Terminal Section of the Male Duodenum Ascending Toward the Jejunum

Description

Superiorly, the fourth (ascending) part of the duodenum rises from the left of the midline toward the duodenojejunal flexure, where it becomes jejunum. Its lumen is positioned posterior to the mesentery of the transverse colon and anterior to the left psoas major and vertebral column, with the duodenum transitioning from a more fixed retroperitoneal segment to the intraperitoneal proximal jejunum. Medially, the root of the mesentery begins just distal to the flexure, while laterally the loop turns into the mobile jejunal coils. Short and steep. Orientation of this terminal duodenum matters because the duodenojejunal flexure is the landmark for the ligament of Treitz and for the boundary between upper and lower gastrointestinal bleeding in clinical algorithms. The ascending duodenum also sits in the corridor where the superior mesenteric artery crosses anterior to the third part, so appreciating the superiorly directed course into the jejunum helps explain superior mesenteric artery syndrome and the obstruction pattern proximal to the transition. Surgeons use this relationship when mobilizing the duodenum (Kocher maneuver) and when exposing the proximal jejunum for anastomosis in pancreaticoduodenectomy or trauma. Use this artwork in gross anatomy and GI teaching modules to anchor the sequence from duodenum to jejunum, and in surgical atlases or operative notes that describe identification of the duodenojejunal junction, ligament of Treitz, and proximal jejunal limb. It also fits radiology education on small-bowel follow-through and CT localization of the duodenojejunal flexure when assessing malrotation or proximal obstruction. Anatomical accuracy verified by SciePro's Medical Advisory Board.