The gastrointestinal (GI) tract in essence is a long tube starting at the mouth and ending at the anal orifice (or, as viewed by gastroenterologists, starting with breakfast and ending at dinner). In the proximal part of the GI tract, food is taken in, and mastication prepares the food for digestion. When mechanical and chemical/enzymatic processes have finished this operation, nutrients can be absorbed by the intestinal mucosa in the form of relatively simple compounds. These then pass into the bloodstream and the lymphatic system, and subsequently become available for cellular needs.

Another function of the GI tract is excretion (e.g., elimination of waste deriving from the diet and products of the body metabolism) in order to avoid toxic elements being present inside the body.

Optimal environmental conditions for the treatment of various food components are produced by the secretory activity of the coating epithelium, the intramural glands, and extramural exocrine glands, such as the salivary glands, the liver, and the pancreas.

Between the inner part (the mucosal surface devoted to absorption) and the outer part (the serosal surface) of the GI tract lie the muscular structures. Some muscles have a circular arrangement (circular muscle), and are able to narrow the lumen. Other muscles are arranged lengthwise (longitudinal muscles), and their constriction causes shortening of the intestine. In some locations, circular muscle predominates and forms valves, the sphincters, whose constriction determines the closure of a passage or a natural opening. The circular and longitudinal muscles constrict and release in coordinated waves (peristaltic waves), thus mixing the contents of the intestine and allowing the progression of food from the mouth to the anus.

Pathologies of the GI tract are due to impairment of one or more of these simple functions (secretion, absorption, motility, etc.), and are divided into organic (e.g., inflammatory bowel disease (IBD) (see 6.30 Emesis/Prokinetic Agents), peptic ulcer, and gastroesophageal reflux disease (GERD) (see 6.28 Inflammatory Bowel Disease; 6.31 Cardiovascular Overview) or functional (e.g., irritable bowel syndrome (IBS) (see 6.29 Irritable Bowel Syndrome) and dyspepsia) types, depending on whether the diagnosis is based on the symptoms only or achieved with the aid of precise clinical tests.

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