The ileum is the last section of the small intestine, separated from the large intestine by the so-called ileocecal valve. On the other side, however, it emerges from the jejunum without a sharp boundary.
What is the ileum?
The ileum, also known as the ileum, represents the third and final part of the small intestine. It follows the jejunum with no discernible boundary and ends before the so-called Bauhin’s valve (ileocecal valve), which separates the small and large intestines. The ileum, together with the jejunum and duodenum, performs the functions of the small intestine. In particular, the ileum and the jejunum together form a functional unit. Their fine tissue structure changes only gradually from the end of the duodenum to the ileocecal valve. Thus, a clear boundary between the two sections of the small intestine cannot be discerned. Within this section, nutrients are absorbed from the food pulp. However, adapted to the change in the composition of the food pulp during the small intestinal passage, the size, shape, and number of the intestinal villi and other fine-tissue structures change, especially within the ileum.
Anatomy and structure
The ileum, along with the jejunum, is attached to the abdomen via the mesenteries. There it is supplied with blood by the ileal arteries, which arise from the superior mesenteric artery. In humans, the ileum has a length of about three meters and thus accounts for 60 percent of the length of the small intestine. The differences between the ileum and the preceding jejunum are inconspicuous. For example, the ileum is somewhat paler and has a slightly smaller diameter. The mesentery of the ileum, however, is somewhat more fatty. Most striking, however, is the fact that the ileum, in contrast to the jejunum terminal, contains a large number of Peyer’s plaques. Peyer’s plaques are closely spaced lymphoid follicles. Their function is to defend against germs ingested with food. Furthermore, the ileum contains only very few cricoid folds, which are responsible for intestinal peristalsis. Towards the end, the intestinal villi also disappear because the food pulp no longer contains any absorbable nutrients before entering the colon. Analwardly, the ileum closes with the so-called Bauhin’s valve (ileocecal valve). The ileocecal valve is a functional sphincter that arises from the ring muscle layers of the scrotum and the appendix (ceacum) preceding the colon. Its purpose is to prevent the reflux of bacteria-rich indigestible food debris from the colon into the sterile ileum.
Function and Tasks
The ileum, like the preceding jejunum, has the task of continuing to absorb nutrients from the food pulp. This requires a large surface area of the intestinal mucosa, which is provided by the intestinal villi and microvilli. However, the intestinal villi become smaller and smaller in the direction of the large intestine, and in the end they disappear completely because there are no more absorbable nutrients in the food pulp in the terminal region of the ileum. Instead, vitamin B12 (cobalamin) and bile acids are absorbed here by the intestinal mucosa in addition to the unchanged absorption of water. Vitamin B12 is responsible for blood formation, cell division and the function of the nervous system. This underlines the special importance of the ileum, because an absorption disorder of vitamin B12 leads to pernicious anemia (malignant anemia). The absorption of vitamin B12 takes place with the help of the intrinsic factor. The intrinsic factor is a glyco-protein that binds cobalamin to protect it from the digestive enzymes pepsin and trypsin produced in the stomach. This protein is in turn produced by the gastric mucosal vestibular cells. Furthermore, the small intestine absorbs a total of 80 percent of the water from the food pulp. However, this applies equally to all sections of the small intestine. Last but not least, defense reactions against bacteria ingested with food take place in the ileum with the help of lymphoid follicles (Peyer’s plaques).
Diseases
Diseases of the ileum usually do not occur in isolation. Other areas of the intestine are usually affected as well. Inflammations of the bowel may be infectious or noninfectious in nature. Rarely can a satisfactory diagnosis be made on the basis of symptoms alone. For example, both small bowel and large bowel inflammation cause similar symptoms. Inflammation of the small intestine is referred to as enteritis. If the stomach is involved, for example, it is gastroenteritis.If the colon is affected in addition to the small intestine, enterocolitis is present. Infectious bacteria that can trigger enteritis include salmonella, shigella, clostridia or Escherichia coli. Viruses, such as rotaviruses, adenoviruses, or noroviruses, also often cause severe inflammation of the small intestine. Non-infectious enterites are caused, for example, by medications, food intolerances, allergies or autoimmune processes. Crohn’s disease and ulcerative colitis, for example, are autoimmune diseases. While Crohn’s disease affects the entire intestine, ulcerative colitis is often limited to the colon. However, ulcerative colitis can also spread to the small intestine. Inflammatory processes in the colon often also affect the Bauhin’s valve between the ileum and colon. When the ileocecal valve is inflamed, it can no longer close properly. As a result, bacteria from the colon spill over into the sterile ileum. Since the ileum is also responsible for the absorption of vitamin B12, its absorption may also be impaired in inflammatory processes in this area. In addition to the absence of intrinsic factor due to gastric disease, this is one of the most common causes of pernicious anemia. Cancers of the ileum are rare because the rapid passage of the food pulp prevents the accumulation of carcinogenic substances in this area.
Typical and common intestinal diseases
- Crohn’s disease (chronic inflammatory bowel disease)
- Inflammation of the intestine (enteritis)
- Intestinal polyps
- Intestinal colic
- Diverticula in the intestine (diverticulosis)
