Ileum
Structure and Function
Structure
General
About 20ft long. The first 2 fifths are jejunum, the rest ileum. Each has distinctive features, but there is a gradual change. The jejunum begins at the duodenojejunal flexure, and the ileum ands at the eleocecal junction. The coils are freely moveable, and attached to the posterior abdominal wall by folds of peritoneum called the mesentery of the small intestine. At the root of the mesentery is the entrance and exit of the superior mesenteric artery and vein, lymph vessels and nerves.
Differences between ileum and jejunum
| Area | Jejunum | Ileum |
|---|---|---|
| Location | Upper part of peritoneal cavity, by the left side of the transverse colon. | Lower part of cavity and in the pelvis. |
| Size | Wider bored, Thicker walled, and redder. Thicker walls because the plicae circulares are larger and more numerous. | Plicae circulares smaller and wider separated – absent in lower part of ileum. |
| Mesentery | Mesentery attached to posterior abdominal wall above and left of aorta. | Mesentery attached below and right of aorta. |
| Vessels | Only one or two arcades, with long branches to the wall. | Numerous short terminal vessels, after 3 or more arcades. |
| Fat | Fat deposited near the root, and is scanty near intestines. | Fat is deposited throughout. |
| Peyer's Patches | No Peyer's patches | Peyer's patches (lumps of lympoid tissue) present on the antimesenteric wall. |
Function
- Digests B12, Folic acid and Vitamin C
- Absorbs pretty much all the nutrients from food.
Arterial and Venous supply
The superior mesenteric artery supplies the whole intestines. The lowest part of the ileum is supplied ileocolic artery. The veins correspond to the branches of the superior mesenteric artery and drain into the superior mesenteric vein.
Nervous Supply
The nerves are derived from the sympathetic and parasympathetic (vagus) nerves from the superior mesenteric plexus.
Lymph
Eventually drain to the superior mesenteric nodes.
Histology
Basically the same as the duodenum except for a few key differences:
- There are no Brunner's glands
- Jejunal villi are long, broad and leaf shaped, whilst ileal ones are shorter and rounded.
- Jejunal crypts are deeper and contain less paneth cells than the ileal ones.
- The jejunum has more plicae than the other areas.
Clinical Conditions
It can be difficult to differentiate between conditions in the different bits of the intestines. Common symptoms are abdominal pain, diarrhoea, flatulence, weight loss, and nutritional deficiencies.
Chronic Infections
Infection with Giardia lambia, or from parasites like hookworms and tapeworms, is a common form of malabsorption in poorer countries. In immunocompromised individuals, microsporidia and cryptosporidia cause incomtractable diarrhoea. Salmonella typhi gains entry to the body through Peyer's patches, which become acutely inflamed and can perforate.
Crohn's disease
About: Areas of inflammation (skip lesions) with patches of healthy tissue between. In the ileum, there is often a palpable mass. Though in 15% of cases there are no GI symptoms, in 80% there is diarrhoea.
Can affect any part of intestine, but in 60% of cases it affects the terminal ileum, causing mucosal ulceration and transmural(?) granulomatous(?) inflammation. Tends to affect terminal ileum, colon and/or anorectum areas preferentially.
Treatment:Steroids promote remission, as do low fat and acid content diets, and then long term prevention is from Azathioprine (AZA) and 6-mercaptopurine (6MP).
Hernia
Loops of small intestine are very mobile, and may be caught in hernial sacs or in adhesions – this can cause intestinal obstruction, which presents with acute, often intense pain, anorexia and vomiting. Has to be relieved surgically.
Meckels diverticulum
Congenital. Found on the antimesenteric border of the ileum around 2 ft from the ileocecal junction. Found in 2% of individuals. May contain ectopic acid secreting gastric mucosa, and so bleeding may occur from a gastric ulcer in its mucous membrane. The pain from it may be confused with the pain from appendicitis.
Neoplasia
Again, not common – in the Far East, where there is a lot of GI infection you get a bit more lymphoma, but not much.