The foods we eat—apples, pepperoni pizzas, leafy green salads—taste good to us, but cannot be used by the body as they are. The nutrition the cells of the body need to keeping growing and working must be in a simple form: amino acids, simple sugars, and fatty acids. It is the job of the digestive system to take the complex organic molecules of the foods we ingest—proteins, carbohydrates, and fats—and break them down into their simple building blocks. This process is called digestion. Once digestion has occurred, the simple molecules (nutrients) are absorbed from the digestion system by the cardiovascular and lymphatic systems and transported to cells throughout the body.

Design: Parts of the Digestive System

The digestive system may be broken into two parts: a long, winding, muscular tube accompanied by accessory digestive organs and glands. That open-ended tube...