ABSTRACT
Study objectives Describe the anatomical and functional characteristics of each of the four layers of the digestive tract wall: mucosa, submucosa, muscularis externa, and serosa Distinguish between the two types of gastrointestinal motility: segmentation and peristalsis Explain how each of the three types of sensory receptors within the digestive tract are stimulated: chemoreceptors, osmoreceptors, and mechanoreceptors Explain how the following mechanisms regulate the activity of the digestive system: intrinsic nerve plexuses, extrinsic autonomic nerves, and gastrointestinal hormones List the components of saliva and their functions Describe how salivary secretion is regulated Explain how swallowing takes place For each of the following organs, esophagus, stomach, small intestine, and large intestine, describe:
Specialized anatomical modications The type of motility and how it is regulated The types of secretions and how they are regulated The digestive processes that take place The absorptive processes that take place Other functions
20. Introduction The overall function of the digestive system is to make ingested food available to the cells of the body. Most ingested food is in the form of very large molecules that must be broken down by mechanical and biochemical processes into their smaller components (see Table 20.1). These smaller units are then absorbed across the wall of the digestive tract and distributed throughout the body. Not all ingested materials may be completely digested and absorbed by the human gastrointestinal tract. For example, cellulose, the brous form of plant carbohydrates, is indigestible by humans. Normally, about 95% of
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ingested food materials are made available for use by the body. Interestingly, as long as food remains within the digestive tract, it is technically outside of the body. Not until the materials have crossed the epithelium that lines the tract are they considered to have “entered” the body.
