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Structure and Function
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The digestive system is also known as the gastrointestinal (GI) system. It includes all the structures of the alimentary canal, from the mouth to the anus, and the accessory organs. The digestive system has two key functions: digestion and excretion. The organs of the GI system break down food into usable nutrients and then eliminate bulk waste in the form of feces.
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We will discuss the parts of the GI system in the same order in which food passes through the system. As we do this, please refer to Figure 9-1 to see the various parts of the GI system.
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Learning Style Tip
After reading each paragraph, summarize the content and key points aloud before moving on to the next paragraph.
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The first or most proximal part of the digestive system is the mouth, also known as the oral or buccal cavity. When we take a bite of food (ingestion) and begin chewing it, our tongue and teeth aid in the process of mechanical digestion as food is broken down into smaller and smaller parts. It is mixed and moistened with saliva, which is secreted from three different salivary glands. Saliva also contains ptyalin, a chemical that starts to break down starches. The tongue helps to form chewed food into a bolus, which is a rounded mass ready to be swallowed. The tongue also allows us to taste food. Specific areas on the tongue identify sweet, salty, sour, and bitter flavors (Fig. 9-2). Because taste can make eating an enjoyable process, there are times when we may overeat. When we eat more calories than we burn through our daily activities or exercise, we gain weight (Box 9-1).
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Box 9-1 Obesity
Our bodies use the foods we eat for energy. When we eat more calories than our bodies burn through physical activity, the extra calories are stored as fat. Overeating and eating unhealthy foods are behaviors that can cause us to become overweight or even obese. In the United States, 1 in 3 adults and 1 in 6 children are obese. For these individuals, the chance of developing other conditions such as heart disease, diabetes, and cancer is greatly increased.
Our environment also plays a role in obesity. Low income families may not be able to afford healthier food choices. Our neighborhoods may not have sidewalks or crosswalks that enable us to exercise safely outdoors. Parks and community centers may be too far from our homes. While we ...