The ileum, jejunum, and duodenum are all parts of the small intestine. The ileum is the last section of the small intestine and is about 3 meters long. The jejunum is the middle section of the small intestine and is about 2.5 meters long. The duodenum is the first section of the small intestine and is about 25 centimeters long. All three sections of the small intestine are responsible for breaking down food and absorbing nutrients.