The duodenum is usually located in the right upper or middle abdomen in a posterior position, immediately behind the abdominal wall. The duodenum is divided into four parts, namely the bulb, the descending part, the horizontal part, and the ascending part, part of which is an interperitoneal organ and part of which is an extraperitoneal organ in a relatively stable position. The entire duodenum is roughly C-shaped, upward through the pylorus and the stomach to the continuation, and downward to the jejunum. The duodenum has openings for the pancreas and bile ducts, and both bile and pancreatic juices are secreted directly into the duodenum. The duodenum is the shortest in length, thickest in diameter, and most posteriorly located in the small intestine. Because the duodenum is connected to the stomach, gastric acid in the stomach may enter the duodenum through the pylorus causing damage to the duodenal mucosa, which can easily induce diseases such as duodenitis or duodenal ulcer and can cause rhythmic abdominal pain, which is manifested as pain on an empty stomach and relieved after eating.