First Portion (Duodenal Bulb)
Second Portion (Descending)
Third Portion (Inferior)
Fourth Portion (Ascending)
Plicae Circulares

The First (Duodenal Bulb) and Fourth (Ascending) Portions of the Duodenum are mobile and contained between the layers of the peritoneum. The Second (Descending) and Third (Inferior) Portions are fixed and retroperitoneal. The first part of the duodenum lies at the level of L1 vertebra, the second to the right side of L2, the third crosses the level of L3, and the fourth is on the left of L2. The duodenum rarely extends below the level of L3-L4, the transumbilical plane. Internally the mucous membrane of most of the duodenum is thrown into numerous circular folds (Plicae Circulares). The walls of the duodenal bulb (cap) are smooth, hence the smooth outline of the full shadow of barium in the duodenal cap at radiographic examination. From the duodenal cap onwards the plicae break up the barium and its shadow.