Understanding Digestion Process: An In-depth Review

1. Digestion • Anatomy of digestive system (review!) • 4 major processes of digestive system: • Motility • Secretion • Digestion • Absorption Gastrointestinal System

3. Motility • Secretion • Digestion • Absorption Motility 2 purposes: • Forward movement of food • Mechanical mixing GI smooth muscles contract spontaneously • Pacemaker cells, (Interstitial Cells of Cajal), connected by gap junctions, generate slow wave potentials • AP’s spread throughout longitudinal muscles (gap junctions)  wave of contraction • Like cardiac muscle, Ca2+ can regulate contraction strength

4. Different Patterns of Contraction Tonic Contractions Sustained contraction, usually in the stomach Phasic Contractions Peristaltic contractions • progressive waves moving along segments of longitudinal layer  forward propulsion • circular layer contracts proximal to bolus • Especially esophagus Segmental contractions • alternate contraction & relaxation lead to mixing • A side effect of narcotics

5. Motility • Secretion • Digestion • Absorption Secretion • 9 L of fluid pass through the GIT (only 2 L from food & drink)  Secretion and Reabsorption important • Ions and water; similar to renal mechanisms • Saliva • Hydrochloric Acid (Parietal Cells) • Bicarbonate (enzyme necessary ?) • Enzymes (zymogens) • Mucus (Goblet cells) • Bile (bile salts – function?) • Net Fluid Balance in GI system

6. Activation of Zymogens / Proenzymes

7. Regulation of GIT • ANS • Parasympathetic (R & R) • Sympathetic • Emotional (cephalic reflexes) • E.g., smell of food • ENS (Enteric Nervous System) • Self-contained (intrinsic) • GI peptides can have regulatory role as hormones or paracrines • E.g., Gastrin, CCK

8. Motility • Secretion • Digestion • Absorption Digestion Overview • Mechanical breakdown and mixing aid enzymatic breakdown • Chewing • Tonic contractions, esp. stomach • Enzymatic breakdown converts macromolecules into absorbable units • Bile emulsifies fats • Optimal pH of enzymes indicates location of activity

9. Motility • Secretion • Digestion • Absorption Absorption Overview • Most nutrient absorption takes place in ? • Fats absorbed into lacteals • Everything else absorbed into portal vein • Alcohol & aspirin across gastric epithelium • Additional: H2O, ions & some vitamins absorbed in ________________ • Mechanisms analogous to renal absorption

10. CHO Digestion & Absorption • ~50% of calories in average American diet • Starch (polysaccharide) and sucrose (disaccharide) • Cellulose (roughage) not digestible • Enzymes: amylases, disaccharidases (maltase, sucrase, lactase) • Absorbed only as monosaccharides (glucose, fructose) • Small intestine

11. Protein Digestion and Absorption • Variable digestibility • 30-60% of protein not from diet • First digestion in Stomach by HCl • Proteases secreted as proenzymes • Pepsin(-ogen), trypsin, etc. • Absorption of single a.a. and di- and tripeptides • Specific receptors required for larger chains • May serve as allergens • DDAVP

12. Lipid (fat) Digestion • Mostly triglycerides in diet • Cholesterol, Fat-soluble vitamins, others. • Combination with bile salts creates an emulsion • Colipase and lipase allow formation of small micelles • Absorption of fat via diffusion across apical CM • Chylomicrons in the cell are absorbed into lacteals

13. Other Stuff • Water soluble vitamins—mediated transport • Fat soluble vitamins via absorption • Water, Ions and Minerals • Various locations and methods, e.g, diffusion, carrier proteins • Nucleic Acids

14. Colon Cancer • 2nd largest cause of cancer deaths • Cellulose (indigestible) = fiber, roughage • Significance of “roughage” in diet??

15. OlestraBiotechnology focus • Consumers want fat in their foods  search for low cal fat substitutes • 1996, Procter & Gamble market Olestra Olestra = Olean = sucrose polyester(sucrose molecule + 6-8 FA) • Cannot be digested, nor absorbed • Unpleasant side effects • Absorption of fat sol. vitamins pot. problem

16. Phases of Digestion/Absorption Cephalic Gastric Intestinal Defecation

17. Cephalic Gastric Intestinal Defecation • Anticipation • Begins with sensory input that is sent to the cerebral cortex and then to the hypothalamus to stimulate the PNS • Salivation, relaxation of the GE sphincter, relaxation of stomach muscles, increase gastric motility • Mastication • Mechanical digestion • Teeth, tongue • Deglutition • Bolus formation for swallowing • Peristalsis in esophagus

22. Salivary Glands • Parotid Glands • Secretes Mucous • Allows for smooth swallowing of bolus • Mumps • Sublingual and Submandibular glands secrete Amylase • Breaks down complex carbohydrates into simple carbohydrates

23. Deglutition

26. Reflux Esophagitis = Heartburn = GERD Gastroesphogeal Reflux Disease Lower esophageal sphincter dysfunction Why reflux against gravity?

27. GERD, cont’d • Intrapleural pressure  during inspiration  Wall of esophagus expands • Subatmospheric pressure in esophageal lumen sucks acidic stomach contents into esophagus • May also occur during pregnancy or obesity

28. Cephalic • Gastric • Intestinal • Defecation • Storage • Relaxation or contraction as needed. • Digestion • HCl - Hydrochloric Acid (Parietal cells) • Pepsin (Chief cells) • Mechanical • Immune Protection • Ingested pathogens • Respiratory mucus

30. Mechanical Digestion • Oblique, Longitudinal and Circular Gastric Muscles • Esophagus does not contain oblique muscles • Allows for gastric motility to occur • Muscles are sensitive to • Parasympathetic input • Gastrin hormone • Rugae are used along with the Gastric Muscles for mechanical digestion

32. Chemical Digestion • Gastric Pits • Invaginations of the stomach wall • Lined with several different types of cells • Chief Cells • At the bottom of the pits • Secretes pepsinogen • Inactive form of the enzyme pepsin • Pepsinogen is converted into pepsin upon mixing with HCl • Pepsin breaks down proteins into amino acids

33. Chemical Digestion Cont. • Parietal Cells • Above Chief Cells • Secret H+ and Cl- into the Gastric Pits • Used for Digestion of foodstuff • Gastric Glands or G cells • Secretes the hormone Gastrin • Gastrin secretion increases when the contents of the stomach is basic • Gastrin secretion decreases when the contents of the stomach is acidic • Gastrin’s functions are • To increase HCl secretion from the Parietal Cells • To increase Gastric Motility

34. Mucous Cells • Secrete Mucous to coat the lumen of the stomach • Prevents ulceration • H. Pylori also responsible for ulcers

36. Question • 18 year old White Female Adult enters into the emergency room. • She has been vomiting for over 72 hours. • You observe she is also hypoventilating. • How is the vomiting and hypoventilation tied together? • List two of the possible problems are that your patient is experiencing. • HINT: Don’t forget the Parietal Cell

38. Cephalic • Gastric • Intestinal – Chyme (ingesta) enters small intestine • Defecation • Neutralization of HCl • NaHCO3 from pancreas • Running Problem: Peptic Ulcer and antacids, including H2 antagonists and proton pump inhibitors • Pancreatic enzymes • Digest protein, CHO • Bile acids • Emulsification of Fat • Absorption of H2O

40. Mechanical Digestion • Peristalsis and Segmentation • Occurs in the small intestine • Food is basically shaken back and forth in small segments along the length of the small intestine

41. Chemical Digestion • During the segmentation process the food scrapes along the tips of the villi and causes cells to explode off • Release Digestive Enzymes • Rapid Mitosis in the Crypts of Lieberchun or Intestinal crypts • Easily affected by chemo/radiation therapy due to rapid mitosis

42. Chemical Digestion • Release of Digestive Enzymes • Pancreas releases digestive enzymes into the duodenum • Release of Bile • Emulsification of Fat • Storage and Release by Gall Bladder • Production of Bile by Liver • Bile contains: • Cholesterol • Bile Salts • Lecithin • Billirubin