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

3. Motility Forward movement of food Mechanical mixing
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. Secretion
Motility
Secretion
Digestion
Absorption
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 Emotional (cephalic reflexes)
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. Digestion Overview Bile emulsifies fats
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. Absorption Overview Most nutrient absorption takes place in ?
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. Olestra Biotechnology focus
Consumers want fat in their foods  search for low cal fat substitutes
1996, Procter & Gamble market Olestra Olestra = Olean = sucrose polyester (sucrose molecule 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. Gastroesphogeal Reflux Disease Lower esophageal sphincter dysfunction
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 Chief Cells
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. Intestinal – Chyme (ingesta) enters small intestine Defecation
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 Release of Bile
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

51. Bacterial fermentation of CHO and proteins
Cephalic
Gastric
Intestinal
Defecation
Bacterial fermentation of CHO and proteins
Lactate, some vitamins are digested and/or absorbed
More H2O absorption
Osmotic diarrhea vs. secretory diarrhea
Osmotic-solutes prevent H2O reabsorption
Lactose intolerance
Secretory- bacterial toxins ("flush out' pathogens)
Defecation Reflex

53. Colon Three Major Functions of the Large Intestine Absorption of H2O
Storage of Feces
Usually 24 hours
Production of Vitamin K
From E. Coli
Used in the Blood Clotting Process

55. Lactose Intolerance Lactose = glucose + galactose
Lactase only found in juvenile mammals
Adaptive (dominant) mutation in populations with dairy-based cultures
Lactose intolerance in
95% of Native Americans,
90% of Asian Americans
70% of African Americans
50% of Mexican Americans