Coiled up in your abdomen, your small and large intestines form a single tube some 28 ft (8 m) long. As food travels through, a collection of chemicals breaks it down into molecules the body can absorb.
The intestines make up the most important part of the digestive system. Here, the process of chemical digestion is completed, and the nutrients released are absorbed into the bloodstream to be carried to other parts of the body.
Like the stomach, the intestines have muscular walls that contract to squeeze food along. It takes only a few hours for a meal to pass through the small intestine, by which time nearly all the nutrients are absorbed. The remains then spend up to a day traveling slowly through the large intestine, where water is absorbed and bacteria help digest tough fibrous matter. The journey ends at the anus, where the undigested remains leave the body as feces.
As soon as food leaves the stomach and enters the small intestine, it is mixed with powerful digestive chemicals from two nearby organs: the gall bladder and the pancreas. The gall bladder secretes a green liquid called bile, which neutralizes stomach acid and turns fats into tiny droplets that are easier to digest.
The pancreas secretes at least seven digestive enzymes. These attack carbohydrate, protein, and fat molecules, breaking them down into smaller units.
Lining the inside wall of the small intestine are millions of tiny, fingerlike growths called villi, each about 0.04 in (1 mm) long. Villi absorb the small food molecules produced by digestion, such as sugars and amino acids. These molecules pass into blood vessels in the villi to be carried away. Together, all the villi provide a huge surface area for absorption to take place. If all your villi were stretched out flat, they would cover the same area as a tennis court.
The intestines run all the way from the stomach to the anus. There are two main sections. The first, longer section is the small intestine and does most of the work of digesting and absorbing food. The second section, called the large intestine, is twice the width of the small intestine and a quarter of its length. It receives watery leftovers from the small intestine and turns them into feces.
The nutrients in food are locked up in giant molecules that our bodies can’t absorb directly. The process of digestion breaks these molecules into smaller molecules that can dissolve in body fluids and enter the blood. The digestive organs produce a range of chemicals called enzymes to break down food. Each enzyme attacks a particular type of food molecule.
Butter and oil are sources of fat. Enzymes in the small intestine break down fat into glycerol and fatty acid molecules.
Meat and cheese are rich in protein. Enzymes in the stomach and small intestine break protein molecules into amino acids.
Foods rich in carbohydrates include pasta, rice, and bread. Enzymes in the mouth and small intestine split large carbohydrate molecules into sugars.