It begins with the duodenal bulb, which is located next to the stomach. The first or superior part of the duodenum is nothing but a continuation of the duodenal end of the pylorus. The second or the descending part is the portion, where the duodenum begins to curve or descend. This is where the common bile duct and the pancreatic duct enter the duodenum.
The third part of the duodenum is the horizontal portion. The fourth or the ascending part, connects to the diaphragm by the ligament of Treitz, and then leads to the jejunum. The duodenum ends at the duodenojejunal flexure, the point where it meets the jejunum.