The digestive system
The function of the digestive system is to make ingested food available to the cells of the body. Most ingested food is in the form of very large molecules that must be broken down by mechanical and biochemical processes into their smaller components (see Table 18.1). These smaller units are then absorbed across the wall of the digestive tract and distributed throughout the body. Not all ingested materials may be completely
digested and absorbed by the human gastrointestinal tract. For example, cellulose, the fibrous form of plant carbohydrates, is indigestible by humans. Normally, about 95% of ingested food materials are made available for use by the body. Interestingly, as long as food remains within the digestive tract, it is technically outside the body. Not until the materials have crossed the epithelium that lines the tract are they considered to have “entered” the body.