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Digestive System Chapter 23

Digestive System Chapter 23. Overview. Digestive Processes. Functional Concepts. Mechanical and Chemical Stimuli Intrinsic and extrinsic Controls Two types of reflexes Short =enteric (local) Long =CNS and autonomic nerves. Digestive Anatomy Overview. Hepatic portal system 4 layers

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Digestive System Chapter 23

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  1. Digestive System Chapter 23

  2. Overview

  3. Digestive Processes

  4. Functional Concepts • Mechanical and Chemical Stimuli • Intrinsic and extrinsic Controls • Two types of reflexes • Short =enteric (local) • Long =CNS and autonomic nerves

  5. Digestive Anatomy Overview • Hepatic portal system • 4 layers • Mucosa -mucous membrane • Submucosa -areolar tissue with blood and lymphatic vessels • Muscularis Externa -circular and longitudinal muscle layers • Serosa -areolar and simple squamous • Enteric Nervous System • Submucosal plexus • Myenteric plexus

  6. Functional Anatomy

  7. Functional Anatomy

  8. Saliva • Greater than 97% water • PH 6.75-7.0 • Electrolytes • Enzymes: Salivary amylase and lingual lipase • Mucin -glycoprotein • Immune proteins: Lysozyme, definsin, IgA • Urea and Uric acid • Naturally occurring bacteria • http://www.textbookofbacteriology.net/normalflora.html

  9. Salivary Control • Intrinsic salivary glands keep mouth moist • Extrinsic glands moisten food • Food entering mouth activates • Parasympathetic innervation triggers enzyme rich saliva • Sympathetic triggers thick saliva (mucin rich) • Chemical and mechanical stimulate nuclei in pons and medulla • Sight and smell of food also activate • Lower GI irritation, spicy food, acidic food increase saliva production

  10. Teeth and Gums • Mechanical breakdown of food • Oral disease may cause: • Atherosclerotic plaques • Clotting within coronary and cerebral arteries

  11. Pharynx and Esophagus • Pharynx contains friction free stratified squamous epithelia • Esophagus is a muscular tube • GERD • Hiatal hernia

  12. Digestive Process:Mouth to Esophagus • Mechanical breakdown • Deglutition (swallowing) uses 22 muscle groups • Buccal phase -voluntary movement within mouth • Pharyngeal-esophageal phase -involuntary movement controlled by pons and medulla

  13. The Stomach

  14. Stomach • Simple columnar epithelium -secretes thick alkaline mucous • Gastric pits and glands • Secretions depend upon location • Mucous neck cells -thin mucous • Parietal cells -secretes H+, Cl-, intrisic factor • Chief cells -pepsinogen and lipases • Enteroendocrine cells -histamine, serotonin, somatostatin, gastrin

  15. Homeostatic Imbalances • Gastric Ulcers • 90% caused by Helicobacter pylori • 10-20% of infected show symptoms • Inhibit or lessen HCl production • Damage stomach epithelium • Disrupt cell junctions • Promote chronic inflammation

  16. Digestive Processes in the Stomach • Protein digestion begins • Alcohol and aspirin absorb into blood stream • Secretion of intrinsic factor • Required for absorbtion of vitamin B12 • B12 is needed for RBC production

  17. Regulation of Gastric Secretion • Neural mechanisms • Hormonal mechanism • Stimuli act on brain, stomach, intestine • Three phases: cephalic, gastric, intestinal • Not mutually exclusive phases

  18. Cephalic (Reflex) Phase • Short, lasting only minutes • Prepares stomach to receive food • Sight and smell of food are the trigger • Stimulates mucous, chief, parietal and G cells • Conditional response • Special senses → hypothalamus → medulla → vagus → enteric plexus → stomach glands

  19. Gastric Phase • 3-4 hours • Neural and hormonal response • Triggers • Distension of stomach • Increase in pH (decrease in acidity) • Presence of undigested materials (proteins)

  20. Gastric Phase • Neural response • Produces mixing waves • Stretch and chemoreceptors trigger short (myenteric) reflexes • Ach from parsympathetic neurons stimulates parietal and chief cells • Increase in HCl and Pepsinogen • Proteins, alcohol, caffine stimulate chemoreceptors

  21. Gastric Phase • Hormonal response • Increasing gastrin levels stimulates parietal and chief cells • Gastrin enters blood stream circulates back to stomach • Increase in HCl and Pepsinogen • Decrease in pH (acidity) • Increase in gastric motility • Alkaline tide

  22. Intestinal Phase • Very long, lasting hours • Controls gastric emptying • Neural and hormonal responses • Neural Responses • Chyme leaving stomach relieves distension • No-longer stimulating stretch receptors • Distension of duodenum by chyme stimulates intestinal stretch receptors =enterogastric reflex • Inhibits medulla (gastrin secretions) and local reflexes (gastric contractions) • Stimulates contraction of pyloric sphincter • Mucous production stimulated in duodenum to protect intestine

  23. Intestinal Phase • Hormonal response • Triggered by the arrival of chyme in duodenum • Arrival of lipids and carbohydrates • Stimulates cholesystokinin (CCK) and gastric inhibitory peptide (GIP) • CCK inhibits gastric secretion of acids and enzymes • GIP inhibits gastric secretion and rate of contraction • Meals low in fat enter intestine quicker than those high in fat • Allows more time for lipids to be broken down by intestine • Decrease in ph stimulates enteroendocrine cells to release secretin • Secretin inhibits parietal and chief cells • Stimulates pancreas to produce bicarbonate • Stimulates liver to secrete bile • Partially digested proteins in duodenum stimulate G cells in duodenal wall. • G cells produce gastrin which circulates back to stomach increasing acid production • Gastric processing meets digestive requirements of specific meals

  24. Gastric Motility and Emptying • Response to distension of stomach • Gastric Contractile Activity • Smooth muscle pacemaker cells • Interstitial cells of Cajal set basic electrical rhythm (BER) • Set maximum rate of contraction • Do not initiate contractions • Regulation of Gastric Emptying • Stomach and duodenum work in conjunction

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