Normally, as food moves along the digestive tract, digestive juices and enzymes digest and absorb calories and nutrients. As food is chewed and swallowed, it moves down the esophagus to the stomach, where acid continues the digestive process.
When the stomach contents move to the duodenum, the first segment of the small intestine, bile and pancreatic juice speed up digestion. Most of the iron and calcium from eaten foods are absorbed in the duodenum. The jejunum and ileum, the remaining two segments of the nearly 20 feet of small intestine, complete the absorption of almost all calories and nutrients. The food particles that cannot be digested in the small intestine are stored in the large intestine until eliminated.
Source: National Institute of Diabetes & Digestive & Kidney Diseases