Big Ideas Digestion. Digestion is the breakdown of food into simpler molecules that can be absorbed by the body. The digestive system is actually a long hollow tube called the gastrointestinal tract, or GI tract. The digestive system includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine.he first task of the digestive system is to break down food into a fine pulp (mechanical digestion). When the food is physical broken down, digestive chemicals break the food down into small molecules (chemical digestion).