The small intestine is made up of the duodenum, jejunum, and ileum. Together with the esophagus, large intestine, and the stomach, it forms the gastrointestinal tract. In living humans, the small intestine alone measures about 6 to 7 meters long. After death, this length can increase by up to half. It has a surface area of over 200 meters.

The internal walls of the small intestine are covered in finger-like tissue called villi. Each of these villi is covered in even smaller finger-like structures called microvilli. These villi and microvilli increase the surface area available for the absorption of nutrients.

In the small intestine food that has already been broken down by chewing and stomach enzymes is further degraded by additional enzymes. Some of these chemicals are secreted in the lumen (the hollow area in the middle of the intestine), but others are transported to the intestine from other organs such as the pancreas and liver. Where absorption takes place depends on the type of nutrient or vitamin being absorbed.

Once fully reduced to a chemical level the molecules that are going to be absorbed pass through the walls of the intestine into the bloodstream. Peristalsis, contraction of the muscle walls, is the force that propels matter through the small intestine. It is a slow process, allowing the food matter to mix with the digestive juices.