The jejunum is part of the digestive system and is the central part of the small intestine. The other two parts of the small intestine are the duodenum and the ileum. The jejunum is therefore located between the duodenum downstream and the ileum upstream. The ileum and jejunum make up most of the small intestine and the mobile part of the small intestine. The jejunum is about 2 meters long and 2 to 4 centimeters in diameter. It is vascularized by the superior mesenteric artery and the smooth muscles are controlled by the autonomic nervous system. The pH is about 7 to 8.

The inner wall of the jejunum consists of a mucosa lined with numerous folds, which allow for better absorption of lipids, carbohydrates, and proteins due to the increase in total internal surface area. It is then formed by a muscularis with circular internal fibers and longitudinal external fibers.