| General principles of digestion
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| Digestion is a sequential, ordered process with links between each stage
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The process of digestion is characterized by several specific stages, which occur in characteristic sequence, allowing the interaction of fluid, pH, emulsifying agents, and enzymes. This, in turn, requires concerted action of the liver, pancreas, gall bladder, and salivary glands. The processes that are involved are outlined in Figure 9.1 and can be summarized as follows:
- lubrication and homogenization of food with fluids secreted by glands of the GI tract, starting in the mouth;
- secretion of enzymes whose prime function is the hydrolytic breakdown of polymeric macromolecules to a mixture of oligomers, dimers and monomers;
- secretion of electrolytes, hydrogen ion and bicarbonate within different regions of the GI tract, to optimize the conditions for enzymic hydrolysis specific to a particular region of the GI tract;
- secretion of bile acids to emulsify dietary lipid, allowing appropriate enzymic hydrolysis and absorption;
- further hydrolysis of oligomers and dimers within the jejunum by membrane-bound surface enzymes;
- specific transport of digested material into enterocytes and thence to blood or lymph.
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| Numerous sections of the gut are involved in these processes and each area contains specialized glands and unique surface epithelial properties, as outlined in Table 9.1. Before signs and symptoms of GI maldigestion or malabsorption occur, there needs to be a considerable impairment of structural/functional relationships within the GI tract.
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| There is considerable functional reserve in all aspects of digestion and absorption. Minor functional loss may go unnoticed by the individual, allowing pathology to progress for some time before being diagnosed.
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| Each of the organs involved in digestion and absorption has the capacity to increase its activity several fold in response to specific stimulation; this adds to the gut's reserve capacity. For pancreatic disease to become manifest, 90% of the pancreatic function has to be destroyed. In addition, the gut can accommodate loss of function of one particular organ. For example, both the pancreas and the small intestine can take over after a total loss of gastric digestion, and lingual lipases can accommodate, in part, some loss of pancreatic lipase production.
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