The small intestine is divided into three parts: duodenum, jejunum and ileum. The duodenum is C-shaped and joins the gastric pylorus to the proximal jejunum by curving around the head of the pancreas. It is 25 cm long and receives the openings of the common bile and pancreatic ducts. The proximal 2.5 cm is covered on its anterior and posterior surfaces by peritoneum, the remainder being retroperitoneal. The duodenum, for purposes of description, is divided into four parts (D1-4). At the duodenojejunal junction the intestine turns forward - this being called the duodenojejunal flexure.

The mucosa of the duodenum is thick and thrown into numerous circular folds called plicae circulares. The common bile duct and the major pancreatic duct pierce the medial wall of D2, approximately half way along its length. At this point there is a small elevation called the major duodenal papilla (see below).

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