The jejunum is the middle of the three parts of the small intestine. The transition from the extraperitoneal ascending part of the duodenum to the intraperitoneal jejunum occurs at the duodenojejunal flexure (at the height of L2). The transition to the ileum is not sharply marked and only visible microscopically.
The jejunum makes up about 2/5 of the total length of the small intestine (1.5 to 3.5 meters). Macroscopically noticeable are the many parallel running circular folds in the mucosa (valves of Kerckring). Like all intraperitoneal organs both the jejunum and ileum are attached to the posterior wall of the abdomen by the mesentery. By this means the entire convolute of the small intestine lies quite flexibly in the abdominal cavity however “framed” by the colon.
The blood supply is carried by around 5 jejunal arteries which are interconnected with the other arteries of the small intestine by numerous arcades. The venous blood drains through the correspondent veins into the superior mesenteric vein. The sympathetic innervation is carried by the nerves of the coeliac plexus and superior mesenteric plexus, the parasympathetic innervation by the vagus nerve (cranial nerve X).
The jejunum has the typical histological pattern as the entire small intestine: mucosa, submucosa, muscularis and serosa. The mucosa is lined by simple columnar epithelium towards the lumen (lamina epithelialis). It contains enterocytes and goblet cells. Characteristic features are the crypts of Lieberkuhn and finger-like villi protruding in the intestinal lumen. Similar to the duodenum paneth cells are found deep in the crypts. The epithelial layer is followed by a connective tissue layer (lamina propria) and a muscle layer (lamina muscularis mucosae). The submucosa consists of loose connective tissue with blood vessels, lymph nodes and the Meissner’s plexus. As usual, the muscularis has an inner circular and outer longitudinal layer of smooth musculature between which the Auerbach’s plexus lies. The entire jejunum is covered by serosa from the outside which consists of simple squamous epithelium and a connective tissue layer underneath (lamina propria serosae). Histologically the jejunum differentiates from the rest of the small intestine by the absence of Brunner’s glands (duodenum) and Peyer’s patch (ileum) however single lymphoid follicles are present.
The main tasks of the jejunum are:
- cleavage of nutrients (e.g. by amylase, proteinase)
- absorption of lipophilic nutrients (proteins, fats, cholesterol and the fat-soluble vitamins A, D, E and K)
- absorption of water (about 90% of the secreted water, 6 to 8 liters/day). This induces an osmotic gradient leading to a paracellular transport of electrolytes, carbohydrates and amino acids.