Duodenum. Endoscope view of the mid-part of a healthy, normal duodenum. The duodenum is the first section of the small intestine, just after the stomach. This section, which comes after the duodenal bulb, is often known as the second duodenum. The circular ridges (light pink) are known as Kerckring's folds or plicae circulares, and are unique to this part of the small intestine.