© 2004, 2002 Elsevier Inc. All rights reserved. Digestion and Absorptio n pavemedicine.com
© 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