Feedback regulation and sensation.

The gut is a long tube; food goes down it and across it. Its purpose is to process the food that is consumed in order to optimize absorption of nutrients. It works rather like the reverse of an assembly line in a manufacturing plant with different regions of the gastrointestinal tube specialized sequentially for storage, acid digestion, alkaline digestion, absorption, fermentation, and disposal of waste products. Like an assembly line, it will only work efficiently if the delivery of material to the next process in coordinated closely with the optimum rate of that process. In the gut, the delivery of material is achieved by the propulsive activity of the gastrointestinal smooth muscle, which is in turn programmed by the enteric nervous system and orchestrated by neurohumoral responses to the content of the digesta. It is a finely tuned system; any disturbance can impair absorption and give rise to abdominal discomfort. The stomach is a key organ in the "gastrointestinal dissembly line," It not only commences the digestive process under highly acidic conditions, it stores the masticated food as in a hopper, delivering the acidic digesta or chyme into the duodenum at a rate that is commensurate with the rate at which it can be digested and the products of digestion can be absorbed. The rate at which material enters the small intestine is crucial to achieving optimum nutrition; too fast, and it overwhelms the functional capacity of the small intestine, causing malabsorption and diarrhoea; too slow, and it inhibits the consumption of another meal. The gastrointestinal tract can normally process three medium-sized meals a day.