DIGESTION AND ABSORPTION

1. © 2004, 2002 Elsevier Inc. All rights reserved.
Digestion
and
Absorption
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2. Digestive System
 Reduces particle size
 Helps to absorb micronutrients and trace elements
 Sets a physical and immunologic barrier

3. Digestion
Breaks down
 Carbohydrates (starch and sugar) → single sugar
molecules
 Proteins amino acids→
 Fats fatty acids, glycerol→

4. Enzymes
 Proteins that break
bonds

5. The Digestive System
Is a long tube
from the
mouth to the
anus

6. Digestive System
 Mouth to anus
 Epithelium lines the lumen
 Barrier to invaders
 Submucosal layer
 Muscularis
 Taste and smell

7. Peristalsis
 Muscular contractions that
move food along throughout
GI tract

8. 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

9. Tongue
Taste receptors: sweet, sour, salty, bitter,
savory
Enzymes to help break down fatty acids

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

11. The Esophagus
 Long tube
 Connects pharynx
to the stomach
 Peristalsis, muscle
contraction
 Lower esophageal
sphincter
 Heartburn
http://www.gerd.com/media/grossovw.gif

12. 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

13. 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

14. 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

15. Physiology of the Stomach

16. Stomach Acid
 Activates digestive enzymes
 Partially digests dietary protein
 Assists in calcium absorption
 Makes dietary minerals soluble for absorption

17. The Small Intestine
 Most digestion and absorption happens here
 About 5-7 meters
 Duodenum (.5 meters)
 Jejunum (2-3 meters)
 Ileum (3-4 meters)

18. The Small Intestine
 Folded walls with villi projections
 Absorptive cells are located on the villi
 Increases intestinal surface area 600x
 Rapid cell turnover

19. The Small Intestine

20. 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

21. Nutrient Absorption

22. Site of Absorption

23. Absorption
Digestion small particles→
End products of digestion:
 Carbohydrates monosaccharides→
 Proteins amino acids→
 Fats glycerol, fatty acids→

24. Absorption
Through small intestine walls
Absorbed into
 Blood – water soluble nutrients
 Lymph – fat soluble nutrients
 Blood liver general circulation→ →
 Liver detoxifies and repackages

25. 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

26. Rectum
 Stool remains
 Stimulates
elimination
 Muscle contraction
 Anal sphincters
 Voluntary control
 Opens for
elimination

27. Accessory Organs
 Salivary glands
 Pancreas
 Gallbladder
 Liver

28. Salivary Glands
Saliva
 Works in mouth
 Moistens
 Salivary amylase
 Digests starch

29. 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

30. The Liver
 Produces bile (fat digestion)
 Enterohepatic circulation

31. The Gallbladder
Stores bile
 Concentrates it
 Releases to small intestine
when needed

32. The Digestive System

33. Sites of Secretion, Digestion, and
Absorption

34. Digestion in the Stomach
 Different nutrients leave the stomach at different rates:
—Carbohydrates first, followed by protein,
fat, and fibrous foods

35. Summary of Enzymatic Digestion
and Absorption
SecretionSecretion EnzymeEnzyme SubstrateSubstrate ActionAction FinalFinal
ProducProduc
tt
SalivaSaliva PtyalinPtyalin StarchStarch HydrolysisHydrolysis
to formto form
dextrinsdextrins
GastricGastric
juicejuice
PepsinPepsin ProteinProtein HydrolysisHydrolysis
of peptideof peptide
bondsbonds
GastricGastric
lipaselipase
FatsFats HydrolysisHydrolysis
into freeinto free
fatty acidsfatty acids

36. Summary of Enzymatic Digestion
and Absorption —cont’d
SecretionSecretion EnzymeEnzyme SubstrateSubstrate ActionAction FinalFinal
ProducProduc
tt
PancreaticPancreatic
exocrineexocrine
secretionsecretion
LipaseLipase FatFat Hydrolysis toHydrolysis to
mono-mono-
glyceridesglycerides
FattyFatty
acidsacids
CholesterolCholesterol
esteraseesterase
CholesterolCholesterol Hydrolysis toHydrolysis to
esters ofesters of
cholesterolcholesterol
and fattyand fatty
acidsacids
Choles-Choles-
terolterol
alpha-alpha-
AmylaseAmylase
Starch,Starch,
dextrinsdextrins
HydrolysisHydrolysis Dextrin,Dextrin,
maltosemaltose

37. Summary of Enzymatic Digestion
and Absorption —cont’d
SecretionSecretion EnzymeEnzyme SubstratSubstrat
ee
ActionAction FinalFinal
ProductProduct
PancreaticPancreatic
exocrineexocrine
secretionsecretion
TrypsinTrypsin ProteinProtein HydrolysiHydrolysi
ss
PolypeptidesPolypeptides
ChymotrypsiChymotrypsi
nn
ProteinProtein HydrolysiHydrolysi
ss
PolypeptidesPolypeptides
Carboxy-Carboxy-
peptidasepeptidase
Polypep-Polypep-
tidestides
HydrolysiHydrolysi
ss
Amino acidsAmino acids
RibonucleasRibonucleas
ee
Ribonu-Ribonu-
cleic acidscleic acids
HydrolysiHydrolysi
ss
Mono-Mono-
nucleotidesnucleotides
ElastaseElastase FibrousFibrous HydrolysiHydrolysi Amino acidsAmino acids

38. Summary of Enzymatic Digestion
and Absorption —cont’d
SecretionSecretion EnzymeEnzyme SubstrateSubstrate ActionAction FinalFinal
ProductProduct
Brush borderBrush border
enzymesenzymes
Carboxy-Carboxy-
peptidase;peptidase;
aminopep-aminopep-
tidase;tidase;
dipeptidasedipeptidase
Polypep-Polypep-
tidestides
HydrolysisHydrolysis AminoAmino
acidsacids
Entero-Entero-
kinasekinase
Trypsino-Trypsino-
gengen
ActivatesActivates
to trypsinto trypsin
Polypep-Polypep-
tidasestidases
andand
peptidespeptides
SucraseSucrase SucroseSucrose HydrolysisHydrolysis Glucose,Glucose,
fructosefructose

39. Enzymatic Digestion and
Absorption —cont’d
SecretioSecretio
nn
EnzymeEnzyme SubstrateSubstrate ActionAction FinalFinal
ProductProduct
BrushBrush
borderborder
enzymesenzymes
IsomaltaseIsomaltase DextrinDextrin HydrolysisHydrolysis GlucoseGlucose
MaltaseMaltase MaltoseMaltose HydrolysisHydrolysis GlucoseGlucose
LactaseLactase LactoseLactose HydrolysisHydrolysis Glucose,Glucose,
galactosegalactose
NucleotidasesNucleotidases Nucleic acidNucleic acid HydrolysisHydrolysis NucleotidesNucleotides
NucleosidasesNucleosidases
andand
phosphorylasephosphorylase
NucleosidaseNucleosidase
ss
HydrolysisHydrolysis Purine andPurine and
pyrimidinepyrimidine
basesbases

40. 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

41. Villi of the Human Intestine

42. Refeeding after Starvation
 GI tract compromised after starvation
 Feeding results in cellular proliferation, which restores
normal gastrointestinal architecture and function after
a few days

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

44. Summary of Fat Absorption

45. Transport Pathways through the Cell
Membrane

46. Large Intestine
 5 feet long
 Bacterial action to produce gases and organic acids
 Nutrients formed here—vitamin K, thiamin, vitamin B12,
riboflavin

47. The Gradual Breakdown of Large
Starch Molecules

48. Process of Digestion of Carbohydrate

49. Sites of Secretion and Absorption into
Gastrointestinal Tract

50. 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

51. 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

52. 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

53. 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

54. 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

55. 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

56. 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

57. Digestion and Absorption
 It’s important to know the physiology and function of
the GI tract in order to understand the implications of
pathophysiology