The small intestine is roughly 4 to 6 m long, and is made up of three parts. In the first part of the small intestine, the duodenum, the important function is the addition of other digestive juices: bile that is produced by the liver, along with a rich soup of enzymes from the pancreas. The digestive juices all act together to break down the food material into simple nutrients.

Most of the absorption of nutrients happened in the jejunum, the second part of the small intestine, and in the ileum, the final and longest part. From these parts of the small intestine the absorbed nutrients are carried to the liver for "processing", and then on to nourish the rest of the body. At the very end of the ileum lies another valve called the ileocecal valve. It marks the end of the small intestine.