The small intestine can be divided into three sections: the duodenum, jejunum, and ileum. The duodenum forms the first section of the small intestine. Partially digested food moves from the stomach through the pyloric sphincter and into the duodenum. The pancreas and liver are also connected to the region. Those two organs release their juices (digestive enzymes and bile, respectively) to mix with the partially digested food.
Trypsinogen and chymotrypsinogen are digestive enzymes released into the duodenum. They are responsible for the digestion of proteins into polypeptides. The polypeptides are further broken down by exopeptidases and dipeptidases into amino acids. Lipase and amylase are additional digestive enzymes released into the duodenum for the digestion of fats and carbohydrates. Bile from the liver and lipase digest fats into glycerol and fatty acids. Amylase helps in the digestion of carbohydrates into glucose.
The jejunum is the middle section of the small intestines. It is responsible for the absorption of nutrients from the digested food. The ileum is the final section of the small intestines and is connected to the large intestines. This section absorbs remaining nutrients that were not absorbed in the jejunum and empties any remaining matter into the large intestine.