The Gastrointestinal System
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