16th February 2024

Small intestines

The duodenum is a 20-30 cm C-shaped small bowel region. The pancreatic head usually lies inside the C-shape of the duodenum. The initial 2-3cm of the 1st part of the duodenum is intraperitoneal. The second part of the duodenum receives the pancreatic and common bile duct at the ampulla of Vater. Just inferior to the ampulla is the junction of the foregut and midgut. The duodenum ends at the ligament of Treitz aka duodenojejunal (DJ) flexure continuing as jejunum.

The remainder of the small bowel is divided into the jejunum, the proximal 2/5th, and the ileum, the latter 3/5th. Small bowel loops contain mucosal folds called valvulae conniventes, which start at the second part of the duodenum, becoming the thickest at the jejunum and then fading away distally towards the terminal ileum. 

These folds make complete circles in the bowel wall and give a characteristic appearance on plain radiographs or barium studies. The ileum enters the caecum at the ileo-caecal valve.