1. 4 major processes of digestive system:
Ch 21: Digestion
Anatomy of digestive system (review!)
4 major processes of digestive system:
Motility
Secretion
Digestion
Absorption
Fort Mackinac
Mackinac Is, MI

2. GI Anatomy
Fig 21-1

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
APs 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
Fig 21-4

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
1/2 from accessory glands
1/2 from digestive epithelial cells
Fig 21-5

6. Regulation of GIT p 689 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
Table 21-1

7. 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

8. 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

9. 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
Fig 21-14

10. 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 (Ch 24)
DDAVP

11. 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
See fig 21-19

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

13. Colon Cancer 2nd largest cause of cancer deaths
Cellulose (indigestible) = fiber, roughage
Significance of “roughage” in diet??
behind lung cancer
derived from initially benign mucosal tumors called pulyps incidence of polyp formation increases with age
multistep process. one gene involved is the p53 (tumor suppressor gene)

14. Phases of Digestion/Absorption
Cephalic
Gastric
Intestinal
Defecation

15. Cephalic
Gastric
Intestinal
Defecation
Anticipation
Salivation
Mastication
Mechanical digestion
Deglutition
Peristalsis in esophagus

16. Reflux Esophagitis = Heartburn = GERD
Lower esophageal sphincter dysfunction
Why reflux against gravity?
See p.700

17. GERD, cont’d Intrapleural pressure  during inspiration
 Wall of esophagus expands
 Subatmospheric pressure in esophageal lumen sucks acidic stomach contents into esophagus

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

19. 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
Emulsion of Fat
Absorption of H2O

20. 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
This can happen when a nonabsorbable, osmotically active substance is ingested ('osmotic diarrhea') or when electrolyte absorption is impaired ('secretory diarrhea'). Most cases of acute and chronic diarrhea are due to the latter mechanism. Secretory diarrhea can result from bacterial toxins, reduced absorptive surface area caused by disease or resection, luminal secretagogues (such as bile acids or laxatives), circulating secretagogues (such as various hormones, drugs, and poisons), and medical problems that compromise regulation of intestinal function.
Fig 21-21

21. Lactose Intolerance Clinical focus p. 707
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
Lactose = galactose + glucose
lactose intolerance is normal, tolerance is abnormal!! undigested lactose in intestine acts osmotially to hold water with it and also candraw water form the ECF into the lumen, resulting in diarrhea. this can be linked to osmotic diuresis in diabetes, where glucose ramining in the lumen of the nephron horld water with it. In addition people complain of cramps and gas.
Not all dairy products elicit same symptoms: lactose mostly absent from hard cheeses and reduced in cottage cheese ad ice cream. Yogurt with acitve bacterial cultures may be tolerated because bacteria in yogurt metabolize lactose.