Functions of the Digestive System

What are the functions of the digestive system?

The need for food presents every animal with at least two challenges. The first is how to obtain it. Once an animal has caught or gathered its food, its body faces a new challenge—how to convert the food into useful molecules. In humans and many other animals, this is the job of the digestive system. The digestive system converts food into small molecules that can be used by the cells of the body. Food is processed by the digestive system in four phases—ingestion, digestion, absorption, and elimination.

Ingestion Naturally, the first step in digestion is getting food into the system. Ingestion, as the process is called, is the process of putting food into your mouth—the opening to the digestive tract.

Digestion As food passes through the digestive system, it is broken down in two ways—by mechanical and chemical digestion. Mechanical digestion is the physical breakdown of large pieces of food into smaller pieces. These smaller pieces can be swallowed and accessed by digestive enzymes. During chemical digestion, enzymes break down food into the small molecules the body can use.

Absorption Once food has been broken into small molecules, it can be absorbed by cells in the small intestine. From the small intestine, the molecules enter the circulatory system, which transports them throughout the body.

Elimination The digestive system cannot digest and absorb all the substances in food that enter the body. Some materials, such as cellulose, travel through the large intestine and are eliminated from the body as feces.

FIGURE 30–10 The Digestive System