jejunum

The jejunum is the middle portion of the small intestine, extending from the duodenojejunal flexure to the ileum. In adults it occupies much of the central and upper abdomen and measures about 2.5 meters in length. It is intraperitoneal and suspended by the mesentery, which contains the vessels, nerves, and lymphatics supplying the segment.

Histologically, the jejunal mucosa is specialized for absorption. It features circular folds (plicae circulares), tall villi, and microvilli on enterocytes, which together amplify the surface area for nutrient uptake. Crypts of Lieberkühn are present between the villi and secrete intestinal juice; goblet cells produce mucus. The brush border enzymes on enterocytes digest disaccharides and other substrates. The jejunum is primarily responsible for absorbing most carbohydrates and amino acids, as well as a large proportion of fat absorbed from micelles. It also reclaims substantial quantities of water and electrolytes, while iron absorption is greatest in the proximal small intestine near the duodenum and proximal jejunum.

The blood supply comes from the superior mesenteric artery, with venous drainage via the superior mesenteric vein into the portal circulation. Lymphatic drainage includes lacteals within the villi, which transport absorbed fats. Innervation involves parasympathetic input from the vagus nerve and sympathetic input through periarterial plexuses, modulating peristalsis and secretion via the enteric nervous system.

Clinical notes include involvement in conditions such as celiac disease and Crohn’s disease, which can affect the jejunum, and surgical resections that may lead to short bowel syndrome with malabsorption.