Humans eat for many different reasons: because they are hungry, because they are bored, because they are stressed, or simply because the food smells and tastes good. The biological reason for eating, however, is to replenish nutrients and to provide energy to support the body’s functions. The task of the digestive system is to break down food into the elements that the body can use and to eliminate as waste whatever is left over.

One might believe that such a basic function would be well understood, with little left to learn. In fact, scientists are constantly discovering new things about how the digestive system does its job and how it interacts with other aspects of the body. Current research may help experts understand elements of health as diverse as how the digestive system affects immunity as well as its relationship to such important health concerns as heart disease, diabetes, and cancer. This book explores what happens to food in its journey through your body.

The entrance to the digestive tract is the mouth. Although little digestion of food actually takes place in the mouth, it contains important structures that aid in and begin the digestive process. These include the teeth, the tongue, and the salivary glands, which together reduce food into small particles and mix it with saliva in order to speed its progression through the digestive system. Human teeth are specifically designed for an omnivorous diet, with different shapes and surfaces for cutting and tearing meat and grinding grains and vegetables. The muscular tongue moves food around the mouth, pushing it toward the esophagus.