Digestion is the process that breaks food down into the molecules that the body needs for all of its metabolic processes. Food is the body's fuel source and the source that the body's molecules and cells use to build and repair the body, but the body cannot use the food unless it has been digested.
The digestive process starts before food even enters the mouth. Just the sight and smell of food can start the production of saliva, which contains enzymes that begin the process of digesting carbohydrates. The whole digestive process is achieved by a combination of both mechanical actions, such as chewing and mixing, and chemical actions, which occur when digestive enzymes begin breaking down the food. Gastric juice secreted in the stomach breaks down protein molecules. Secretions from the pancreas break fats down into fatty acids and glycerol. Once the digested food is broken down into molecules that are small enough to be carried through the blood stream, the body can begin to absorb its nutrients. Most of the nutrient absorption takes place in the small intestine, a tube between 15 and 30 feet in length that is coiled in the human abdomen. Food molecules are absorbed or transported across the walls of the large intestine and into the bloodstream, where they are carried away to be used by the cells of the body.