In lieu of an abstract, here is a brief excerpt of the content:

109 7 The mammalian gastrointestinal system is also known as the digestive tract or the enteric system. It is a complex system performing mechanical, secretory, digestive, absorptive, and excretory functions. Each of these is under the influence of local gastrointestinal reflexes as well as central feedback control mechanisms . Consider, for example, the medical problems a person might have if she ate three meals per day for several days without having a bowel movement. To prevent the intestinal storage of food wastes and the pathogens they support, a wide variety of gastrointestinal reflexes exist. In the well-tuned, normally functioning mammalian digestive tract, shortly after the consumption of one meal a defecation reflex is activated, causing contraction of the colon and initiating a bowel movement. Nearly simultaneously, mechanical distension of the stomach initiates a gastro-ileal reflex. This causes the ileum to contract and to empty its contents into the colon. Emptying of the ileum makes room upstream as the stomach passes its contents into the duodenum. In many humans who have bowel movements at approximately twentyfour -hour intervals, the gastro-ileal reflex is activated within thirty to sixty minutes after ingestion of the morning meal. This means the colon has space to accommodate another meal when it is ingested. In all mammals, there is a centrally located satiety center in the brain stem. Usually about four to six hours after the last meal, sensations of hunger, often originating in the stomach, signal to the person that it is time to eat again. Some refer to these abdominal sensations as hunger pains or hunger pangs. Their specific cause is not known. However, consumption of a meal resolves the sensations of hunger, and that individual usually will not be motivated to seek food for another four to six hours. Of course the normal physiology of hunger and satiety is influenced by disease and other interventions. Any person who has experienced food poisoning such as that caused by E. coli or salmonella knows that during the period of The Gastrointestinal System vomiting and diarrhea and for a day or two thereafter the last thing one thinks of is eating more food. Other changes in behavior can also influence digestion and bowel regularity. Examples include travel, fasting and feasting, and prescription as well as over-the-counter medications. Simply modifying one’s eating habits can markedly influence behavior of the gastrointestinal system. My wife and fifteen-year-old son ate considerably less than I did. I decided one time to reduce my daily calorie intake. My plan was to observe them for one month at all meals we ate together. After they had taken their portions, I would take mine based on my visual estimate of theirs. I did this for one month. Many things happened. I lost five pounds and my slacks fit more loosely. Most interestingly , my gastrointestinal functions changed markedly. For example, bowel movements became less frequent and my stools less bulky. I attributed these changes to the reduced daily intake of calories; nothing else had changed in my life during that period. I enjoyed that act of self-discipline and learned a few more things about myself. Our diets and changes in them affect us differently. The numbers, distributions , and physiological efficacies of membrane-bound digestive enzymes, among other physiological characteristics of the enteric system, differ from person to person. Thus, two people making the same change in their similar diets cannot expect the same outcome in terms of pounds lost. Components of the Gastrointestinal (GI) or Enteric System The GI system begins with the oral cavity and ends at the anus (see figure .). The entire system is designed to mix and churn, store and propel, and digest and absorb. These processes begin in the mouth and end in the colon or cecum (for guinea pigs and other mammals). In the healthy individual, they continue for a lifetime. In the person with an unhealthy GI tract, hyperalimentation or feeding via intravenous infusions might be required and can sustain life for prolonged periods. We can identify points of demarcation along the entire length of the GI tract that help define functions. One such set of points is called sphincters. A sphincter is a band of visceral smooth muscle—sometimes skeletal muscle, other times a mixture of visceral smooth and skeletal muscle—that circumscribes a small but well-defined segment of the GI tract. Sphincters are also comprised of longitudinal and circular smooth muscle cells. These form separate layers of muscle that...

Share