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Anatomy Chapter 16.docx

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Biological Sciences
Connie Soros

Chapter 16 Digestive System • Basics - function: transfer nutrients, H2O and electrolytes from food to body 1. food is an energy source and contains basic building blocks of body tissues - 4 digestive processes 1. motility a. muscular contractions that move food through digestive tract (propulsive) and mix food 2. secretion a. digestive juices (enzymes, bile, mucus, hormones) 3. digestion a. breaking down large molecules into smaller units 4. absorption a. substances moved from digestive tract to blood or lymph - major and accessory organs - layers of digestive tract wall - intrinsic nerve plexuses 1. network of nerves in digestive tract wall (enteric nervous system) 2. influence all digestive tract activity a. mainly coordinate activities 3. influenced by extrinsic nerves - extrinsic nerves (ANS) 1. influences motility and secretion a. modify activity of intrinsic plexuses b. act directly on glands and smooth muscle - receptors and reflexes 1. digestive tract contains a. chemoreceptors b. mechanoreceptors c. osmoreceptors 2. stimulation of receptors initiates reflexes a. short reflexes - entirely within intrinsic nerves b. long reflexes - also involve ANS - gastrointestinal hormones 1. produced in mucosa, released to blood in response to local chemical changes or nerve stimulation • Mouth - mastication (chewing) 1. mixes and breaks down food - saliva produced by salivary glands 1. begins digestion of carbohydrate with salivary amylase 2. moistens food with mucus for easy swallowing 3. lysozyme lyses bacteria 4. produced in response to stimulation from chemoreceptors and pressure receptors in mouth, or seeing/smelling food • Pharynx and Esophagus - pathways to stomach - • Stomach - 3 major functions 1. store food and release to duodenum at the appropriate rate 2. secrete HCl and enzymes to begin protein digestion (continue carbohydrate digestion with salivary enzymes) 3. mix food with gastric secretions to make chyme - 4 aspects of motility 1. gastric filling a. plasticity - stomach can stretch without increasing tension b. receptive relaxation - eating triggers reflex relaxation 2. gastric storage a. food stored mainly in body of stomach 3. gastric mixing a. mostly in antrum (thicker muscle layer) b. peristaltic contractions 4. gastric emptying a. peristaltic waves push some chyme into duodenum b. influencing factors (stomach) (1) amount of chyme - more chyme increases emptying via direct effect on smooth muscle, intrinsic and extrinsic neurons, hormone gastrin c. influencing factors (duodenum) * (1) stimuli such as fat, acid, increased osmolarity and distention in duodenum trigger slowing of gastric emptying via neural and hormonal responses (2) neural response - short and long reflexes (enterogastric reflex) (3) hormonal response - enterogastrones released to blood act on stomach (secretin, cholecystokinin or CCK, gastric inhibitory peptide or GIP) - gastric secretions come from gastric pits/gastric glands 1. oxyntic mucosa (body and fundus) a. surface epithelial cells secrete thick alkaline mucus that protects stomach from acid and digestive enzymes b. mucous neck cells produce watery, lubricating mucus, also divide rapidly and differentiate into other cell types (entire mucosa replaced every 3 days) c. parietal cells secrete HCl and intrinsic factor (1) HCl activates digestive enzymes and produces optimal pH for protein digestion; breaks down connective tissue and muscle; kills bacteria (2) intrinsic factor allows absorption of vitamin B12 d. chief cells secrete pepsinogen (inactive so cells won't be digested), which is converted to protein digestive enzyme pepsin 2. pyloric gland area (antrum)
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