Ch 21: DigestionCh 21: Digestion

Anatomy of digestive system (review!)

4 major processes of digestive system:

1.1. MotilityMotility

2.2. SecretionSecretion

3.3. DigestionDigestion

4.4. AbsorptionAbsorption

Fort MackinacMackinac Is, MI

GI AnatomyGI Anatomy

Fig 21-1

MotilityMotility

2 purposes:2 purposes:

Forward movement of foodForward movement of foodMechanical mixingMechanical mixing

GI smooth muscles contract GI smooth muscles contract spontaneouslyspontaneously

Pacemaker cells, (Interstitial Cells of Pacemaker cells, (Interstitial Cells of Cajal), connected by gap junctions, Cajal), connected by gap junctions, generate slow wave potentialsgenerate slow wave potentials

APs spread throughout longitudinal APs spread throughout longitudinal muscles (gap junctions) muscles (gap junctions) wave of wave of contractioncontraction

Like cardiac muscle, CaLike cardiac muscle, Ca2+ 2+ can regulate can regulate contraction strengthcontraction strength

1.1. MotilityMotility2.2. SecretionSecretion 3.3. DigestionDigestion 4.4. AbsorptionAbsorption

Different Patterns of ContractionDifferent Patterns of Contraction

TonicTonic Contractions Contractions Sustained contraction, usually in the Sustained contraction, usually in the stomachstomach

PhasicPhasic Contractions ContractionsPeristaltic contractionsPeristaltic contractions

progressive waves moving along segments of progressive waves moving along segments of longitudinal layer longitudinal layer forward propulsion forward propulsion

circular layer contracts proximal to circular layer contracts proximal to bolusbolusEspecially esophagusEspecially esophagus

Segmental contractionsSegmental contractions alternate contraction & relaxation lead to mixingalternate contraction & relaxation lead to mixing A side effect of narcoticsA side effect of narcotics

Fig 21-4

SecretionSecretion

9 L of fluid pass through the GIT 9 L of fluid pass through the GIT (only 2 L from food & drink) (only 2 L from food & drink) Secretion and Reabsorption Secretion and Reabsorption importantimportant

Ions and water; similar to renal Ions and water; similar to renal mechanismsmechanisms

SalivaSaliva Hydrochloric Acid (Parietal Hydrochloric Acid (Parietal

Cells)Cells) Bicarbonate Bicarbonate (enzyme necessary ?) (enzyme necessary ?) Enzymes Enzymes (zymogens)(zymogens) Mucus Mucus (Goblet cells)(Goblet cells) Bile Bile (bile salts – function?) (bile salts – function?)

Net Fluid Balance in GI systemNet Fluid Balance in GI system

Fig 21-5

1.1. MotilityMotility2.2. SecretionSecretion 3.3. DigestionDigestion 4.4. AbsorptionAbsorption

Regulation of GIT Regulation of GIT p 689p 689

ANSANS Parasympathetic (R & R)Parasympathetic (R & R) SympatheticSympathetic

Emotional (cephalic reflexes)Emotional (cephalic reflexes) E.g., smell of foodE.g., smell of food

ENS (Enteric Nervous System)ENS (Enteric Nervous System) Self-contained (intrinsic)Self-contained (intrinsic)

GI peptides can have regulatory GI peptides can have regulatory role as hormones or paracrinesrole as hormones or paracrines E.g., Gastrin, CCKE.g., Gastrin, CCK

Table 21-1Table 21-1

Digestion OverviewDigestion Overview

Mechanical breakdown and mixing aid Mechanical breakdown and mixing aid enzymatic breakdownenzymatic breakdown

ChewingChewing

Tonic contractions, esp. stomachTonic contractions, esp. stomach

Enzymatic breakdown converts Enzymatic breakdown converts macromolecules into absorbable unitsmacromolecules into absorbable units

Bile emulsifies fatsBile emulsifies fats

Optimal pH of enzymes indicates location of Optimal pH of enzymes indicates location of activityactivity

1.1. MotilityMotility2.2. SecretionSecretion 3.3. DigestionDigestion 4.4. AbsorptionAbsorption

Absorption OverviewAbsorption Overview

Most nutrient absorption takes place in ?Most nutrient absorption takes place in ? Fats absorbed into lactealsFats absorbed into lacteals Everything else absorbed into portal veinEverything else absorbed into portal vein

Alcohol & aspirin across gastric epitheliumAlcohol & aspirin across gastric epithelium

Additional: HAdditional: H22O, ions & some vitamins O, ions & some vitamins

absorbed in ________________absorbed in ________________

Mechanisms analogous to renal absorptionMechanisms analogous to renal absorption

1.1. MotilityMotility2.2. SecretionSecretion 3.3. DigestionDigestion 4.4. AbsorptionAbsorption

CHO CHO Digestion & AbsorptionDigestion & Absorption

~50% of calories in average American ~50% of calories in average American dietdiet

Starch (polysaccharide) and sucrose Starch (polysaccharide) and sucrose (disaccharide)(disaccharide)

Cellulose (roughage) not digestibleCellulose (roughage) not digestible

Enzymes: Enzymes: amylases, disaccharidasesamylases, disaccharidases (maltase, sucrase, lactase)(maltase, sucrase, lactase)

Absorbed only as monosaccharides Absorbed only as monosaccharides (glucose, fructose)(glucose, fructose)

Small intestineSmall intestine

Fig 21-14

Protein Digestion and AbsorptionProtein Digestion and Absorption

Variable digestibilityVariable digestibility30-60% of protein not from diet30-60% of protein not from dietFirst digestion in Stomach by HClFirst digestion in Stomach by HClProteases secreted as proenzymesProteases secreted as proenzymes

Pepsin(-ogen), trypsin, etc.Pepsin(-ogen), trypsin, etc.

Absorption of single a.a. and di- and Absorption of single a.a. and di- and tripeptidestripeptides

Specific receptors required for larger Specific receptors required for larger chainschains

May serve as May serve as allergens allergens (Ch 24)(Ch 24)DDAVPDDAVP

Lipid (fat) DigestionLipid (fat) Digestion

Mostly triglycerides in dietMostly triglycerides in diet Cholesterol, Fat-soluble Cholesterol, Fat-soluble

vitamins, others.vitamins, others.

Combination with bile salts Combination with bile salts creates an emulsioncreates an emulsion

Colipase and lipase allow Colipase and lipase allow formation of small micellesformation of small micelles

Absorption of fat via Absorption of fat via diffusion across apical CMdiffusion across apical CM

Chylomicrons in the cell are Chylomicrons in the cell are absorbed into lactealsabsorbed into lacteals

See fig 21-19

Other StuffOther Stuff

Water soluble vitamins—Water soluble vitamins—mediated transportmediated transport

Fat soluble vits. via absorptionFat soluble vits. via absorption

Water,Ions and MineralsWater,Ions and Minerals Various locations and methods, Various locations and methods,

e.g, diffusion, carrier proteinse.g, diffusion, carrier proteins

Nucleic AcidsNucleic Acids

Colon CancerColon Cancer

22ndnd largest cause of cancer deaths largest cause of cancer deaths

Cellulose (indigestible) = fiber, Cellulose (indigestible) = fiber, roughageroughage

Significance of “roughage” in diet??Significance of “roughage” in diet??

Phases of Digestion/AbsorptionPhases of Digestion/Absorption

1.1. CephalicCephalic

2.2. GastricGastric

3.3. IntestinalIntestinal

4.4. DefecationDefecation

1.1. CephalicCephalic

2.2. GastricGastric

3.3. IntestinalIntestinal

4.4. DefecationDefecation

•Anticipation

•Salivation

•Mastication

•Mechanical digestion

•Deglutition

•Peristalsis in esophagus

Reflux EsophagitisReflux Esophagitis = = Heartburn = GERDHeartburn = GERD

Lower esophageal Lower esophageal sphincter dysfunctionsphincter dysfunction

Why reflux against gravity?Why reflux against gravity?

See p.700

1.1. Intrapleural pressure Intrapleural pressure during inspiration during inspiration

Wall of esophagus Wall of esophagus expands expands

Subatmospheric Subatmospheric pressure in pressure in esophageal lumen sucks esophageal lumen sucks acidic stomach contents into acidic stomach contents into esophagusesophagus

GERD, cont’d

1.1. CephalicCephalic2.2. GastricGastric3.3. IntestinalIntestinal4.4. DefecationDefecation

•Storage

•Relaxation or contraction as needed.

•Digestion

•HCl (Parietal cells)

•Pepsin (Chief cells)

•Mechanical

•Immune Protection

•Ingested pathogens

•Respiratory mucus

1.1. CephalicCephalic2.2. GastricGastric3.3. Intestinal – Intestinal – Chyme (ingesta) enters small intestineChyme (ingesta) enters small intestine4.4. DefecationDefecation

o Neutralization of HCl

o NaHCO3 from pancreas

o Running Problem: Peptic Ulcer and antacids, including H2 antagonists and proton pump inhibitors

o Pancreatic enzymes

o Digest protein, CHO

o Bile acids

o Emulsion of Fat

o Absorption of H2O

Bacterial fermentation of CHO and proteins Bacterial fermentation of CHO and proteins

Lactate, some vitamins are digested and/or absorbedLactate, some vitamins are digested and/or absorbed

More HMore H22O absorptionO absorption

Osmotic diarrhea vs. secretory diarrheaOsmotic diarrhea vs. secretory diarrhea Osmotic-solutes prevent HOsmotic-solutes prevent H22O reabsorptionO reabsorption

Lactose intoleranceLactose intolerance

Secretory- bacterial toxins ("flush out' pathogens)Secretory- bacterial toxins ("flush out' pathogens) Defecation ReflexDefecation Reflex

Fig 21-21

1.1. CephalicCephalic2.2. GastricGastric3.3. IntestinalIntestinal4.4. DefecationDefecation

Lactose IntoleranceLactose Intolerance

Lactose = glucose + galactoseLactose = glucose + galactose

Lactase only found in juvenile Lactase only found in juvenile mammalsmammals

Adaptive (dominant) mutation in Adaptive (dominant) mutation in populations with dairy-based populations with dairy-based cultures cultures

Lactose intolerance in Lactose intolerance in 95% of Native Americans,95% of Native Americans, 90% of Asian Americans90% of Asian Americans 70% of African Americans70% of African Americans 50% of Mexican Americans50% of Mexican Americans

Clinical focus p. 707