Horizontal Part of the Duodenum Seen in Anterior Aspect

Description

Running transversely across the posterior abdominal wall, the third (horizontal, inferior) part of the duodenum (D3) spans from right to left at approximately the L3 level, positioned inferior to the head of the pancreas and continuous laterally with the descending duodenum. Anteriorly, D3 lies deep to the root of the mesentery and is commonly crossed by the superior mesenteric artery and vein as they descend into the mesentery. Posterior relations include the abdominal aorta and inferior vena cava near the midline, with the right ureter and psoas major lying more laterally; the transition toward the duodenojejunal flexure occurs superiorly and to the left as the bowel turns cranially. An anterior aspect of D3 matters because this is the segment trapped in classic superior mesenteric artery syndrome, where a narrowed aortomesenteric angle compresses the duodenum between the SMA anteriorly and the aorta posteriorly, producing postprandial pain, early satiety, and bilious vomiting. It is also a key operative and radiologic landmark when evaluating distal duodenal obstruction or when mobilizing the duodenum during a Kocher maneuver, since D3’s fixed retroperitoneal position contrasts with the mobile jejunum beyond the ligament of Treitz. Relationships here explain why retroperitoneal hematoma or pancreatic head pathology can present with duodenal narrowing on CT. Ideal for gastrointestinal anatomy teaching in gross lab and prosection guides, this illustration also supports surgical texts covering duodenal exposure, mesenteric root orientation, and bypass planning (duodenojejunostomy) for SMA syndrome. Editorial uses include radiology primers correlating axial CT findings to anterior surface anatomy of the duodenum and mesenteric vessels. Anatomical accuracy verified by SciePro's Medical Advisory Board.