© 2004, 2002 Elsevier Inc. All rights reserved.

Digestion Digestion and and

AbsorptionAbsorption

Digestive SystemDigestive System

Reduces particle sizeReduces particle size Helps to absorb micronutrients Helps to absorb micronutrients

and trace elementsand trace elements Sets a physical and immunologic Sets a physical and immunologic

barrierbarrier

DigestionDigestion

Breaks downBreaks down Carbohydrates (starch and sugar) Carbohydrates (starch and sugar)

→→ single sugar molecules single sugar molecules Proteins Proteins → amino acids→ amino acids Fats → fatty acids, glycerolFats → fatty acids, glycerol

EnzymesEnzymes

Proteins Proteins that break that break bondsbonds

The Digestive SystemThe Digestive System

Is a long tube Is a long tube from the from the mouth to the mouth to the anusanus

Digestive SystemDigestive System

Mouth to anusMouth to anus– Epithelium lines the lumenEpithelium lines the lumen– Barrier to invadersBarrier to invaders– Submucosal layerSubmucosal layer– Muscularis Muscularis

Taste and smellTaste and smell

PeristalsisPeristalsis

Muscular Muscular contractions contractions that move food that move food along along throughout GI throughout GI tracttract

The MouthThe Mouth

Mastication: chewingMastication: chewing SalivaSaliva

– Salivary glands produce about 1.5 liters of Salivary glands produce about 1.5 liters of saliva dailysaliva daily

– Salivary amylase (ptyalin) begins to break Salivary amylase (ptyalin) begins to break down starch (inactivated in the stomach)down starch (inactivated in the stomach)

– Salivary lipase: begins fat digestion, but has Salivary lipase: begins fat digestion, but has minimal effectminimal effect

– Mucus to lubricate the food for easier Mucus to lubricate the food for easier swallowingswallowing

– Lysozyme to kill bacteria Lysozyme to kill bacteria

TongueTongue– Taste receptors: sweet, sour, salty, Taste receptors: sweet, sour, salty,

bitter, savorybitter, savory– Enzymes to help break down fatty acidsEnzymes to help break down fatty acids

SwallowingSwallowing Bolus is the food swallowed at one Bolus is the food swallowed at one

timetime Epiglottis blocks windpipe, prevents Epiglottis blocks windpipe, prevents

chokingchoking

The EsophagusThe Esophagus

Long tubeLong tube Connects pharynx Connects pharynx

to the stomachto the stomach Peristalsis, Peristalsis,

muscle muscle contractioncontraction

Lower esophageal Lower esophageal sphinctersphincter

HeartburnHeartburn

http://www.gerd.com/media/grossovw.gif

The StomachThe Stomach

Lower esophageal sphincter and Lower esophageal sphincter and pyloric sphincter control entry and exit pyloric sphincter control entry and exit from stomachfrom stomach

Storage capacity of ~4 cupsStorage capacity of ~4 cups Secretion of hydrochloric acid, Secretion of hydrochloric acid,

enzymes, including a protease, gastric enzymes, including a protease, gastric lipase; mucus, the hormone gastrin, lipase; mucus, the hormone gastrin, and intrinsic factor (B12)and intrinsic factor (B12)

Stomach secretes about 2000-2500 ml Stomach secretes about 2000-2500 ml of gastric juice dailyof gastric juice daily

StomachStomach

pH of stomach is low, 1 to 4; kills many pH of stomach is low, 1 to 4; kills many bacteria and micro-organismsbacteria and micro-organisms

Achlorhydria, gastrectomy, GI Achlorhydria, gastrectomy, GI dysfunction or disease dysfunction or disease ↑ risk of ↑ risk of bacterial overgrowth in the intestinesbacterial overgrowth in the intestines

Protein digestion begins with Protein digestion begins with pepsinogen which is converted to active pepsinogen which is converted to active pepsin in acidic stomach environmentpepsin in acidic stomach environment

Chief cells secrete acid-stable gastric Chief cells secrete acid-stable gastric lipase, which has minor effect on lipase, which has minor effect on shorter-chain triglyceridesshorter-chain triglycerides

StomachStomach

Most of a liquid meal exits in 1 to Most of a liquid meal exits in 1 to 2 hours and most of a solid meal 2 hours and most of a solid meal in 2 to 3 hoursin 2 to 3 hours

Formation of chymeFormation of chyme Mucus layer protects stomach Mucus layer protects stomach

from digestionfrom digestion Very little absorption of nutrientsVery little absorption of nutrients

Physiology of the Physiology of the Stomach Stomach

Stomach AcidStomach Acid

Activates digestive enzymesActivates digestive enzymes Partially digests dietary proteinPartially digests dietary protein Assists in calcium absorptionAssists in calcium absorption Makes dietary minerals soluble for Makes dietary minerals soluble for

absorptionabsorption

The Small IntestineThe Small Intestine

Most digestion and absorption Most digestion and absorption happens herehappens here

About 5-7 metersAbout 5-7 meters– Duodenum (.5 meters)Duodenum (.5 meters)– Jejunum (2-3 meters)Jejunum (2-3 meters)– Ileum (3-4 meters)Ileum (3-4 meters)

The Small IntestineThe Small Intestine

Folded walls with villi projectionsFolded walls with villi projections Absorptive cells are located on Absorptive cells are located on

the villithe villi Increases intestinal surface area Increases intestinal surface area

600x 600x Rapid cell turnoverRapid cell turnover

The Small IntestineThe Small Intestine

Nutrient AbsorptionNutrient Absorption Passive diffusion: driven by Passive diffusion: driven by

concentration; fats, water, some concentration; fats, water, some mineralsminerals

Active absorption: uses energy; Active absorption: uses energy; glucose and amino acidsglucose and amino acids

Phagocytosis and pinocytosis: Phagocytosis and pinocytosis: absorptive cells engulf compounds, absorptive cells engulf compounds, generally larger molecules, as in generally larger molecules, as in immune substances in breast milkimmune substances in breast milk

Nutrient AbsorptionNutrient Absorption

Site of AbsorptionSite of Absorption

AbsorptionAbsorption

Digestion Digestion → small particles→ small particles

End products of digestion:End products of digestion: Carbohydrates → Carbohydrates →

monosaccharidesmonosaccharides Proteins → amino acidsProteins → amino acids Fats → glycerol, fatty acidsFats → glycerol, fatty acids

AbsorptionAbsorption

Through small intestine wallsThrough small intestine walls

Absorbed intoAbsorbed into Blood – water soluble nutrientsBlood – water soluble nutrients Lymph – fat soluble nutrientsLymph – fat soluble nutrients

Blood Blood → liver → general circulation→ liver → general circulation– Liver detoxifies and repackagesLiver detoxifies and repackages

The Large IntestineThe Large Intestine

~3 1/2 feet in length~3 1/2 feet in length No villi or enzymes presentNo villi or enzymes present Little digestion occursLittle digestion occurs Absorption of water, some Absorption of water, some

minerals, vitaminsminerals, vitamins Bacteria break down fiber; produce Bacteria break down fiber; produce

Vitamin KVitamin K Formation of feces for eliminationFormation of feces for elimination

RectumRectum

Stool remainsStool remains Stimulates Stimulates

eliminationelimination Muscle Muscle

contractioncontraction Anal sphinctersAnal sphincters

– Voluntary controlVoluntary control– Opens for Opens for

eliminationelimination

Accessory OrgansAccessory Organs

Salivary glandsSalivary glands PancreasPancreas GallbladderGallbladder LiverLiver

Salivary GlandsSalivary Glands

SalivaSaliva Works in mouthWorks in mouth MoistensMoistens Salivary amylaseSalivary amylase

– Digests starchDigests starch

The PancreasThe Pancreas

Produces glucagon and insulin Produces glucagon and insulin (endocrine)(endocrine)

Manufactures digestive enzymesManufactures digestive enzymes→ → small intestinesmall intestine

Secretes pancreatic juicesSecretes pancreatic juices Bicarbonate needed to neutralize Bicarbonate needed to neutralize

chyme when it enters small chyme when it enters small intestineintestine

The LiverThe Liver

Produces bile (fat digestion)Produces bile (fat digestion) Enterohepatic circulationEnterohepatic circulation

The GallbladderThe Gallbladder

Stores bileStores bile Concentrates itConcentrates it Releases to Releases to

small intestine small intestine when neededwhen needed

The Digestive SystemThe Digestive System

Sites of Secretion, Digestion, Sites of Secretion, Digestion, and Absorptionand Absorption

Digestion in the Digestion in the StomachStomach Different nutrients leave the Different nutrients leave the

stomach at different rates: stomach at different rates: ——Carbohydrates first, followed by Carbohydrates first, followed by

protein, protein, fat, and fibrous foods fat, and fibrous foods

Summary of Enzymatic Summary of Enzymatic Digestion and Digestion and AbsorptionAbsorption

SecretioSecretionn

EnzymeEnzyme SubstratSubstratee

ActionAction Final Final ProducProductt

SalivaSaliva PtyalinPtyalin StarchStarch Hydrolysis Hydrolysis to form to form dextrinsdextrins

Gastric Gastric juicejuice

PepsinPepsin ProteinProtein Hydrolysis Hydrolysis of peptide of peptide bondsbonds

Gastric Gastric lipaselipase

FatsFats Hydrolysis Hydrolysis into free into free fatty acidsfatty acids

Summary of Enzymatic Summary of Enzymatic Digestion and Digestion and Absorption Absorption —cont’d—cont’d

SecretioSecretionn

EnzymeEnzyme SubstratSubstratee

ActionAction Final Final ProducProductt

Pancreatic Pancreatic exocrine exocrine secretionsecretion

LipaseLipase FatFat Hydrolysis Hydrolysis to mono-to mono-glyceridesglycerides

Fatty Fatty acidsacids

CholesterCholesterol ol esteraseesterase

CholesterolCholesterol Hydrolysis Hydrolysis to esters of to esters of cholesterol cholesterol and fatty and fatty acidsacids

Choles-Choles-terolterol

alpha-alpha-AmylaseAmylase

Starch, Starch, dextrinsdextrins

HydrolysisHydrolysis Dextrin, Dextrin, maltosemaltose

Summary of Enzymatic Summary of Enzymatic Digestion and Digestion and Absorption Absorption —cont’d—cont’d

SecretioSecretionn

EnzymeEnzyme SubstratSubstratee

ActionAction Final Final ProductProduct

Pancreatic Pancreatic exocrine exocrine secretionsecretion

TrypsinTrypsin ProteinProtein HydrolysHydrolysisis

PolypeptidPolypeptideses

ChymotrypChymotrypsinsin

ProteinProtein HydrolysHydrolysisis

PolypeptidPolypeptideses

Carboxy-Carboxy-peptidasepeptidase

Polypep-Polypep-tidestides

HydrolysHydrolysisis

Amino Amino acidsacids

RibonucleaRibonucleasese

Ribonu-Ribonu-cleic acidscleic acids

HydrolysHydrolysisis

Mono-Mono-nucleotidesnucleotides

ElastaseElastase Fibrous Fibrous proteinprotein

HydrolysHydrolysisis

Amino Amino acidsacids

Summary of Enzymatic Summary of Enzymatic Digestion and Digestion and Absorption Absorption —cont’d—cont’d

SecretionSecretion EnzymeEnzyme SubstratSubstratee

ActionAction Final Final ProductProduct

Brush Brush border border enzymesenzymes

Carboxy-Carboxy-peptidase; peptidase; aminopep-aminopep-tidase; tidase; dipeptidasdipeptidasee

Polypep-Polypep-tidestides

HydrolysiHydrolysiss

Amino Amino acidsacids

Entero-Entero-kinasekinase

Trypsino-Trypsino-gengen

Activates Activates to trypsinto trypsin

Polypep-Polypep-tidases tidases and and peptidespeptides

SucraseSucrase SucroseSucrose HydrolysiHydrolysiss

Glucose, Glucose, fructosefructose

Enzymatic Digestion and Enzymatic Digestion and AbsorptionAbsorption —cont’d—cont’d

SecretioSecretionn

EnzymeEnzyme SubstrateSubstrate ActionAction Final Final ProductProduct

Brush Brush border border enzymesenzymes

IsomaltaseIsomaltase DextrinDextrin HydrolysiHydrolysiss

GlucoseGlucose

MaltaseMaltase MaltoseMaltose HydrolysiHydrolysiss

GlucoseGlucose

LactaseLactase LactoseLactose HydrolysiHydrolysiss

Glucose, Glucose, galactosegalactose

NucleotidaseNucleotidasess

Nucleic acidNucleic acid HydrolysiHydrolysiss

NucleotideNucleotidess

NucleosidasNucleosidases and es and phosphorylaphosphorylasese

NucleosidasNucleosidaseses

HydrolysiHydrolysiss

Purine and Purine and pyrimidine pyrimidine basesbases

Features of Digestion Features of Digestion and Absorptionand Absorption 92% to 97% of the diet is 92% to 97% of the diet is

absorbedabsorbed Small intestine—key role in Small intestine—key role in

digestion and absorptiondigestion and absorption Intestines: 7 meters longIntestines: 7 meters long

——Villi and microvilli/brush borderVilli and microvilli/brush border

Villi of the Human IntestineVilli of the Human Intestine

Refeeding after Refeeding after StarvationStarvation GI tract compromised after GI tract compromised after

starvationstarvation Feeding results in cellular Feeding results in cellular

proliferation, which restores proliferation, which restores normal gastrointestinal normal gastrointestinal architecture and function after a architecture and function after a few daysfew days

LipidsLipids

The major portion of fat digestion The major portion of fat digestion takes place in the small intestine takes place in the small intestine as a result of pancreatic lipaseas a result of pancreatic lipase

Summary of Fat Summary of Fat AbsorptionAbsorption

Transport Pathways through the Transport Pathways through the Cell MembraneCell Membrane

Large IntestineLarge Intestine

5 feet long5 feet long Bacterial action to produce gases Bacterial action to produce gases

and organic acidsand organic acids Nutrients formed here—vitamin K, Nutrients formed here—vitamin K,

thiamin, vitamin B12, riboflavinthiamin, vitamin B12, riboflavin

The Gradual Breakdown of The Gradual Breakdown of Large Starch MoleculesLarge Starch MoleculesThe Gradual Breakdown of The Gradual Breakdown of Large Starch MoleculesLarge Starch Molecules

Process of Digestion Process of Digestion of Carbohydrateof Carbohydrate

Sites of Secretion and Absorption Sites of Secretion and Absorption into Gastrointestinal Tractinto Gastrointestinal Tract

NeurotransmittersNeurotransmitters

Alpha-aminobutyric acid: relaxes LESAlpha-aminobutyric acid: relaxes LES Norepinephrine: decreases motility, Norepinephrine: decreases motility,

increases contractions of sphincters, increases contractions of sphincters, inhibits secretionsinhibits secretions

Acetylcholine: increases motility, Acetylcholine: increases motility, relaxes sphincters, stimulates secretionsrelaxes sphincters, stimulates secretions

Neurotensin: inhibits release of gastric Neurotensin: inhibits release of gastric emptying and acid secretionemptying and acid secretion

Neuropeptide-Y: stimulates feeding Neuropeptide-Y: stimulates feeding behaviorbehavior

Hormones and GI Hormones and GI Function: GastrinFunction: Gastrin Released from gastric mucosa Released from gastric mucosa

and duodenum in presence of and duodenum in presence of peptides and amino acids; peptides and amino acids; stimulates secretion of stimulates secretion of hydrochloric acid and hydrochloric acid and pepsinogen; increases gastric pepsinogen; increases gastric antral motility, increases LES antral motility, increases LES tonetone

Hormones and GI Hormones and GI function: Secretinfunction: Secretin Secretin: from duodenal mucosa Secretin: from duodenal mucosa

in presence of acid in small in presence of acid in small intestine; stimulates pancreas to intestine; stimulates pancreas to secrete water and bicarb; also secrete water and bicarb; also insulin and pancreatic enzymesinsulin and pancreatic enzymes

Hormones and GI Hormones and GI function: Cholecystokinin function: Cholecystokinin (CCK)(CCK) Released from proximal small Released from proximal small

bowel in presence of peptices, bowel in presence of peptices, amino acids, fat, HCL, stimulates amino acids, fat, HCL, stimulates pancreas to release pancreatic pancreas to release pancreatic enzymesenzymes

Hormones and GI Hormones and GI function: Gastric function: Gastric Inhibitory Polypeptide Inhibitory Polypeptide (GIP)(GIP) Released from small intestine in Released from small intestine in

presence of glucose and fat; presence of glucose and fat; inhibits gastrin-stimulated release inhibits gastrin-stimulated release of gastric acidof gastric acid

Hormones and GI Hormones and GI Function (Motilin)Function (Motilin) Released from stomach and small Released from stomach and small

and large intestine in presence of and large intestine in presence of biliary and pancreatic secretions; biliary and pancreatic secretions; promotes gastric emptying and promotes gastric emptying and increases GI motility increases GI motility

Hormones and GI Hormones and GI function: Somatostatinfunction: Somatostatin Released from stomach, Released from stomach,

pancreas, and upper small pancreas, and upper small intestine in presence of acidity intestine in presence of acidity and products of protein and fat and products of protein and fat digestion; inhibits release of digestion; inhibits release of gastrin, motilin, and pancreatic gastrin, motilin, and pancreatic secretions; decreases motility of secretions; decreases motility of GI tractGI tract

Digestion and Digestion and AbsorptionAbsorption It’s important to know the It’s important to know the

physiology and function of the GI physiology and function of the GI tract in order to understand the tract in order to understand the implications of pathophysiologyimplications of pathophysiology