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The digestive or gastrointestinal system is roughly defined as the anatomical component from the mouth to the anus, including organs responsible for transit, mechanical breakdown, digestion and absorption of foodstuffs, as well as the efficient elimination of solid waste. Included are the mouth and dentitia, pharynx and esophagus, stomach, small intestine, liver, gall bladder, pancreas, large intestine, rectum, and anus. As with the complex integration and control of metabolism, the digestive system is, itself, under the influence of neurological and hormonal regulation that both activates and inhibits many of its complex actions. Most of these complex actions are, themselves, simple biochemical processes of proteins, carbohydrates, lipids, and nucleosides/nucleotides to include enzyme reactions with associated activating and inhibitory molecules, membrane-spanning protein channels, and pumps all leading to the production and storage of energy and essential building blocks for current or future use.


The digestive system is the collection of organs responsible for the digestion of ingested foods and liquids (Figure 11-1). This system is classically considered to start at the mouth and continue via the esophagus, stomach, small and large intestines, and end at the rectum/anus. Besides their mechanical and enzymatic breakdown of foodstuffs, considerable contributions toward digestion are supplied from the liver and pancreas.

Figure 11-1.

Overview of the Digestive System. Components of digestion and transport of food from the mouth to the rectum/anus are shown, including a summary of their contributions and the average amount of time for food to reach their location after ingestion. [Reproduced with permission from Kibble JD and Halsey CR: The Big Picture: Medical Physiology, 1st edition, McGraw-Hill, 2009.]


Teeth provide the initial mechanical breakdown of food via chewing or mastication and, therefore, the formation and health of teeth is important. Proteins serve as an important structural component of teeth enamel. Like bone, teeth require the collagen-like proteins amelogenin (over 90% composed of proline, glutamine, and histidine amino acids), ameloblastin, enamelin, and tuftelin (a phosphorylated glycoprotein), which organize, initiate, and direct calcium phosphate crystal formation and help anchor teeth to the gums.

The possible breakdown of enamel is equally important and is also dependent on biochemical processes. Dental plaque formation and tooth decay both rely on enzymatic processes for formation as well as prevention. Plaque is caused mainly by the normal oral bacteria Streptococcus mutans, Lactobacillus acidophilus, Fusobacterium nucleatum, Actinomyces viscosus, and Nocardia spp. When these organisms form a layer on teeth, those closest to the teeth exist in an oxygen-deficient environment and convert to anaerobic respiration for energy production. This enzymatic process turns carbohydrates into lactic acid from pyruvate (Chapter 6) and results in a pH of below 5.5, leading to tooth decay, the demineralization process ...

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