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The Gastrointestinal System
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The gastrointestinal system consists of the gastrointestinal tract and the accessory exocrine glands. The gastrointestinal tract includes the mouth, the esophagus, the stomach, the small intestine, and the large intestine. The major accessory glands are the salivary glands, the liver, the gallbladder, and the pancreas.
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The major functions of the gastrointestinal system are assimilation of nutrients and excretion of waste products via the biliary system. Movement of food through the gastrointestinal system (motility) is carefully coordinated with the delivery of appropriate fluid and enzyme solutions (secretion) so that the macromolecules in food can be hydrolyzed (digestion) and the nutrient molecules, which are liberated, can be transported into the circulatory system (absorption). Coordinating multiple organs and physiologic processes is a significant challenge that must be solved to achieve the overall functions of the gastrointestinal system. Elaborate control mechanisms are provided by the enteric nervous system (ENS), a large intrinsic network of neurons in the wall of the gastrointestinal tract, and by several hormones.
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Figure 7-1 provides an overview of the main functions of each area of the gastrointestinal system during the passage of food. The following sequence of events results in the efficient assimilation of nutrients from food:
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Chewing (mastication) of food breaks down the food to create a bolus that is suitable for swallowing. Saliva lubricates food and provides enzymes for digestion. It takes about 10 seconds for swallowed food to travel down the esophagus to the stomach.
Depending on its composition, food can remain in the stomach for about 1–4 hours. Stomach motility mixes and grinds food into small particles suitable for delivery to the small intestine via the pyloric sphincter. Exocrine secretions from the stomach mucosa help to dilute and dissolve food; gastric acid assists in dissolving and denaturing the components of food.
Entry of food into the small intestine is coordinated with the delivery of major exocrine secretions from the biliary system and the pancreas. The pancreas is essential for digestion because it produces numerous enzymes. The pancreas also secretes HCO3−, which neutralizes acid from the stomach. Contractions of the gallbladder deliver stored bile to the intestine. Bile acids are the major organic component of bile and are important for lipid assimilation.
Food moves through the small intestine within 7–10 hours. Motility patterns in the fed state mix food with digestive enzymes and distribute nutrients throughout the absorptive surface. All significant absorption of nutrients occurs in the small intestine.
Transit through the large intestine, from the cecum to the sigmoid colon, usually occurs over a period of 12–24 hours. The functions of the large intestine include fluid and electrolyte transport and fermentation of undigested carbohydrates (e.g., cellulose). Storage of fecal waste occurs in the distal large intestine; elimination of fecal waste typically occurs within 1–3 ...