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The Digestive System

This article provides an overview of the digestive system, its mechanisms for nourishing the body, and the process of digestion in the GI tract. It covers the structure and functions of the upper and lower GI tract, as well as the accessory organs such as the liver, gallbladder, and pancreas. Additionally, it explains the regulation of digestive processes through regulatory peptides and neural regulation.

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The Digestive System

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  1. The Digestive System • Mechanism for nourishing the body • Most nutrients in food require either degradation or release prior to absorption • Digestion occurs in GI Tract • Digestion • Mechanical • chewing and peristalsis • Chemical • enzymes, HCl

  2. Overview of GI Tract • Upper GI Tract • Mouth and pharnyx provide entryway • Lead to Esophagus • Esophagus wall consists of same layers found in remainder of GI tract • epithelium (mucosa), submucosa, muscularis mucosa, circular and longitudinal muscles • Secretory glands found throughout GI except colon • Exocrine glands and Endocrine glands

  3. Upper GI Tract Continued • Sphincters • circular muscles located throughout digestive tract • Lower Esophageal Sphincter • allows passage of food into stomach • Pyloric Sphincter • Controls release of chyme from stomach to duodenum • Stomach • J shaped organ • volume ranges from 50 to 1500 mL (2 -52 oz)

  4. Lower GI tract and Accessory Organs • Small Intestine • duodenum, jejunum and ileum • main site for nutrient digestion and absorption • duodenum receives secretions from liver, gallbladder and pancreas • Liver • hepatocytes make bile from cholesterol • right and left hepatic bile ducts join to form common hepatic duct • unites with cystic duct leading to gallbladder

  5. Lower GI tract and Accessory Organs • Gallbladder • capacity to hold 40-50 mL of bile • functions to concentrate and store bile • Pancreas • 2 types of active tissue • Acini or ducted exocrine tissue • produces digestive enzymes • secreted into small ducts within pancreas which leads to common hepatic bile duct • empties into duodenum • Ductless endocrine tissue • secretes hormones, glucagon and insulin

  6. Structure of SI • Epithelial surface (mucosa) structured to maximize surface area • Large folds of mucosa (folds of Kerckring) • Villi • projections lined with 100s of absorptive cells • contain blood capillaries and central lacteal • Microvilla • extensions of plasma membrane of absorptive cells • possess surface coat of glycocalyx • forms brush border • most digestive enzymes produced by SI found here.

  7. Structure of SI • Crypts of Lieberkuhn • pits located between villi • epithelial cells in these crypt migrate upward and out of crypts toward tips of villi • turnover every 3-5 days • Cells include • paneth cells secrete proteins with unknown function • globlet cells secrete mucus • Enterochromaffin cells with endocrine functions

  8. Structure of SI • Lymphoid Tissues • mucosa and submucosa • Peyer’s Patches • T-lymphocytes and B-lymphocytes • provide defense against bacteria and foreign bodies • Ileocecal valve • allows unabsorbed materials to leave ileum and enter cecum

  9. Colon • Cecum • Ascending, transverse and descending sections • Sigmoid sections • Haustra or pouches • created by contraction of 1 or 3 muscular strips (tenaie coli) along with contraction of circular muscles • Absorbs water and lytes

  10. Regulation Of Digestive Process • Regulatory Peptides (GI hormones and neuropeptides) • Gastrin (hormone) • produced mainly by gastric cells • released upon entry of food into stomach • stimulates HCl release • gastric and intestinal motility • pepsinogen release • Cholecystokinin (CCK, hormone) • produced by SI cells • released upon entry of chyme into stomach • amino acids, fat • targets pancreas and gallbladder

  11. Regulatory Peptides • Secretin • produced by cells of SI • secreted in response to chyme (acid) into duodenum • targets pancreas • may inhibit GI motility • Gastric Inhibitory Polypeptide (GIP) • produced by SI cells • Inhibits gastric secretions and motility • Simulates intestinal and insulin secretions

  12. Regulatory Peptides • Somatostatin • Produced by pancreatic and SI cells • Paracrine • released by endocrine cells but diffuse through extracellular space to target tissue • Enters gastric juice • Inhibits gastrin secretion, as well as GIP and secretin • Inhibits gastric acid release, gastric motility, pancreatic exocrine secretions and gall bladder secretions

  13. Neural Regulation • Enteric Nervous System • located in wall of GI tract • begins in esophagus and ends at anus • controls peristaltic activity • affected by parasympathetic and sympathetic nervous systems • Acetylcholine increase GI motility • Norepinephrine and epinephrine inhibit GI activity • Also influences GI secretions

  14. Neurocrines (Regulatory Peptides) • Neurocrines • Peptides originating from nerves of gut • Vasoactive Intestinal Peptide (VIP) • stimulates intestinal secretions • relaxes GI sphincters • inhibits gastric acid secretion • stimulates bicarbonate release • Gastrin Releasing Peptide (GRP) • Stimulates release of HCl, gastrin and CCK • Neurotensin (Bombesin) • Mediates gastric emptying, gastric acid secretion and SI motility

  15. Process of Digestion • Oral Cavity and Salivary Glands • mastication of food • mixed with salivary gland secretions • parotid glands • water, lytes and enzymes • submandibular and sublingual glands • water, lytes, enzymes and mucus • Enzymes • alpha amylase and lingual lipase

  16. Process of Digestion • Esophagus • Entry of food results in peristalsis • acetylcholine • LES • LES pressure decreases upon swallowing • Neural and hormonal regulation • functions to prevent gastric reflux

  17. Process of Digestion • Stomach • Body extends from LES to angular notch • includes fundus • serves as reservoir as well as producer of gastric juice • Antrum (Pyloric Portion) • extends from angular notch to duodenum • grinds and mixes food with digestive juice (chyme) • provides strong peristalsis

  18. Stomach • Gastric Juices • produced by 3 different gastric glands • Cardiac glands (fundus) • mucus cells (HCO3 and mucus) and endocrine cells • no parietal (oxyntic cells) • Parietal glands (body) • mucus cells, oxyntic cells (secrete HCl and IF), chief cells secrete pepsinogen upon stimulation by acetylcholine • Pyloric glands (antrum) • both mucus and oxyntic cells • endocrine G cells that produce gastrin • Acetylcholine stimulates chief, oxyntic and other gastric cells

  19. Gastric Juices • HCl • activates inactive zymogen pepsinogen to active pepsin • denatures proteins • serves as bactericide • Mucus • lubricates and protects gastric lining • Intrinsic Factor (IF) • necessary for vitamin B12 absorption

  20. HCl • Release from oxyntic cells stimulated by • acetylcholine, gastrin, CCK, histamine • Histamine • secreted from mast cells • paracrine that binds to H2 receptors on parietal cells to stimulate HCl release • mechanism used in drug therapy to lower acid • drugs prevent histamine from binding to H2 receptors • decreases acid release

  21. Gastric Emptying • GI hormones and neuropeptides affect a pacemaker • determines frequency and rate of contractions • 1-5 mL chyme allowed to enter duodenum every 30 seconds • Fat appears to slow gastric emptying • Inhibitors • Secretin, GIP, Somatostatin, CCK

  22. Small Intestine • Chyme has low pH • SI protected by pancreatic secretions and secretions from Brunner’s glands • Secretin and CCK • Digestive juices • Glands within crypts of Lieberkuhn • Pancreas • Responsible for enzymes that digest 50% CHO, 50% protein, 90% lipids

  23. Digestive Juice • Pancreatic Proteases • trypsinogen, chymotrypsinogen, procarboxypeptidases, proelastase, collagenase • secreted in vesicles • must be activated • trypinsogen converted to trypsin by enteropeptidase (enterokinase) and by free trypsin • hydrolyze peptide bonds either internally or from ends • mono, di and some tri can be absorbed • Brush border aminopeptidases to hydrolyze further

  24. Digestive Juice • CHO • Pancreatic alpha amylase • hydrolyzes alpha 1-4 bonds • Alpha dextrinase to hydrolyze 1-6 branches • Brush border enzymes (maltase, sucrase, lactase) • Lipids • Pancreatic lipase hydrolyzes TG to yield MG, FFA and glycerol • Bile required

  25. Bile Synthesis • Made in liver cells from cholesterol • cholesterol oxidized • chenodeoxycholic acid and cholic acid • conjugated to glycine or taurine • glycocholic acid and taurocholic acid • conjugation of bile acids with amino acids improves its ability to form micelles • cholesterol and PLs secreted into bile • bile acid dependent frx • Water, Lytes and bilirubin secreted in bile

  26. Bile Storage • Gallbladder • Concentrates bile by removing 90% of water • Stores bile • Stimulated to release bile by CCK • CCK secreted in response to amino acids and lipids • Bile is secreted into duodenum

  27. Function of Bile • Digestion • Bile acids are amphipathic • Decrease surface tension of fat • Permits emulsification of fat • Increases exposure of area of lipids • Allow digestive enzymes (lipase) to get close to fat • Absorption • Micelle Formation

  28. Micelles • Bile acid monomers from simple micelles • Hydrophobic centers, hydrophilic periphery • Fatty acids and MAGs enter micelles • Mixed Micelles • Micelles travel to brush border • MAGs and fatty acids diffuse through unstirred water layer and into enterocytes • Bile acids released for reuse

  29. Enterohepatic Circulation • > 90% of bile acids secreted in duodenum are reabsorbed into ileum • Enters portal vein for transport via blood back to liver • Reabsorbed bile acids are secreted in bile along with newly made bile acids • New bile mixed with recirculated bile is sent via cystic duct for storage in gallbladder • Pool of bile (2-4 g) may recycle 1-2 times/meal

  30. Secondary Bile Acids • Bile acids not reabsorbed in ileum may be deconjugated by bacteria in colon • deconjugated bile acids form secondary bile acids • cholic acid is converted to deoxycholic acid which may be reabsorbed • chenodeoxycholic acid is converted to lithocholic acid which is excreted in feces • ~ 0.5 g of bile salts are lost in feces daily

  31. Decreasing Blood Cholesterol • Only route of cholesterol excretion is in feces • By increasing excretion of bile in feces, one can lower blood cholesterol levels • necessitates use of body cholesterol for synthesis of new bile acids • drugs (powdered resins) bind bile and enhance excretion from body • soluble fiber behaves like resins

  32. Role of Intestinal Brush Borderin Digestion • CHO Digestion • Isomaltase • 1-6 bonds in oligosaccharides and dextrins • Sucrase • alpha 1-4 bonds in sucrose and maltose • Glucoamylase, glucosidase, maltase • alpha 1-4 bonds in oligosaccharides, maltotriose and maltose • Maltase, Lactase and Sucrase

  33. Role of Intestinal Brush Borderin Digestion • Protein Digestion • Aminopeptidases hydrolyze N terminal amino acids from oligopeptides, tripeptides and dipeptides • Tripeptidases • Dipeptidases

  34. Absorption • Begins in duodenum • Continues throughout jejunum and ileum • May be accomplished by • simple diffusion • facilitated diffusion • active transport • pinocytosis or endocytosis • paracellular absorption • Mechanism depends on • nutrient solubility; electrical gradient; size

  35. Colon • Greater absorption of sodium, chloride and water • Secretion of bicarbonate into colon • neutralize acids produced by colonic anaerobic bacteria via action on CHO • also synthesize vitamin K, biotin and folate • Acids produced in colon include short chain fatty acids • acetate, butyrate, propionate • may be absorbed by colonocyte • Progressive dehydration of unabsorbed materials • 1 L of chyme reduced to < 200 mL of defecated material

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