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Chapter 24 The Digestive System

Chapter 24 The Digestive System. Structure Gross Anatomy Histology Function Mechanical Chemical Development Disorders. Overview of GI tract Functions. Mouth---bite, chew, swallow Pharynx and esophagus----transport Stomach----mechanical disruption; absorption of water & alcohol

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Chapter 24 The Digestive System

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  1. Chapter 24The Digestive System • Structure • Gross Anatomy • Histology • Function • Mechanical • Chemical • Development • Disorders Tortora & Grabowski 9/e 2000 JWS

  2. Overview of GI tract Functions • Mouth---bite, chew, swallow • Pharynx and esophagus----transport • Stomach----mechanical disruption; absorption of water & alcohol • Small intestine--chemical & mechanical digestion & absorption • Large intestine----absorb electrolytes & vitamins (B and K) • Rectum and anus---defecation Tortora & Grabowski 9/e 2000 JWS

  3. Layers of the GI Tract 1. Mucosal layer 2. Submucosal layer 3. Muscularis layer 4. Serosa layer Tortora & Grabowski 9/e 2000 JWS

  4. Mucosa • Epithelium • stratified squamous(in mouth,esophagus & anus) = tough • simple columnar in the rest • secretes enzymes and absorbs nutrients • specialized cells (goblet) secrete mucous onto cell surfaces • enteroendocrine cells---secrete hormones controlling organ function • Lamina propria • thin layer of loose connective tissue • contains BV and lymphatic tissue • Muscularis mucosae---thin layer of smooth muscle • causes folds to form in mucosal layer • increases local movements increasing absorption with exposure to “new” nutrients Tortora & Grabowski 9/e 2000 JWS

  5. Submucosa • Loose connective tissue • containing BV, glands and lymphatic tissue • Meissner’s plexus--- • parasympathetic • innervation • vasoconstriction • local movement by muscularis mucosa smooth muscle Tortora & Grabowski 9/e 2000 JWS

  6. Muscularis • Skeletal muscle = voluntary control • in mouth, pharynx , upper esophagus and anus • control over swallowing and defecation • Smooth muscle = involuntary control • inner circular fibers & outer longitudinal fibers • mixes, crushes & propels food along by peristalsis • Auerbach’s plexus (myenteric)-- • both parasympathetic & sympathetic innervation of circular and longitudinal smooth muscle layers Tortora & Grabowski 9/e 2000 JWS

  7. Serosa • An example of a serous membrane • Covers all organs and walls of cavities not open to the outside of the body • Secretes slippery fluid • Consists of connective tissue covered with simple squamous epithelium Tortora & Grabowski 9/e 2000 JWS

  8. Peritoneum • Peritoneum • visceral layer covers organs • parietal layer lines the walls of body cavity • Peritoneal cavity • potential space containing a bit of serous fluid Tortora & Grabowski 9/e 2000 JWS

  9. Parts of the Peritoneum • Mesentery • Mesocolon • Lesser omentum • Greater omentum • Peritonitis = inflammation • trauma • rupture of GI tract • appendicitis • perforated ulcer Tortora & Grabowski 9/e 2000 JWS

  10. Greater Omentum, Mesentery & Mesocolon Tortora & Grabowski 9/e 2000 JWS

  11. Lesser Omentum Tortora & Grabowski 9/e 2000 JWS

  12. Peritonitis • Acute inflammation of the peritoneum • Cause • contamination by infectious microbes during surgery or from rupture of abdominal organs Tortora & Grabowski 9/e 2000 JWS

  13. Mouth • Lips and cheeks-----contains buccinator muscle that keeps food between upper & lower teeth • Vestibule---area between cheeks and teeth • Oral cavity proper---the roof = hard, soft palate and uvula • floor = the tongue Tortora & Grabowski 9/e 2000 JWS

  14. Pharyngeal Arches • Two skeletal muscles • Palatoglossal muscle • extends from palate to tongue • forms the first arch • posterior limit of the mouth • Palatopharyngeal muscle • extends from palate to pharyngeal wall • forms the second arch • behind the palatine tonsil Tortora & Grabowski 9/e 2000 JWS

  15. Salivary Glands • Parotid below your ear and over the masseter • Submandibular is under lower edge of mandible • Sublingual is deep to the tongue in floor of mouth • All have ducts that empty into the oral cavity Tortora & Grabowski 9/e 2000 JWS

  16. Composition and Functions of Saliva • Wet food for easier swallowing • Dissolves food for tasting • Bicarbonate ions buffer acidic foods • bulemia---vomiting hurts the enamel on your teeth • Chemical digestion of starch begins with enzyme (salivary amylase) • Enzyme (lysozyme) ---helps destroy bacteria • Protects mouth from infection with its rinsing action---1 to 1 and 1/2qts/day Tortora & Grabowski 9/e 2000 JWS

  17. Salivary Gland Cellular Structure • Cells in acini (clusters) • Serous cells secrete a watery fluid • Mucous cells (pale staining) secrete a slimy, mucus secretion Tortora & Grabowski 9/e 2000 JWS

  18. Salivation • Increase salivation • sight, smell, sounds, memory of food, tongue stimulation---rock in mouth • cerebral cortex signals the salivatory nuclei in brainstem---(CN 7 & 9) • parasympathetic nn. (CN 7 & 9) • Stop salivation • dry mouth when you are afraid • sympathetic nerves Tortora & Grabowski 9/e 2000 JWS

  19. Mumps • Myxovirus that attacks the parotid gland • Symptoms • inflammation and enlargement of the parotid • fever, malaise & sour throat (especially swallowing sour foods) • swelling on one or both sides • Sterility rarely possible in males with testicular involvement (only one side involved) • Vaccine available since 1967 Tortora & Grabowski 9/e 2000 JWS

  20. Structure and Function of the Tongue • Muscle of tongue is attached to hyoid, mandible, hard palate and styloid process • Papillae are the bumps---taste buds are protected by being on the sides of papillae Tortora & Grabowski 9/e 2000 JWS

  21. Tooth Structure • Crown • Neck • Roots • Pulp cavity Tortora & Grabowski 9/e 2000 JWS

  22. Composition of Teeth • Enamel • hardest substance in body • calcium phosphate or carbonate • Dentin • calcified connective tissue • Cementum • bone-like • periodontal ligament penetrates it What is the gingiva? Tortora & Grabowski 9/e 2000 JWS

  23. Dentition • Primary or baby teeth • 20 teeth that start erupting at 6 months • 1 new pair of teeth per month • Permanent teeth • 32 teeth that erupt between 6 and 12 years of age • differing structures indicate function • incisors for biting • canines or cuspids for tearing • premolars & molars for crushing and grinding food Tortora & Grabowski 9/e 2000 JWS

  24. Primary and Secondary Dentition Tortora & Grabowski 9/e 2000 JWS

  25. Digestion in the Mouth • Mechanical digestion (mastication or chewing) • breaks into pieces • mixes with saliva so it forms a bolus • Chemical digestion • amylase • begins starch digestion at pH of 6.5 or 7.0 found in mouth • when bolus & enzyme hit the pH 2.5 gastric juices hydrolysis ceases • lingual lipase • secreted by glands in tongue • begins breakdown of triglycerides into fatty acids and glycerol Tortora & Grabowski 9/e 2000 JWS

  26. Pharynx • Funnel-shaped tube extending from internal nares to the esophagus (posteriorly) and larynx (anteriorly) • Skeletal muscle lined by mucous membrane • Deglutition or swallowing is facilitated by saliva and mucus • starts when bolus is pushed into the oropharynx • sensory nerves send signals to deglutition center in brainstem • soft palate is lifted to close nasopharynx • larynx is lifted as epiglottis is bent to cover glottis Tortora & Grabowski 9/e 2000 JWS

  27. Esophagus • Collapsed muscular tube • In front of vertebrae • Posterior to trachea • Posterior to the heart • Pierces the diaphragm at hiatus • hiatal hernia or diaphragmatic hernia Tortora & Grabowski 9/e 2000 JWS

  28. Histology of the Esophagus • Mucosa = stratified squamous • Submucosa = large mucous glands • Muscularis = upper 1/3 is skeletal, middle is mixed, lower 1/3 is smooth • upper & lower esophageal sphincters are prominent circular muscle • Adventitia = connective tissue blending with surrounding connective tissue--no peritoneum Tortora & Grabowski 9/e 2000 JWS

  29. Physiology of the Esophagus - Swallowing • Voluntary phase---tongue pushes food to back of oral cavity • Involuntary phase----pharyngeal stage • breathing stops & airways are closed • soft palate & uvula are lifted to close off nasopharynx • vocal cords close • epiglottis is bent over airway as larynx is lifted Tortora & Grabowski 9/e 2000 JWS

  30. Swallowing • Upper sphincter relaxes when larynx is lifted • Peristalsis pushes food down • circular fibers behind bolus • longitudinal fibers in front of bolus shorten the distance of travel • Travel time is 4-8 seconds for solids and 1 sec for liquids • Lower sphincter relaxes as food approaches Tortora & Grabowski 9/e 2000 JWS

  31. Gastroesophageal Reflex Disease • If lower sphincter fails to open • distension of esophagus feels like chest pain or heart attack • If lower esophageal sphincter fails to close • stomach acids enter esophagus & cause heartburn (GERD) • for a weak sphincter---don't eat a large meal and lay down in front of TV • smoking and alcohol make the sphincter relax worsening the situation • Control the symptoms by avoiding • coffee, chocolate, tomatoes, fatty foods, onions & mint • take Tagamet HB or Pepcid AC 60 minutes before eating • neutralize existing stomach acids with Tums Tortora & Grabowski 9/e 2000 JWS

  32. Anatomy of Stomach • Which side is it on? • Size when empty? • large sausage • stretches due to rugae • Parts of stomach • cardia • fundus---air in x-ray • body • pylorus---starts to narrow as approaches pyloric sphincter • Empties as small squirts of chyme leave the stomach through the pyloric valve Tortora & Grabowski 9/e 2000 JWS

  33. Pylorospasm and Pyloric Stenosis • Abnormalities of the pyloric sphincter in infants • Pylorospasm • muscle fibers of sphincter fail to relax trapping food in the stomach • vomiting occurs to relieve pressure • Pyloric stenosis • narrowing of sphincter indicated by projectile vomiting • must be corrected surgically Tortora & Grabowski 9/e 2000 JWS

  34. Histology of the Stomach Tortora & Grabowski 9/e 2000 JWS

  35. Mucosa & Gastric Glands • Hydrochloric acid converts pepsinogen from chief cell to pepsin • Intrinsic factor • absorption of vitamin B12 for RBC production • Gastrin hormone (g cell) • “get it out of here” • release more gastric juice • increase gastric motility • relax pyloric sphincter • constrict esophageal sphincter preventing entry Tortora & Grabowski 9/e 2000 JWS

  36. Submucosa Tortora & Grabowski 9/e 2000 JWS

  37. Muscularis • Three layers of smooth muscle--outer longitudinal, circular & inner oblique • Permits greater churning & mixing of food with gastric juice Tortora & Grabowski 9/e 2000 JWS

  38. Serosa • Simple squamous epithelium over a bit of connective tissue • Also known as visceral peritoneum Tortora & Grabowski 9/e 2000 JWS

  39. Physiology--Mechanical Digestion • Gentle mixing waves • every 15 to 25 seconds • mixes bolus with 2 quarts/day of gastric juice to turn it into chyme (a thin liquid) • More vigorous waves • travel from body of stomach to pyloric region • Intense waves near the pylorus • open it and squirt out 1-2 teaspoons full with each wave Tortora & Grabowski 9/e 2000 JWS

  40. Physiology--Chemical Digestion • Protein digestion begins • HCl denatures (unfolds) protein molecules • HCl transforms pepsinogen into pepsin that breaks peptides bonds between certain amino acids • Fat digestion continues • gastric lipase splits the triglycerides in milk fat • most effective at pH 5 to 6 (infant stomach) • HCl kills microbes in food • Mucous cells protect stomach walls from being digested with 1-3mm thick layer of mucous Tortora & Grabowski 9/e 2000 JWS

  41. Regulation of Gastric Secretion and Motility • Cephalic phase • Gastric phase • Intestinal phase Tortora & Grabowski 9/e 2000 JWS

  42. Cephalic Phase = “Stomach Getting Ready” • Cerebral cortex =sight, smell, taste & thought • stimulate parasympathetic nervous system • Vagus nerve • increases stomach muscle and glandular activity Tortora & Grabowski 9/e 2000 JWS

  43. Gastric Phase = “Stomach Working” • Nervous control keeps stomach active • stretch receptors & chemoreceptors provide information • vigorous peristalsis and glandular secretions continue • chyme is released into the duodenum • Endocrine influences over stomach activity • distention and presence of caffeine or protein cause G cells secretion of gastrin into bloodstream • gastrin hormone increases stomach glandular secretion • gastrin hormone increases stomach churning and sphincter relaxation Tortora & Grabowski 9/e 2000 JWS

  44. Intestinal Phase = “Stomach Emptying” • Stretch receptors in duodenum slow stomach activity & increase intestinal activity • Distension, fatty acids or sugar signals medulla • sympathetic nerves slow stomach activity • Hormonal influences • secretin hormone decreases stomach secretions • cholecystokinin(CCK) decreases stomach emptying • gastric inhibitory peptide(GIP) decreases stomach secretions, motility & emptying Tortora & Grabowski 9/e 2000 JWS

  45. Absorption of Nutrients by the Stomach • Water especially if it is cold • Electrolytes • Some drugs (especially aspirin) & alcohol • Fat content in the stomach slows the passage of alcohol to the intestine where absorption is more rapid • Gastric mucosal cells contain alcohol dehydrogenase that converts some alcohol to acetaldehyde-----more of this enzyme found in males than females • Females have less total body fluid that same size male so end up with higher blood alcohol levels with same intake of alcohol Tortora & Grabowski 9/e 2000 JWS

  46. Regulation of Gastric Emptying • Release of chyme is regulated by neural and hormonal reflexes • Distention & stomach contents increase secretion of gastrin hormone & vagal nerve impulses • stimulate contraction of esophageal sphincter and stomach and relaxation of pyloric sphincter • Enterogastric reflex regulates amount released into intestines • distension of duodenum & contents of chyme • sensory impulses sent to the medulla inhibit parasympathetic stimulation of the stomach but increase secretion of cholecystokinin and stimulate sympathetic impulses • inhibition of gastric emptying Tortora & Grabowski 9/e 2000 JWS

  47. Vomiting (emesis) • Forceful expulsion of contents of stomach & duodenum through the mouth • Cause • irritation or distension of stomach • unpleasant sights, general anesthesia, dizziness & certain drugs • Sensory input from medulla cause stomach contraction & complete sphincter relaxation • Contents of stomach squeezed between abdominal muscles and diaphragm and forced through open mouth • Serious because loss of acidic gastric juice can lead to alkalosis Tortora & Grabowski 9/e 2000 JWS

  48. Anatomy of the Pancreas • 5" long by 1" thick • Head close to curve in C-shaped duodenum • Main duct joins common bile duct from liver • Sphincter of Oddi on major duodenal papilla • Opens 4" below pyloric sphincter Tortora & Grabowski 9/e 2000 JWS

  49. Histology of the Pancreas • Acini- dark clusters • 99% of gland • produce pancreatic juice • Islets of Langerhans • 1% of gland • pale staining cells • produce hormones Tortora & Grabowski 9/e 2000 JWS

  50. Composition and Functions of Pancreatic Juice • 1 & 1/2 Quarts/day at pH of 7.1 to 8.2 • Contains water, enzymes & sodium bicarbonate • Digestive enzymes • pancreatic amylase, pancreatic lipase, proteases • trypsinogen---activated by enterokinase (a brush border enzyme) • chymotrypsinogen----activated by trypsin • procarboxypeptidase---activated by trypsin • proelastase---activated by trypsin • trypsin inhibitor---combines with any trypsin produced inside pancreas • ribonuclease----to digest nucleic acids • deoxyribonuclease Tortora & Grabowski 9/e 2000 JWS

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