Digestion & Absorption
Mechanical processing
Chemical digestion
Organs
Hormones
Why do we eat?
• Ultimately, food provides us with:
– Energy (ATP) for metabolic processes
• Catabolic: deconstructing macromolecules
• Anabolic: constructing macromolecules
– Building blocks for growth and maintenance
of body tissues (anabolism)
• Carbon & Nitrogen
– Essential Nutrients that we cannot construct,
but play vital roles in metabolism or
homeostatic function
Essential Nutrients
•
•
•
•
Essential Amino acids (AA)
Essential Fatty Acids
Vitamins
Minerals
Amino Acids
• Building blocks of all
proteins
• Integral component of
nucleotides
• Animals are incapable
of constructing ALL 20
• They eat what they
cannot build
Micronutrients
• Vitamins: Organic molecules
– “Large”
– Carbon is a major component
• Minerals: Inorganic molecules
– “Small”
– Single atoms; often charged
Ca2+
Na+,
• ALL are essential for metabolic processes
Food molecules must be digested
• Carbs, proteins and fats are very large molecules
• Goals of digestion:
– Brainstorm goals
Vitamins &
minerals
Growth
&
repair
Activity
Three Digestive processes
• Digestion
– Types
• Mechanical& Chemical
– Enzymes
• Carbohydrases, proteases, lipases
– Digestive organs & Accessory digestive organs
• Organ anatomy reveals function
• Absorption
– Absorptive organs
• Organ anatomy reveals function
• Elimination
– What is eliminated?
Overview of Digestion
GI Tract
• Gastrointestinal
(GI) tract: series of
organs (essentially
sacs) arranged in a
long tube
– Stomach, small
intestine, large
intestine
– Organs separated
by circular muscles
(sphincters)
How are different regions of the GI
prepared before each meal?
• Cephalic phase
– Thinking about food
• Gastric phase
– Mechanical manipulation of food
– Hormonal
• Intestinal phase
– Hormonal
Cephalic (Brain) phase
• Anticipatory phase
– Sensations (smell, sight,
tactile), plus association of
previous meals stimulate MO
– MO sends signals along Vagus
nerve to stomach secretory
cells
– Stimulate cells to produce
gastric secretions
• Subsequent positive feedback
of gastrin
Digestion begins in mouth
• mechanical processing- Chewing
• Chemical digestion - Salivary glands
– Saliva lubricates mouth and food (mucin & H20), protects
mouth (Lysozyme & antibodies)
– Salivary amylase - Enzyme begins chemical digestion of
carbohydrates
Teeth allow mechanical processing
• Specialization for different
food types
–
–
–
–
Incisors: Clip & scrape
Canines: puncture & hold
Premolars: Slice or cut
Molars: grind & crush
Moves down esophagus
Through the esophageal sphincter
Into Stomach
Longitudinal
Circular
Diagonal
Three layers
of muscle
Stomach jobs
• Mechanical digestion
– mixes food with gastric juice (highly acidic! Death to invaders!
Activation of enzymes!) to produce chyme
– Hydrochloric acid (HCl) denatures food proteins & activates
enzymes
• Chemical digestion of proteins and fats by gastric juices
– Pepsin
– Gastric lipase (little)
• Protection of cells by secreting mucus
• Absorption of littleH2O; mostly fatty acids and drugs
Cells of Gastric pits
• Heavily infolded rugae
allow stretch
• 4 cell types
– Mucous: mucus
– Parietal: HCl & intrinsic
factor
– Chief: pepsinogen
– Endocrine: regulatory
hormones (gastrin)
Stomach secretions
• Pepsin: breaks covalent bonds between protein
monomers (Amino Acids).
• HCl: converts pepsinogen to pepsin; low pH kills
microorganisms
• Intrinsic factor: binds to vitamin B12 to promote
absorption in SI
• Gastrin: hormone that increases gastric secretions;
influenced by “hunger”
• Mucus: lubricates and protects cells from chyme (acidic)
& pepsin
Gastric pits
Gastric phase of digestion
• Promoted by presence of
food in stomach
– Signal: Stretch stimulates
local & CNS reflexes
(receptor type?)
– Response: Increases HCl &
pepsinogen secretion
– Signal: Peptides binding
– Response: HCl secretion
More stomach jobs
• Mix and churn the digesting mass into chyme:
semi-solid product of mechanical and chemical
digestion
• Absorb some H2O, fatty acids and drugs
• Store food and regulate release to small intestine
To Small Intestine
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•
•
•
Longest part of GI tract
Three sections: duodenum, jejunum, ileum
Chyme enters through pyloric sphincter
Chemical digestion continues (most takes place
here) using pancreatic enzymes and mechanical
digestion using bile
Bridge Question
• Why is the stomach separated on both ends by a
sphincter?
1. To prevent gastric juice from leaking into and irritating
the esophagus
2. To prevent food from leaving the stomach too fast.
3. To prevent gastric juice from leaking into and irritating
the small intestine
4. All of the above.
Intestinal phase
• Food entering duodenum
inhibits gastric secretions:
– Chyme (pH < 2.0)
stimulates secretion of
secretin
– Lipids stimulate secretion
of Cholecystokinin (CCK) &
GIP
– These signals also
stimulate neuronal
inhibition locally & via the
MO
Show “Three phases of gastric
secretion” vid
Small intestine + Accessories
Accessory digestive organs help
• Liver: produces &
secretes bile, which
emulsifies fats
• Gall bladder: stores bile
• Pancreas: produces &
secretes proteases,
peptidases, lipase,
nucleases & amylase
Bile mechanically separates fats
• Made from cholesterol & amino acid
• Hydrophobic portions surround fats; hydrophilic portions face
watery environment.
Control of Bile secretion
• Nervous: direct
stimulation of gall
bladder
• Hormonal: A “full”
duodenum releases:
– Secretin & bile salts:
stimulates release of bile
(liver) & of bicarbonate
(Duodenum & pancreas)
– CCK: stimulates release of
bile by gall bladder
Control of pancreatic secretions
• Hormonal
– Secretin & CCK increases
secretion of pancreatic
enzymes
• Nervous
– Direct Vagus stimulation
increases enzyme
secretion
The pH Scale
pH’s of common substances:
Basic
14
Concentrated lye
13
Oven cleaner
12
11
Household ammonia
10
6
Baking soda
Bile
Pancreatic juice
Blood
Water
Saliva
Urine
5
Coffee
4
Orange juice
3
Vinegar
2
Lemon juice
Gastric juice
9
8
pH neutral
7
1
Acidic
0
Battery acid
Small intestine
digests and absorbs
• Absorption: the process of moving molecules
across a cell membrane and into a cell
– Small amount of absorption occurs in the stomach
– Most absorption of nutrients occurs in the small
intestines