Chapter Notes 24 Digestive System(2).docx - THE DIGESTIVE...

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THE DIGESTIVE SYSTEM The digestive system or gastrointestinal (GI) tract is composed of: 1) Digestive tract: A tube extending from the mouth to the anus. Oral cavity Oropharynx Esophagus Stomach Small intestine Large intestine 2) Accessory organs: Glands located outside the digestive tract that secrete fluids into the digestive tract to aid in digestion: Pancreas, Liver, Gall bladder FUNCTIONS OF THE GI TRACT 1) Ingestion: Movement of food from mouth to stomach. Includes deglutition (swallowing) 2) Propulsion: Movement of nutrients from one end of the digestive tract to the other. Mechanisms of propulsion: Peristaltic waves: Occurs from the esophagus through the small intestine. Functions like a zipper. Rolling waves of smooth muscle contraction propel the bolus forward. Bolus can travel the length of the esophagus in 6-10 seconds! Peristalsis also relaxes sphincters/valves that separate each organ of the GI tract Mass movements: Occur in the large intestine. While occurring via the same mechanism as peristalsis, mass movements occur less frequently but move food further down the GI tract. 3) Mixing: Contractions mix the food up with digestive juices within one location. They differ from peristalsis in that mixing waves do not propel the food further down the GI tract. Mixing contractions are called mixing waves in the stomach and segmental contractions in the small intestine. 4) Secretion: Secretions lubricate, liquefy, buffer, and digest food. Types of secretions include mucus, water, bile, and enzymes for digestion. 5) Digestion: Mechanisms of digestion include mechanical digestion (mastication/chewing or mixing/segmentation) and chemical digestion (digestive enzymes). Water and vitamins are not broken down. Hydrolysis: Cleavage of chemical bonds by addition of a water molecule. Hydrolysis by enzymes is the mechanism by which chemical digestion occurs. a) Carbohydrates are digested into monosaccharides (like glucose). Monosaccharides are used by the whole body for energy but are primarily stored in the liver and skeletal muscle in the form of glycogen. b) Proteins are digested into amino acids . Amino acids are used by the entire body to make new proteins.
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c) Triglycerides are digested into fatty acids and glycerol . Fat is stored in the form of triglycerides. 7) Absorption : Movement of molecules out of the digestive tract and into circulation or the lymphatic system. Absorption occurs through passive and active transport processes. 8) Elimination or defecation: Removal of waste products. Process occurs in large intestine. Water and salts are largely re-absorbed leaving waste semi-solid (feces). REGULATION OF THE DIGESTIVE SYSTEM Stimuli to increase digestion and motility (movement) within the digestive tract: 1) Chemoreceptors: 1) Taste receptors in the oral cavity 2) Olfactory receptors in the nasal cavity 3) Lipid and H+ receptors in the small intestine 2) Mechanoreceptors: Stretch (distension) receptors throughout the digestive tract 3) Thoughts of food Control Center: 1) Local: Enteric nervous system. You have likely never heard of the enteric nervous system.
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