Within the gastrointestinal system, the small intestine is made up of two separate parts — the duodenum and the ileum. The duodenum is the shorter of the two parts and is where most digestion takes place. The ileum is significantly longer and is largely concerned with the absorption of the nutrients and molecules released during digestion.
The duodenum is attached to the stomach by the pyloric sphincter. This acts as a method of controlling the flow of material from the stomach into the small intestine. Chyme — partially digested food — can only pass in one direction through the sphincter, from the stomach into the small intestine.
Within the stomach, digestion begins when gastric juice is mixed thoroughly with food that has been ingested. The food is broken down into smaller pieces and a creamy mixture is formed, which is called chyme. Over three to four hours after food has been eaten, the chyme is gradually released into the duodenum. This allows the small intestine to work on small amounts of food at a time, ensuring thorough digestion as well as a continuous supply of food that will be absorbed between meals.
Digestion occurs within the duodenum through the addition of different digestive juices. The three sources of digestive juices that carry out this process are the liver, the pancreas and the wall of the small intestine. The walls of the small intestine are folded and characterized by villi — small finger-like projections. Villi contain smooth muscle cells, which allow them to contract and relax so that food is further mixed with the digestive juices, and also moved through the small intestine.
The three digestive juices that are released into the duodenum are bile, pancreatic juice and intestinal juice. Bile is produced by the liver. It contains mineral salts and bile salts, which help to regulate the pH of the chyme and also break down fats and oils into smaller molecules for easier digestion. There are no digestive enzymes within bile.
As well as containing mineral salts, pancreatic juice contains several different enzymes. Each of these enzymes works on a different type of food molecule to break it down into its constituent parts. As the food moves through the duodenum, it is thoroughly mixed with these enzymes so that when it reaches the ileum, it is broken down into small molecules that can then be absorbed into the body.
The intestinal digestive juices contain mucus, mineral salts and enzymes. The mucus and mineral salts are produced by Brunner’s glands, and the enzymes are produced at the tips of the villi within the duodenum. The mucus and mineral salts help lubricate the duodenum and regulate the pH, while the enzymes carry out digestion by breaking down proteins, lipids and carbohydrates.
