22.8–22.13: We extract energy and nutrients from food.

A kingfisher dives to catch a fish.
22.8: We convert food into nutrients in four steps.
Figure 22.16: There are four steps in the body’s processing of food.

The human digestive system is like an assembly line running backward. Imagine starting with an assembled car and dismantling it into its many parts: the tires, doors, windshield, steering wheel. The food entering the digestive system is like the intact car. It enters the assembly line and then passes through four distinct phases, during which the food is progressively chewed up and broken down, the nutrients absorbed by the body, and the non-usable portion of the raw materials discarded as waste products (FIGURE 22-16).

In the next several sections, we examine in detail the four stages in the processing of food: ingestion, digestion, absorption, and elimination. Throughout these sections, it will be useful to picture the digestive system as a long tube with an opening at each end, the mouth and the anus, and some glands along the way that produce the necessary chemicals to help the process along.

TAKE-HOME MESSAGE 22.8

The digestive process in humans includes four distinct phases during which food is progressively chewed up, broken down, and absorbed by the body, after which the non-usable portion of the raw materials is discarded as waste.

The four phases of the digestive process are ingestion, digestion, absorption, and elimination. In a single sentence, what is accomplished during this process?

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