Download The Small Bowel - Learning Central

Digestion
&Absorption
of every day foods
© CU/SONMS/nutrition/CSSAN
Outcomes
• Describe the relative position of gastrointestinal structure
and organs
• Define the model of motility with the GI system
• Briefly explain the key functions of secretion
• Describe the role of saliva
• Describe the optimal environment and role of the main
digestive enzymes
• Explain where most of the digestion and absorption of
nutrient occurs in relation to macronutrients
• Briefly explain neuronal and endocrine control of digestion
absorption and absorption of nutrients
• Briefly describe aerobic and anaerobic forms of energy
production from food
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UWCM/SONMS/nutrition/MJohn
The Digestive tract
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Movement of food through the GI tract.
Motility:
–Ingestion: Taking food into the mouth.
–Mastication: Chewing food and mixing it with saliva.
–Deglutition: Swallowing food.
–Peristalsis: Rhythmic wave-like contractions, moving food.
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Key Functions• Mechanical processing and motilitybreaking, mixing propelling food
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Key Functions: Secretion
• Releasing of digestive enzymes
• Digesting- chemical breakdown of food
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Key Functions: Absorption• Passage of digested nutrients/fluid across
the GI wall - to blood/lymph
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Key Functions: Elimination
• Expulsion of undigested and unabsorbed
residues from the gut
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Digestion
&
absorption
of beans on
toast
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UWCM/SONMS/nutrition/MJohn
What does saliva do to food
•Lubricates
– Thins -dilutes for swallowing
•Anti acid function,
– buffers acids,
• particularly important when
there are acids of bacteria
•Digests: Amylase (Lipase)
•Bacteriostat Bacteriolytic
– Acts on endogenous bacteria
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UWCM/SONMS/nutrition/MJohn
Does
anything
happen
here?
Holding position
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Food chunks up
here and amylase
continues to work
until food drops into
the stomach
Beans & Toast near cardiac Sphincter
Amylase
continues to
work - no acid
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UWCM/SONMS/nutrition/MJohn
Stomach
Mucosa
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hydrochloric acid
• activates pepsinogen
• Optimal pH (2-3) for enzymes
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The chief (zymogenic) cells
Release pepsinogen & prorennin
Into the acid environment
•Pepsinogen >>>>>>Pepsin
•Prorennin >>>>>> Rennin
–
convert proteins to polypeptides
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UWCM/SONMS/nutrition/MJohn
Digestion of Beans & Toast
Pepsin
Bread - carbohydrates =
no action
Proteins Beans proteins to peptides
Churning turns beans
on toast into chyme
Butter protein to
peptides
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UWCM/SONMS/nutrition/MJohn
Mucous cells
gastric acid can damage the mucosa
• Mucous is a protective mucosal barrier against
acid and the proteolytic enzymes in gastric juice.
– Aspirin, alcohol and an excess of gastric acid
can cause chronic cellular damage and
eventually gastric erosions or ulcers
– If the mucous layer is damaged or reduced
then mucous & blood supply is increased
(prostaglandin).
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UWCM/SONMS/nutrition/MJohn
The Duodenum
• Brunners glands’at the top ‘
-produce a thick tenacious, alkaline mucous
protection
•Gall bladder-bile
•Pancreatic juice
via the ampulla.
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UWCM/SONMS/nutrition/MJohn
Control of Pancreatic Secretion
CCK stimulates
•contraction of gall
bladder
•secretion of
pancreatic enzymes
Secretin stimulates
• water & Bicarb
secretion from the
pancreas
•potentiates CCK
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Beans & Toast in the Duodenum
Carbohydrates in Bread
& Beans
Pancreatic amylase
Protein in Beans &
butter Pancreatic Proteolytic
enzymes
Fat in beans & butter
Beans on toast
is now chyme
Pancreatic Lipase
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Pancreatic Enzymes
• Pancreatic amylase continues digesting
polysaccharides >>>>>> disaccharides
 brush border enzymes convert
 disaccharides to monosaccharides
• Proteolytic enzymes
– Trypsin, Chymotrypsin - Carboxypolypeptidase
 break peptide bonds in polypeptides >>> amino acids
• Pancreatic lipase & intestinal lipase strip fatty acids
from the glycerol backbone.
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UWCM/SONMS/nutrition/MJohn
BILE
Bile salts emulsify fats
Bile contains Cholesterol
important in making a fat concentration
gradient that allows fatty acids to enter the cell
Bile emulsifies the fats in the bean sauce
and butter
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UWCM/SONMS/nutrition/MJohn
Layers of the Small Bowel
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Cells of the mucosal
epithelium – Intestinal Lipase works on
Absorption cells:
These form the brush
border which also
contains several
enzymes.
The Brush
Border
lipds from butter
– Sucrase act on sucrose
from the beans & bead
– Lactase works on Lactose
from the from butter
– Maltase work on maltose
from bread Think about
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the role of
fibre
Enlarged Villus of Small Bowel
Brush
Border cell
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The fate of Beans & Toast
Proteins
Polypeptides
Tri-peptides
Di-peptides
Amino acids
Carbohydrates
Polysaccharides
Disaccharides
Sucrose
 Maltose
 Lactose
Monosaccharides
 Glucose
 Fructose
 Galactose
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UWCM/SONMS/nutrition/MJohn
Fat
Glycerol and
fatty acids
• Small and
long chain
fatty acids
"brush border" cell & fat absorption
• 1,000 microvilli per cell
• Micelle = aggregations of
– free fatty acids,
– mono-glycerides in the middle
– bile salts at polar ends enabling solution in water.
• Micelles 'shuttle’ fatty acids & glycerol to the
brush border where they can be absorbed into
the enterocyte
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UWCM/SONMS/nutrition/MJohn
Think about
the role of
fibre
Diffusion
Process
(Micelle)
Butter fat?
Vegetable fat?© in chyme
UWCM/SONMS/nutrition/MJohn
Now Chyme
Absorption of Nutrients via Small bowel
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Active
transport
mechanism
with NA +
or H+
Vegetable
protein in
Beans?
Chyme
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UWCM/SONMS/nutrition/MJohn
Energy production from food
With Oxygen
Carbohydrate to
Glucose molecule
Without Oxygen
Carbohydrate to
Glucose molecule
2 Pyruvate molecules
2 Pyruvate molecules
4 Acetyl CoA
Molecules
Lactate
16 times MORE energy
16 times LESS energy
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UWCM/SONMS/nutrition/MJohn
Homeostatic controls
Controls over digestive system act before
food is absorbed into internal environment:
• Nervous system- thought of food
• Nerve plexuses in gut wall - distension by
food (? air)
• Endocrine system - e.g. responses to low
blood sugar
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Endocrine Control
Specialised glands secrete
hormones into blood to reach
target organs
• chemical messenger e.g. pancreas
• pituitary
• thyroid etc.
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4 Gastrointestinal Hormones
• Gastrin- secreted by endocrine cells in stomach
lining when amino acids & peptides are in
stomach. Stimulates secretion of acid into
stomach.
• Secretin- peptide hormone. Stimulates the
pancreas to secrete bicarbonate
• CCK (cholecystokinin)- enhances actions of
secretion & stimulates gallbladder contractions
• GIP (glucose insulinotropic peptide)- released in
presence of glucose & fat in small intestine.
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Stimulates insulinUWCM/SONMS/nutrition/MJohn
secretion