the abdomen (umbilical region) and on the left side (left lateral region). The ileum is positioned in the lower area and more on the right side (right lateral, right inguinal and hypogastric regions). The loops of the ileum also extend in the pelvis. The jejunum is located between the duodenum and the ileum. But the ileum is situated between the jejunum and the cecum.

The jejunum forms around two-fifths, while the ileum is three-fifths of the total length of the small intestine. The small intestine is around 5 - 6 meters long. The jejunum is about 2.5 meters, but the ileum is 3.5 meters long. Therefore, the ileum is the longest part of the small intestine. The narrowest portions of the small intestine can be found at the terminal part of the ileum.

The jejunum begins at the duodenojejunal flexure that projects at the second lumbar vertebra (L2) level. The transition zone between the ileum and jejunum is not very well demarcated. However, the jejunum and ileum do not look the same, and knowing the differences between them, makes it easier to distinguish both parts. The ileum ends at the ileocecal junction with the ileocecal orifice and valve in the right iliac fossa.

The ileocecal valve demarcates the transition zone between the small and large intestines. The ileocecal valve is a functional valve that prevents the intestinal content backflow from the large intestine into the small intestine. But it can not control movements from the ileum to the cecum. The ileocecal valve is made up of the circular muscle fibers of the cecum and ileum.

The mesentery with its root attaches the ileum and jejunum to the posterior wall of the abdominal cavity along an oblique line. The oblique line starts from the left side of the second lumbar vertebra (L2), crosses the spine and ends in the right iliac fossa.

 

Differences between jejunum and ileum

As previously mentioned, the transition zone between the jejunum and ileum is not very well-demarcated and expressed. However, both parts have slightly different appearances.

  • The jejunum is broader and better perfused with thicker intestinal walls. Therefore, it is more red than pink in color. The diameter of the ileum is smaller, the walls are thinner, and it looks much paler and pinker.
  • The jejunum has fewer arterial loops, but they appear more significant than those of the ileum. The arterial arcades of the jejunum are simpler. The vasa recta of the jejunum are longer and lesser in number.
  • The mesentery of the jejunum contains less fat, and it appears semi-translucent or transparent between the vasa recta. This characteristic is the reason why the mesentery of the jejunum between the vasa recta is called the peritoneal window. The ileum does NOT contain the mentioned window, and its mesentery is opaque.

 

Anatomical relations

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

  • Posterior to the jejunum and ileum are located the organs of the retroperitoneal space.
  • The anterior aspect of the intestines is covered by the greater omentum.
  • The loops of the jejunum and ileum lie next to the ascending colon and cecum on the right side, but the loops on the left side connect with the descending colon and sigmoid colon.
  • Superior to the jejunum and ileum are the transverse colon and mesocolon.
  • And finally, inferior, the loops of the small intestine slip into the pelvis.

 

Jejunum and ileum histology

The walls of the jejunum and ileum are composed of four histological layers:

  • Mucosa
  • Submucosa
  • Muscular layer
  • Serosa

 

Mucosa

The mucosa is composed of the epithelium and the lamina propria. It is lined by the simple columnar epithelium. Underneath the epithelium is located lamina propria made of connective tissue. The epithelial cells of the intestines are called enterocytes. Besides the enterocytes, the epithelium also contains the goblet cells producing mucus and the endocrine cells producing hormones. The amount of the goblet cells increases towards the ileum.

Together with the submucosa, the mucosa creates circular mucosal folds called the valves of Kerckring. The jejunum contains relatively high circular folds, but towards the ileum, they become lower and almost disappear at the terminal part of the ileum. Besides the folds, another typical characteristic of the small intestines is the villi.

The villi are mucosal projections covered by the epithelium. The jejunum has long finger-like villi, but they are more conical-shaped and shorter in the ileum. The villi contain an epithelial layer, connective tissue with smooth muscle fibers and many blood and lymphatic vessels. The apical ends of the enterocytes are covered by other projections - the microvilli. They appear as finger-like or hair-like tiny and thin protrusions.

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 crypts with Paneth cells that protect the intestines from various antigens. The base of the crypt contains simple tubular glands, also known as the intestinal Lieberkuhn crypts. The intestinal crypts contain the following cells:

  • Epithelium stem cells - provide the regeneration of epithelium;
  • Enterocytes - secrete and absorb multiple substances;
  • Goblet cells - produce mucus;
  • Paneth cells - produce antibacterial substances;
  • Endocrine cells - provide the secretion of hormones.

The mucosal connective tissue (lamina propria) contains fibroblasts, macrophages, plasmocytes, lymphocytes, mast cells, and many others. The mucosa also contains solitary lymphoreticular intestinal tissue. The mucosa of the ileum has large lymphoreticular tissue aggregates called the Peyer's patches. Usually, they continue in the submucosa. The Peyer's patches are the typical characteristic of the ileum. Areas containing Peyer's 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 consists of loose connective tissue that contains adipocytes, nerve fibers, lymphatic and blood vessels. Within the submucosa are located two submucosal nerve plexuses - Meissner's and Henle's plexuses.

 

Muscular layer

The muscular layer can be divided into two portions. The inner part is the circular layer, while the outer part consists of the longitudinal muscle fibers. Therefore it is called the longitudinal layer. The circular layer is thicker than the longitudinal, and between both is located the myenteric nerve plexus - Auerbach's plexus.

 

Serosa

The serosa is formed by the visceral layer of the peritoneum that covers the jejunum and ileum from all sides.

 

Jejunum and ileum functions

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

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

 

Neurovascular supply of jejunum and ileum

Arterial blood supply

The arterial blood supply to the jejunum and ileum is primarily provided by the jejunal and ileal arteries of the superior mesenteric artery, which is a branch of the abdominal aorta. Also, the ileocolic artery of the superior mesenteric artery supplies the terminal part of the ileum.

The jejunal and ileal arteries anastomose and form many arterial loops called arcades. The arcades give off straight arteries that go within the mesentery towards the intestine. These straight arteries are called the vasa recta.

 

Venous drainage

The venous drainage of the jejunum and ileum is provided by small jejunal and ileal veins that carry blood to the superior mesenteric vein. And further, the superior mesenteric vein drains into the portal vein.

 

Lymphatic drainage

The lymph from both small intestine parts is drained to the superior mesenteric lymph nodes.

 

Innervation

The parasympathetic innervation to the jejunum and ileum is provided by the vagus nerve (CN X). But the sympathetic innervation comes from the celiac plexus and superior mesenteric plexus.

 

Enteritis

Enteritis is an inflammation of the small intestine, and it is a pretty common condition that is most commonly caused by various infections. Inflammation can spread, and it can also affect the stomach, and then it is called gastroenteritis. Besides the stomach, inflammation can also spread to the large intestine, and then it is known as enterocolitis.

 

Risk factors

Many risk factors cause enteritis, and the most common causes include the following:

  • Viruses, parasites and bacteria
  • Various medications (mostly nonsteroidal anti-inflammatory drugs such as ibuprofen)
  • Radiation therapy (usually manifests weeks after the treatment)
  • Drugs (cocaine)
  • Excessive alcohol consumption
  • Poor blood circulation in the small intestine
  • Various inflammatory bowel diseases (Crohn's disease, celiac disease)

Bacterial enteritis usually manifests as food poisoning caused by contaminated water, food or bad hygiene. Eating raw meat, drinking unpasteurized milk or eating food that has not been kept in suitable conditions can lead to inflammation. The most common bacteria causing enteritis include E. coli, S. aureus, Salmonella and C. jejuni.

 

Symptoms

Symptoms of enteritis include diarrhea, vomiting, nausea, weakness, fatigue, appetite changes and appetite loss. Sometimes abdominal cramps, pain and low-grade fever can be present.

 

Treatment

Enteritis usually is a self-limiting disease, it lasts only a few days, and there is no need for a specific treatment. However, antibiotic treatment might be needed as the causes include bacteria. The best possible treatment is drinking lots of water frequently in small sips as the fluid loss (caused by vomiting, diarrhea and sweating) can lead to dehydration. Also, taking some rest and eating small portions of easily digestible food can speed up the recovery.

 

Prevention

The best possible prevention is practicing good hygiene, washing hands frequently and properly preparing and cooking food. The food must be stored under the right conditions. Also, avoiding alcohol and nonsteroidal anti-inflammatory drugs can prevent enteritis.