Jejunum

The jejunum is the second of the three parts of the small intestine. It starts at the duodenojejunal flexure, a sharp bend in the upper left abdomen where the duodenum ends, and makes up roughly two-fifths of the small intestine beyond the duodenum. It blends gradually into the ileum with no sharp border. Unlike most of the duodenum, it hangs in movable loops from the mesentery, a fan-shaped fold that ties it to the back wall of the abdomen and carries its vessels and nerves.

Its lining is shaped for absorption. Tall circular folds called plicae circulares line the inner wall, and they are covered in villi whose cells bear even finer microvilli, greatly enlarging the surface. Enzymes on these cells finish digesting sugars and proteins, and the products, with fats, vitamins, minerals and much water, pass into the blood or, for fats, into lymph vessels called lacteals. Compared with the ileum, the jejunum has a thicker, redder wall, more crowded folds, longer straight arteries (vasa recta) and fewer arterial loops (arcades).

Diseases that damage this lining, such as celiac disease or Crohn disease, can cause malabsorption, a poor uptake of nutrients. The name comes from Latin jejunus, empty or fasting, reportedly because early anatomists found it empty after death.