The ascending part is the fourth and final portion of the duodenum. It is up to 2 inches (5 cm) long and distally joins with the jejunum.

The ascending part starts on the left side of the abdominal aorta and third lumbar vertebra (L3) and ascends laterally to the second lumbar vertebra (L2). Further, the ascending part makes a sharp turn in an antero-inferior direction and forms the duodenojejunal flexure (Read more!), which marks the transition site between the duodenum and jejunum.

The ascending portion of the duodenum becomes more and more invested in the peritoneum towards the jejunum to the point that the duodenojejunal flexure is suspended from the retroperitoneal space by a thin and triangular-shaped double fold of the peritoneum called the suspensory ligament of the duodenum (ligament of Treitz).

The ligament of Treitz is a fibromuscular structure, meaning that it also contains muscle fibers, and it is actually composed of two muscular parts joined by the connective tissue. Therefore, it is sometimes referred to as the suspensory muscle of the duodenum. This ligament is also used as a theoretical border between the upper part and lower part of the gastrointestinal tract.

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Ascending part of duodenum