© 2004, 2002 Elsevier Inc. All rights reserved.

Digestion and

Absorptionpavemedicine.com

Digestive System

Reduces particle size Helps to absorb micronutrients and trace

elements Sets a physical and immunologic barrier

Digestion

Breaks down Carbohydrates (starch and sugar) → single sugar

molecules Proteins → amino acids Fats → fatty acids, glycerol

Enzymes

Proteins that break bonds

The Digestive System

Is a long tube from the mouth to the anus

Digestive System

Mouth to anus Epithelium lines the lumen Barrier to invaders Submucosal layer Muscularis

Taste and smell

Peristalsis

Muscular contractions that move food along throughout GI tract

The Mouth Mastication: chewing Saliva

Salivary glands produce about 1.5 liters of saliva daily

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

Salivary lipase: begins fat digestion, but has minimal effect

Mucus to lubricate the food for easier swallowing

Lysozyme to kill bacteria

TongueTaste receptors: sweet, sour, salty, bitter,

savoryEnzymes to help break down fatty acids

Swallowing Bolus is the food swallowed at one time Epiglottis blocks windpipe, prevents choking

The Esophagus

Long tube Connects pharynx

to the stomach Peristalsis, muscle

contraction Lower esophageal

sphincter Heartburn

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

The Stomach

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

Storage capacity of ~4 cups Secretion of hydrochloric acid,

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

Stomach secretes about 2000-2500 ml of gastric juice daily

Stomach

pH of stomach is low, 1 to 4; kills many bacteria and micro-organisms Achlorhydria, gastrectomy, GI dysfunction or disease ↑ risk of bacterial overgrowth in the intestines Protein digestion begins with pepsinogen which is converted to active pepsin in acidic stomach environment Chief cells secrete acid-stable gastric lipase, which has minor effect on shorter-chain triglycerides

Stomach

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

Formation of chyme Mucus layer protects stomach from digestion Very little absorption of nutrients

Physiology of the Stomach

Stomach Acid

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

The Small Intestine

Most digestion and absorption happens here About 5-7 meters

Duodenum (.5 meters) Jejunum (2-3 meters) Ileum (3-4 meters)

The Small Intestine

Folded walls with villi projections Absorptive cells are located on the villi Increases intestinal surface area 600x Rapid cell turnover

The Small Intestine

Nutrient Absorption Passive diffusion: driven by concentration; fats, water, some

minerals Active absorption: uses energy; glucose and amino acids Phagocytosis and pinocytosis: absorptive cells engulf

compounds, generally larger molecules, as in immune substances in breast milk

Nutrient Absorption

Site of Absorption

Absorption

Digestion → small particlesEnd products of digestion: Carbohydrates → monosaccharides Proteins → amino acids Fats → glycerol, fatty acids

Absorption

Through small intestine wallsAbsorbed into Blood – water soluble nutrients Lymph – fat soluble nutrients

Blood → liver → general circulation Liver detoxifies and repackages

The Large Intestine ~3 1/2 feet in length No villi or enzymes present Little digestion occurs Absorption of water, some minerals, vitamins Bacteria break down fiber; produce Vitamin K Formation of feces for elimination

Rectum

Stool remains Stimulates

elimination Muscle contraction Anal sphincters

Voluntary control Opens for

elimination

Accessory Organs

Salivary glands Pancreas Gallbladder Liver

Salivary Glands

Saliva Works in mouth Moistens Salivary amylase

Digests starch

The Pancreas

Produces glucagon and insulin (endocrine) Manufactures digestive enzymes→ small intestine Secretes pancreatic juices Bicarbonate needed to neutralize chyme when it

enters small intestine

The Liver

Produces bile (fat digestion) Enterohepatic circulation

The GallbladderStores bile Concentrates it Releases to small intestine

when needed

The Digestive System

Sites of Secretion, Digestion, and Absorption

Digestion in the Stomach

Different nutrients leave the stomach at different rates: —Carbohydrates first, followed by protein,

fat, and fibrous foods

Summary of Enzymatic Digestion and Absorption

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 Digestion and Absorption —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 Digestion and Absorption —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 Digestion and Absorption —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 Absorption —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 and Absorption

92% to 97% of the diet is absorbed Small intestine—key role in digestion and absorption Intestines: 7 meters long

—Villi and microvilli/brush border

Villi of the Human Intestine

Refeeding after Starvation

GI tract compromised after starvation Feeding results in cellular proliferation, which

restores normal gastrointestinal architecture and function after a few days

Lipids

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

Summary of Fat Absorption

Transport Pathways through the Cell Membrane

Large Intestine

5 feet long Bacterial action to produce gases and organic

acids Nutrients formed here—vitamin K, thiamin,

vitamin B12, riboflavin

The Gradual Breakdown of Large Starch Molecules

Process of Digestion of Carbohydrate

Sites of Secretion and Absorption into Gastrointestinal Tract

Neurotransmitters

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

increases contractions of sphincters, inhibits secretions

Acetylcholine: increases motility, relaxes sphincters, stimulates secretions

Neurotensin: inhibits release of gastric emptying and acid secretion

Neuropeptide-Y: stimulates feeding behavior

Hormones and GI Function: Gastrin

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

Hormones and GI function: Secretin Secretin: from duodenal mucosa in presence of

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

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

of peptices, amino acids, fat, HCL, stimulates pancreas to release pancreatic enzymes

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

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

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

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

Hormones and GI function: Somatostatin Released from stomach, pancreas, and upper

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

Digestion and Absorption

It’s important to know the physiology and function of the GI tract in order to understand the implications of pathophysiology