 Duodenum, the duodenum is the first section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear, and the terms anterior intestine or proximal intestine may be used instead of duodenum. In mammals the duodenum may be the principal site for iron absorption. The duodenum precedes the jujonum and ileum and is the surest part of the small intestine. In humans, the duodenum is a hollow jointed tube about 25-38 cm 10-15 inches long connecting the stomach to the jujonum. It begins with the duodenal bulb and ends at the subspensory muscle of duodenum. It can be divided into four parts.