Introduction
Every cell of the body needs nourishment, yet most cells S cannot leave their position in the body and travel to a food source, so the food must be converted to a usable form and delivered. The digestive system, with the help of the circula- tory system, acts as a gigantic "meals on wheels," providing nour- ishment to over a hundred trillion customer" celis in the body. It also has its own quality control and waste disposal system The digestive system provides the body with water, electro- lytes, and other nutrients. To do this, the digestive system is special- ized to ingest food, propel it through the digestive tract, digest it, and absorb water, electrolytes, and other nutrients from the lumen of the gastrointestinal tract. Once these useful substances are absorbed, they are transported through the circulatory system to cells, where they are used. The undigested portion of the food is moved through the digestive tract and eliminated through the anus.