1. Gastrointestinal Physiology
Ebaa M Alzayadneh, DDS, PhD
Tortora

2. Organs of the digestive system
Main
Accessory Digestive
Teeth tongue salivary glands liver gall bladder pancreas

3. 6 functions of the digestive system
Ingestion
Secretion of water, acid, buffers, and enzymes into lumen
Mixing and propulsion
Digestion
Mechanical digestion: churns food
Chemical digestion: hydrolysis
Absorption : into blood or lymph
Defecation : elimination of feces

4. Copyright 2009, John Wiley & Sons, Inc.
Layers of GI tract

5. Layers of GI tract from lower esophagus to anal canal
Copyright 2009, John Wiley & Sons, Inc.
Layers of GI tract from lower esophagus to anal canal
Mucosa
Inner lining of mucous membrane; epithelium and enteroendocrine cells, lamina propria of connective tissue, thin layer of smooth muscle cells (muscularis mucosa).
Submucosa
Connetive tissue binds mucosa to muscularis, blood vessels, lymphatic and neurons.
Muscularis
Skeletal in mouth, esophegus and external anal sphincter
Rest of GI; Smooth muscle cells: inner circular and outer longtudinal. Involuntary contractions mix and propel. In between the two muscularis is the myenteric plexus.
Serosa
Visceral peritonium
except esophagus

6. Organization of the enteric nervous system
Enteric nervous system “Brain of the gut”
Myenteric(plexus of Auerobach)
Submucosal (plexus of Meissner
sensory neurons, motor neurons and interneurons
Myenteric: GI motility
motor neurons of submucosal plexus control secretions from mucosal epithelial glands.
sensory neurons of submucosal epithelial. Some are chemoreceptors or stretch receptors
Interneurons interconnect the 2 plexuses
ENS can be independent but still subject to regulation of ANS
Autonomic Nervous
System
Parasympathetic; Vagus and sacral spinal (increase activity)
Sympathetic;
Thoracic and lumbar spinal
(decrease activity)

7. Mouth Oral or buccal cavity
Formed by cheeks, hard and soft palates, and tongue
Oral cavity proper is a space that extends from gums and teeth to fauces (opening between oral cavity and oropharynx)
Salivary glands release saliva
Ordinarily, just enough is secreted to keep mouth and pharynx moist and clean
When food enters mouth, secretion increases to lubricate, dissolve and begin chemical digestion
3 pairs of major salivary glands secrete most of the saliva
Parotid, submandibular, and sublingual

8. Structures of the mouth (oral cavity)

9. The three major salivary glands- parotid, sublingual, and submandibular

10. Saliva Saliva Salivation Mostly water 99.5%
0.5% solutes – ions, dissolved gases, urea, uric acid, mucus, immunoglobulin A, lysozyme, and salivary amylase (acts on starch)
Not all salivary glands produce the same saliva
Salivation
Controlled by autonomic nervous system
Parasympathetic stimulation promotes secretion of moderate amount of saliva
Sympathetic stimulation decreases salivation

11. Digestion in the mouth Mechanical digestion in the mouth
Chewing or mastication
Food manipulated by tongue, ground by teeth, and mixed with saliva
Forms bolus
Chemical digestion in the mouth
Salivary amylase secreted by salivary glands acts on starches
Only monosaccharides can be absorbed
Continues to act until inactivated by stomach acid
Lingual lipase secreted by lingual glands of tongue acts on triglycerides
Becomes activated in acidic environment of stomach

12. Pharynx Passes from mouth into pharynx 3 parts Nasopharynx Oropharynx
Functions only in respiration
Oropharynx
Digestive and respiratory functions
Laryngopharynx

13. Esophagus
Secretes mucous, transports food – no enzymes produced, no absorption
Mucosa – protection against wear and tear
Submucosa
Muscularis divided in thirds
Superior 1/3 skeletal muscle
Middle 1/3 skeletal and smooth muscle
Inferior 1/3 smooth muscle
2 sphincters – upper esophageal sphincter (UES) regulates movement into esophagus, lower esophageal sphincter (LES) regulates movement into stomach
Adventitia – no serosa – attaches to surroundings

14. Deglutition Act of swallowing
Facilitated by secretions of saliva and mucus
Involves mouth, pharynx, and esophagus
3 stages
Voluntary – bolus passed to oropharynx
Pharyngeal – involuntary passage through pharynx into esophagus(the bolus stimulates receptors in the oropharynx to deglut. Center in medulla oblongata and lower pons-soft palate, uvula and epiglottis close off. And upper esophageal sphincter relaxes.
Esophageal – involuntary passage through esophagus to stomach
Peristalsis pushes bolus forward

15. Deglutition (swallowing)
relaxes

16. External and internal anatomy of the stomach
FUNCTIONS
Mix to form chyme
Reservoir :most distensible part of the GI
gastric juice: HCl, pepsin, Intrensic factor, gastric lipase.
Gastrin

17. Histology of the stomach

18. Mechanical and Chemical Digestion
Mechanical digestion
Mixing waves – gentle, rippling peristaltic movements – creates chyme
Chemical digestion
Digestion by salivary amylase continues until inactivated by acidic gastric juice
Acidic gastric juice activates lingual lipase
Digest triglycerides into fatty acids and diglycerides
Parietal cells secrete H+ and Cl- separately but net effect is HCl
Kills many microbes, denatures proteins and stimulates bile and pancreatic juice
Parietal cells secrete intrinsic factor necessary for vitamin B12 absorption.

19. Chemical Digestion Chemical digestion
Pepsin secreted by chief cells digest proteins
Secreted as pepsinogen ( become active at very acidic)
Why it doesn’t digest the stomach or the chief cells?
PH and mucus!
Gastric lipase splits triglycerides into fatty acids and monoglycerides
Small amount of nutrient absorption
Some water, ions, short chain fatty acids, certain drugs (aspirin) and alcohol
Gastrin hormone by G cells: stimulates both parietal and chief cells (HCl and Pepsinogen respectively) , contracts LES and relaxes pyloric s. .

20. Pancreas Lies posterior to greater curvature of stomach
Pancreatic juice secreted into pancreatic duct and accessory duct and to small intestine (duedenum)
Pancreatic duct joins common bile duct and enters duodenum at hepatopancreatic ampulla
Histology
99% of cells are acini
Exocrine
Secrete pancreatic juice – mixture of fluid and digestive enzymes
1% of cells are pancreatic islets (islets of Langerhans)
Endocrine
Secrete hormones glucagon, insulin, somatostatin, and pancreatic polypeptide

21. Relation of the pancreas to the liver, gallbladder, and duodenum

22. Pancreatic juice Sodium bicarbonate – buffers acidic stomach chyme
ml daily
Mostly water, ALKALINE
Sodium bicarbonate – buffers acidic stomach chyme
Enzymes
Pancreatic amylase ( FOR starch)
Proteolytic enzymes – trypsin (secreted as trypsinogen), chymotrypsin (chymotrypsinogen), carboxypeptidase (procarboxypeptidase), elastase (proelastase)
Pancreatic lipase: The principal lipase in adults
Ribonuclease and deoxyribonuclease (for nucleic acids)
Trypsinogen activated by brush border enzyme enterokinase. Trypsin activates Chymotrypsinogen, procarboxypeptidase and proelastase.
Trypsin inhibitor

23. Histology of the Liver

24. Gallbladder
Contraction of smooth muscle fibers eject contents of gall bladder into cystic duct
Functions to store and concentrate bile produced by the liver until it is needed in the small intestine
Jaundice: yellowish pigmentation (bilirubin)

25. Role and composition of bile
Hepatocytes secrete mL of bile daily
Mostly water, bile salts, cholesterol, lecithin, bile pigments and several ions
Partially excretory product/ partially digestive secretion
Bilirubin – principal bile pigment
Derived from the heme of recycled RBCs
Breakdown product stercobilin gives feces brown color
Bile salts play a key role in emulsification
absorption of lipids

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Liver functions
Carbohydrates metabolism: glucose production from glycogen, amino acids, lactic acid, and other sugars.
Lipid metabolism: lipoproteins, store TG, break FA, synthesis of cholestrol.
Protein metabolism: deamination
Drugs and toxins metabolism and detoxification
Bilirubin excretion
Bile salts production and excretion:
Storage: glycogen and vitamins (A, B12, D, E, K), iron
Phagocytosis: aged blood cells and bacteria
Vit. D activation

27. Small intestine 3 regions – duodenum, jejunum, and ileum Same 4 layers
Mucosa
Absorptive cells (digest and absorb), goblet cells (mucus), intestinal glands (intestinal juice), Paneth cells (lysozyme), and enteroendocrine cells ( S cells -secretins, CCK cells,K cells- GIP)
Abundance of MALT and Peyer’s patches (lymphatic follicles)
Submucosa
Duodenal glands secrete alkaline mucus
Muscularis
Serrosa
Completely surrounds except for major portion of the duodenum

28. Anatomy of the small intestine

29. Special structural features increase surface area for digestion and absorption
Circular folds
Permanent ridges of mucosa and submucosa
Cause chyme to spiral
Villi
Fingerlike projections of mucosa
Contains arteriole, venule, blood capillary, and lacteal
Microvilli
Projects of apical plasma membrane of absorptive cells
Brush border has brush border enzymes

30. Histology of the small intestine

31. Histology of the duodenum and ileum

32. Intestinal juice and brush-border enzymes
1-2L daily
Contains water and mucus, slightly alkaline
Provide liquid medium aiding absorption
Brush border enzymes
Inserted into plasma membrane of absorptive cells
Some enzymatic digestion occurs at surface rather than just in lumen
α-dextrinase, maltase, sucrase, lactase, aminopetidase, dipeptidase, nucleosidases and phosphatases

33. Mechanical Digestion Governed by myenteric plexus Segmentations
Localized, mixing contractions to bring chyme to mucosa
Does not move chyme along the Intestine.
Migrating motility complexes (MMC)
A type of peristalsis
Pushes food forward to slowly migrate chyme down the small intestine

34. Chemical digestion Carbohydrates Pancreatic amylase
α-dextrinase ( to glucose), sucrase (to glucose and fructose), lactase (to glucose and galactose), maltase (to glucose) in brush border
Ends up with monosaccharides which can be absorbed
Proteins
Trypsin, chymotrypsin, carboxypeptidase, and elastase from pancreas (from proteins to peptides)
Aminopeptidase and dipeptidase in brush border (to amino acids)

35. Lipids and Nucleic Acids
Pancreatic lipase most important in triglyceride digestion (to fatty acids and monoglycerides)
Emulsification by bile salts increases surface area
Amphipathic – (hydrophobic and hydrophilic )
Nucleic acids
Ribonuclease and deoxyribonuclease in pancreatic juice
Nucleosidases and phosphatases in brush border ( to pentose, phosphates and nitrogenous base)

36. Absorption: Monosaccharides
All dietary carbohydrates digested are absorbed
Only indigestible cellulose and fibers left in feces
Absorbed by facilitated diffusion or active transport into blood
Amino acids, dipetides and tripeptides
Most absorbed as amino acids via active transport into blood

37. Lipids All dietary lipids absorbed by simple diffusion
Short-chain fatty acids go into blood for transport
Long-chain fatty acids and monoglycerides via:
Large and hydrophobic
Bile salts form micelles to ferry them to absorptive cell surface
Reform into triglycerides and coated by proteins forming chylomicrons
Leave cell by exocytosis
Enter lacteals to eventually enter blood with protein coat of chylomicron keeping them suspended and separate

38. Absorption

39. Absorption Electrolytes Vitamins Water From GI secretions or food
Sodium ions (Na+) reclaimed by active transport
Other ions also absorbed by active transport
Vitamins
Fat-soluble vitamins A, D, E, and K absorbed by simple diffusion and transported with lipids in micelles
Most water-soluble vitamins also absorbed by simple diffusion
Water
9.3L comes from ingestion (2.3L) and GI secretions (7.0L)
Most absorbed in small intestine, some in large intestine
Only 100ml excreted in feces
All water absorption by osmosis

40. Large intestine
Overall function to complete absorption, produce certain vitamins, and form and expel feces
4 major regions – cecum, colon, rectum, and anal canal
Ileocecal sphincter between small and large intestine
Colon divided into ascending, transverse, descending and sigmoid
Opening of anal canal (anus) guarded by internal anal sphincter of smooth muscle and external anal sphincter of skeletal muscle

41. Anatomy of the large intestine

42. Digestion of the Large Intestine
Mechanical digestion
Haustral churning
Peristalsis
Mass peristalsis – drives contents of colon toward rectum
Chemical digestion
Final stage of digestion through bacterial action
Ferment carbohydrates, produce some B vitamins and vitamin K
Mucus but no enzymes secreted
Remaining water absorbed along with ions and some vitamins

43. Phases of digestion Cephalic phase Gastric phase Intestinal phase
Smell, sight, thought or initial taste of food activates neural centers – prepares mouth and stomach for food to be eaten
Gastric phase
Neural and hormonal mechanisms promote gastric secretion ( by gastrin) and motility
Intestinal phase
Begins when food enter small intestine
Inhibitory that slows exit of chyme from stomach
Stimulates flow of bile and pancreatic juice

44. The gastric phase of digestion