Transcript Digestion and Absorption Review

Digestion
and
Absorption
© 2004, 2002 Elsevier Inc. All rights reserved.
Digestive System
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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
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Proteins that
break bonds
The Digestive System
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Is a long tube
from the mouth
to the anus
Digestive System
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Mouth to anus
– Epithelium lines the lumen
– Barrier to invaders
– Submucosal layer
– Muscularis
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Taste and smell
Peristalsis
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Muscular
contractions that
move food along
throughout GI
tract
The Mouth
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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
Tongue
– Taste receptors: sweet, sour, salty,
bitter, savory
– Enzymes to help break down fatty acids
Swallowing
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Bolus is the food swallowed at one time
Epiglottis blocks windpipe, prevents
choking
The Esophagus
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Long tube
Connects pharynx
to the stomach
Peristalsis, muscle
contraction
Lower esophageal
sphincter
Heartburn
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The Stomach
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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
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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
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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
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Activates digestive enzymes
Partially digests dietary protein
Assists in calcium absorption
Makes dietary minerals soluble for
absorption
The Small Intestine
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Most digestion and absorption happens
here
About 5-7 meters
– Duodenum (.5 meters)
– Jejunum (2-3 meters)
– Ileum (3-4 meters)
The Small Intestine
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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
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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 particles
End products of digestion:
 Carbohydrates → monosaccharides
 Proteins → amino acids
 Fats → glycerol, fatty acids
Absorption
Through small intestine walls
Absorbed into
 Blood – water soluble nutrients
 Lymph – fat soluble nutrients
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Blood → liver → general circulation
– Liver detoxifies and repackages
The Large Intestine
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~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
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Stool remains
Stimulates
elimination
Muscle contraction
Anal sphincters
– Voluntary control
– Opens for
elimination
Accessory Organs
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Salivary glands
Pancreas
Gallbladder
Liver
Salivary Glands
Saliva
 Works in mouth
 Moistens
 Salivary amylase
– Digests starch
The Pancreas
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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
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Produces bile (fat digestion)
Enterohepatic circulation
The Gallbladder
Stores bile
 Concentrates it
 Releases to small
intestine when
needed
The Digestive System
Sites of Secretion, Digestion, and
Absorption
Digestion in the Stomach
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Different nutrients leave the stomach
at different rates:
—Carbohydrates first, followed by protein,
fat, and fibrous foods
Summary of Enzymatic
Digestion and Absorption
Secretion Enzyme
Substrate
Action
Saliva
Ptyalin
Starch
Hydrolysis
to form
dextrins
Gastric
juice
Pepsin
Protein
Gastric
lipase
Fats
Hydrolysis
of peptide
bonds
Hydrolysis
into free
fatty acids
Final
Product
Summary of Enzymatic
Digestion and Absorption
—cont’d
Secretion
Enzyme
Substrate Action
Pancreatic
exocrine
secretion
Lipase
Fat
Final
Product
Hydrolysis to Fatty
monoacids
glycerides
Cholesterol Cholesterol
esterase
Hydrolysis to Cholesesters of
terol
cholesterol
and fatty
acids
alphaAmylase
Hydrolysis
Starch,
dextrins
Dextrin,
maltose
Summary of Enzymatic
Digestion and Absorption
—cont’d
Secretion Enzyme
Substrate Action
Pancreatic
exocrine
secretion
Protein
Hydrolysis Polypeptides
Chymotrypsin Protein
Hydrolysis Polypeptides
Carboxypeptidase
Hydrolysis Amino acids
Trypsin
Polypeptides
Final
Product
Ribonuclease Ribonu-cleic Hydrolysis Monoacids
nucleotides
Elastase
Fibrous
protein
Hydrolysis Amino acids
Summary of Enzymatic
Digestion and Absorption
—cont’d
Secretion
Enzyme
Substrate Action
Brush border CarboxyPolypepenzymes
peptidase; tides
aminopeptidase;
dipeptidase
Hydrolysis
Final
Product
Amino
acids
Enterokinase
Trypsino-gen Activates
to trypsin
Polypeptidases
and
peptides
Sucrase
Sucrose
Glucose,
fructose
Hydrolysis
Enzymatic Digestion and
Absorption —cont’d
Secretion Enzyme
Substrate
Action
Brush
border
enzymes
Isomaltase
Dextrin
Final
Product
Hydrolysis Glucose
Maltase
Maltose
Hydrolysis Glucose
Lactase
Lactose
Hydrolysis Glucose,
galactose
Nucleotidases
Nucleic acid
Hydrolysis Nucleotides
Nucleosidases Nucleosidases Hydrolysis Purine and
and
pyrimidine
phosphorylase
bases
Features of Digestion and
Absorption
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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
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GI tract compromised after starvation
Feeding results in cellular proliferation,
which restores normal gastrointestinal
architecture and function after a few
days
Lipids
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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
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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
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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
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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
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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)
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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)
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Released from small intestine in
presence of glucose and fat; inhibits
gastrin-stimulated release of gastric
acid
Hormones and GI
Function (Motilin)
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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
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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
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It’s important to know the physiology
and function of the GI tract in order to
understand the implications of
pathophysiology