Copyright © 2004, Elsevier Science (USA) All rights reserved.
In the vertebrate body plan, the gastrointestinal (GI) tract is a tube highly specialized for processing ingesta and absorbing nutrients into the body. As such, the inside of the tube or lumen is frequently considered to be outside the body itself. Often called the digestive system, the GI system has four major activities, of which digestion is only one. These are motility, secretion, digestion, and absorption.
Motility refers to the contractions of the muscles of the GI tract that result in the orderly movement of ingested material from the mouth to the anus. The contractions of the GI tract serve not only to propel material but also to mix and reduce the size of its particles. The movement of material through the tract is regulated to optimize the time needed for both digestion and absorption.
The second major activity of the GI tract is secretion. Secretions consist primarily of water, electrolytes, enzymes, and mucus and are provided both by the mucosal lining of the tract and by accessory glands. The salivary glands, pancreas, and liver produce highly specialized and essential secretions that enter the lumen of the GI tract through ducts. The secretory activities of these glands and those present within the mucosa itself are finely regulated.
Digestion refers to the chemical breakdown of food into molecules able to cross the mucosa and gain entry to the blood. Most digestion is accomplished by enzymes either secreted into the lumen by the various glands or inserted into the surface of the small intestinal mucosa as integral parts of the plasma membranes of the enterocytes. Gastric acid accounts for some digestion as well. Digestion per se is not regulated, but the availability of the digestive enzymes depends in part on the regulation of secretion.
The final function of the GI system is the absorption of nutrients, vitamins, minerals, water, and electrolytes.
Although there is little evidence that absorptive processes are regulated over the short term, motility and secretion are closely regulated to optimize digestion and, in turn, absorption.
The regulation and integration of motility and secretion are mediated, in all known instances, by the action of chemicals on receptors of target cells within the digestive tract. These chemicals are delivered either by nerves and are termed neurocrines, by the bloodstream and termed hormones, or by diffusion through the interstitial fluid and termed paracrines. In the last two instances, the chemicals are peptides synthesized by endocrine cells within the epithelial layer of the mucosa.
This chapter presents an overview of the neural and chemical regulation of GI function. In subsequent chapters in this section, regulation of the motility and secretion of the specific organs is covered in detail. The material in this chapter is a framework for that discussion.
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