Ascending part of duodenum

The ascending part of duodenum (a.k.a. the fourth part of duodenum) is it's final segment that begins at the left side of the abdominal aorta as a continuation of the horizontal (third) part and ascends superiorly to end at the duodenojejunal flexure opposite the level of the second lumbar vertebra (L2), where it becomes continuous with the jejunum. This segment is approximately 2.5-5 cm long and is predominantly retroperitoneal.

Anatomical relations of ascending part of duodenum

Posteriorly, it is related to the left psoas major muscle, the left sympathetic trunk, left renal vessels, and the left margin of the abdominal aorta, while anteriorly it is crossed by the root of the mesentery and related to loops of jejunum.

Suspensory ligament of Treitz

The duodenojejunal flexure is supported by the suspensory muscle of the duodenum, commonly known as the ligament of Treitz, a fibromuscular band extending from the right crus of the diaphragm to the flexure. It helps maintain the angle between the duodenum and jejunum.