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Chapter 6

Chapter 6. DIGESTION & ABSORPTION. § 1. Introduction  Concept of digestion & absorption * Mechanical & chemical digestion  Characteristics of digestive smooth muscles * General characteristics * Electrophysiological characteristics Resting potential (RP)

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Chapter 6

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  1. Chapter 6 DIGESTION & ABSORPTION § 1. Introduction Concept ofdigestion&absorption *Mechanical & chemical digestion Characteristics of digestive smooth muscles *General characteristics *Electrophysiological characteristics Resting potential (RP) Slow wave or basic electrical rhythm (BER) Action potential (AP)

  2. Electrical activities & contraction of digestive smooth muscles

  3. Secretion ofdigestive glands *Main functions of digestive fluids Gastrointestinal innervation *Intrinsic (or enteric) nervous system Submucosal& myenteric plexus *Extrinsic nervous system Parasympathetic & sympathetic nerves Gastrointestinal hormones *General functions *Brain-gut peptides

  4. Intrinsic (or enteric) nervous system

  5. Extrinsic nervous system

  6. § 2. Digestion in the mouth & esophagus Saliva & its function *Nature & Composition of saliva pH 6.6~7.1, 99% water Salivary amylase, lysozyme, salt, etc. *Functionsof saliva Moisten the cavity & solve food; keep the mouth clean; anti-bacteria; digest starch *Regulation of salivary secretion Pure neuroregulation Conditioned & unconditioned reflex

  7. Mastication or chewing & its effects *Definition *Effects Break up large food particles; mixes the food with saliva; aid swallowing Deglutition or swallowing *Definition *Process 1st, 2nd & 3rd phase Peristalsis & lower esophageal sphincter

  8. Peristalsis of the esophagus

  9. § 3. Digestion in the stomach Gastric juice & its secretion *Nature, Compositions & functions pH 0.9~1.5, colorless, 1.5~2.5 L/day Hydrochloric acid(HCl) Secreted by parietal cells Basic secretion: 0~5 mmol/h Max secretion: 20~25 mmol/h Mechanism of HCl secretion Functions: see next page

  10. Relationship between max volume of HCl secretion & the number of parietal cells

  11. Functions of HCl: Turn pepsinogen into pepsin & provide a medium of low pH favoring pepsin action; Kill many swallowed virulent organisms; Promote the secretion of pacreas & bile; Promote the absorption of calcium & ferrum Pepsinogen Secreted bychief cells & activate by HCl Function: digest proteins into proteoses, peptones, polypeptides, with little free amino acids

  12. Mucus Secreted by neck & surface mucous cells, cardiac & pyloric glands Function: form mucus-bicarbonate barrier Intrinsic factor Secreted by parietal cells Function:Promote the absorption of Vit B12 *Regulation of gastric secretion Endogenous releasing substances Acetylcholine (ACh), gastrin & histamine Somatostatin (SST) (release-inhibiting)

  13. Mucus-bicarbonate barrier

  14. Action of acetylcholine, gastrin & histamine on the perietal cells

  15. Gastric secretion in the digestive phase Cephalic phase Mechanisms: Conditioned & Unconditioned reflex Vagal efferent & with Gastrin secretion through gastrin-releasing peptide (GRP) Experiment: Sham feeding by Pavlov Characteristics: Large quantity (30%) High acidity & digestive power

  16. Experiment of Sham feeding by Pavlov

  17. Gastric phase Mechanisms: Distensionof gastric fundus & body initiating vagovagal & local plexus reflexes Distension of pylorus initiating a release of gastrin through intrinsic plexus Chemical stimulation of G cells initiaying a release of gastrin Experiment:Pavlov pouch Characteristics: Large quantity (60%) High acidity & digestive power

  18. Heidenhain (R) & Pavlov (L) pouch

  19. Intestinal phase Mechanisms: Mainly humoral regulation Chemical & Mechanical stimulation initiating releases of Gastrin, Entero- oxyntin & Other humoral factors Experiment: Pavlov pouch Characteristics: Small quantity (10%) Lower acidity & digestive power

  20. Cephalic, gastric, & intestinal phase of gastric juice secretion

  21. Summary of cephalic, gastric, & intestinal phase of gastric juice secretion

  22. Inhibitory regulation Hydrochloric acid (HCl) A typical example of negative feedback Conditions & Mechanisms: pH ≤ 1.2~1.5 in the gastric antrum Inhibition of G cells, Release of SST pH ≤ 2.5 in the duodenum Release of secretin, bulbogastrone Fat: Initiating release of enterogastrone Hypertonic solution:Entero-gastric reflex

  23. Gastric motility & emptying * Main types of gastric motility Tonic contraction Maintaining the shape & place of stomach, as well as the intragastric pressure Receptive relaxation Definition & its innervation Peptide or NO –ergic vagal fibers Gastric peristalsis Initiate from middle stomach, 3 times/min Mix & grind food into chyme & help empty

  24. *Gastric emptying & its control Definition & some rules Carbohydrate > proteins > lipids, Mix 4~6 h Discontinuous emptying Adapting for digestion & absorption Control of emptying Promotion in stomach: vagovagal & plexus reflex, gastrin release Inhibition in duodenum: entero-gastric reflex & enterogastrone

  25. *Gastric motility in the digestive interphase Migrating motor complex (MMC) Phase I: resting phase, 90~120 min Phase II: irregular spike & sporadic peristalsis, 30~45 min Phase III: large amount of spikes & regular strong contraction, 5~10 min Phase IV: transition period, about 5 min Significance: act as a street cleaner Vomiting Definition: a reflex, benefit & harm

  26. Migrating motor complex (MMC)

  27. § 4. Digestion in the intestine Pancreatic juice& its secretion *Nature, Compositions & functions pH 7.8~8.4, colorless & odourless, 1~2 L/day Bicarbonate (HCO3) Neutralize HCl & provide a weak basic medium favoring digestive enzyme action Pancreatic enzymes: amylase, lipase, colipase, trypsinogen & chymtrypsinogen, etc. Turn trypsinogen into trypsin by entero- kinase, turn chymtrypsinogen into chym- trypsin by trypsin Trypsin inhibitor: a polypeptide

  28. *Regulation of pancreatic secretion Nervous regulation Vagus nerve: ACh, gastrin Characteristics: H2O & HCO3, enzymes Sympathetic nerve: ACh, NA Characteristics: weak effect Humoral reulation Secretin: H2O & HCO3, enzymes Cholecystokinin (CCK): Characteristics: H2O & HCO3, enzymes Feedback:CCK-releasing peptide

  29. Bilesecretion&gallbladder emptying *Nature, Compositions & functions Hepatic bile: pH 7.4, golden yellow Bladder bile: pH 6.8, color become darker Compositions: H2O, ions, bile acid, bile pigment, fatty acid, cholesterol, lecithin, mucoprotein, etc., but no enzyme Functions of bile (mainly by bile salt): Fat emulsification; lipid absorption; Promote the absorption of fat-soluble Vits

  30. *Control of bile secretion & gallbladder emptying Nervous regulation Vagus nerve: ACh, gastrin Hepatic bile secretion (small amounts) Gallbladder contraction (slightly) Humoral reulation Gastrin:direct to hepatic cells & gallbladder; indirect to stomachHClsecretin  Secretin: act to bile duct & not to hepatic cells, so: H2O & HCO3, bile salt () Cholecystokinin (CCK): gallbladder contraction & Oddi’s sphincter dialation Bile salt: enterohepatic circulation of bile salt

  31. Enterohepatic circulation of bile salt

  32. Secretionofthe small intestine * Intestinal glands Brunner’s gland (basic mucus) Lieberkuhn Crypt (main part) *Nature, Compositions & functions pH 7.6, iso-osmotic solution, 1~3 L/day Enterokinase, the only one secreted Oligopeptidase,oligosaccharidase those contained in the epithelial cells * Regulation of secretion of small intestine Mainly through plexus reflex initiated by inflation of the intestine

  33. Movements ofthe small intestine * Movement in the digestive interphase MMC similar to that in the stomach * Main movement forms in digestive phase Tonic contraction: the same as in stomach Segmentation: peculiar to the small intestine Peristalsis: 0.5~2.0 cm/s Peristaltic rush: 2~25 cm/s * Regulation of small intestinal movement

  34. Segmentation of the small intestine

  35. * Regulation of small intestinal movement Intrinsic plexus: receivemechanical & chemical stimuli and then cause peristalsis Extrinsic plexus: parasympathetic nerve intestinal activities; sympathetic never intestinal activities Humoral factors: gastrin, CCK, Enk, 5-HT, etc. Functions ofthe ileocecal sphincter * Permit some of chyme to squirt into the cecum each time when a peristalsis wave reaches it * Prevent reflux of colonic contents into the ileum

  36. § 5. Digestion in the colon Main functionof the colon Absorb water Absorb Vit B & K synthesized by bacteria Formation & temporarily storage feces Secretionof the colon pH 8.3~8.4, Mucus & HCO3 Protect mucosa against injuries & lubricate feces Movementsof the colon Haustral to and fro movement, Segmentation or Polyhaustral pushing movement, Peristalsis, Mass peristalsis

  37. Feces &Defecation *Feces:contain inorganic material, undigested plant fibers, bacteria, water, etc. *Defecation:a reflex initiated by distension of the rectum with feces Reflex process, Primary & higher centers Action of colonic bacteria *Benefit:synthesis Vit. Bco & K *Harm:produce some poisonous substances Action of dietary fibers on the intestinal function Reduce absorption of water; Enhance colonic movement; Increase the volume of feces; Low the caloric ratio of the food

  38. § 6. Absorption in the small intestine Sites of absorption Oral cavity & Stomach: little Duodenum & Upper jejunum: most nutrients Ileum: bile salts & Vit. B12 Colon: water & electrolytes Proofs as the main absorptive region Huge absorptive surface (200 m2) Plenty of capillaries & lymph capillaries Large quantity of digestive fluid (6~8 L/day) Food has almost completely been digested

  39. Sites of absorption

  40. Enlargement of the surface area of the intestine

  41. Absorption of main nutrients *Water 8 L/day, passive & iso-osmotic absorbed Different absorbability in different parts *Inorganic slats Sodium: 95%~99%, jejunum>ileum>colon active transport Ferrum: 1/10, mainly in duodenum & jejunum, transferrin dependent, active transport Calcium: promote by Vit. D, active transport Anions: mainly Cl  & HCO3, passive transport

  42. *Carbohydrate Absorptive form: monosarccharide Mechanism: secondary active transport *Protein Absorptive form: amino acid Mechanism: secondary active transport *Fats Absorptive form: glycerol, monoglyceride, fatty acid, cholesterol Mechanism: passive diffusion Pathway: blood & lymph

  43. Digestion & absorption of fats in the intestine

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