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Human
DigestiveDigestive
System YouTube.flv System
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5 Digestion Processes (IDAAE)
• Ingestion: taking in of food into the body.
• Digestion: breaking down of food into simpler
substances
• Absorption: diffusion of food from small
intestine into the blood
• Assimilation: using digested nutrients to make
new material
• Egestion: removal of undigested waste material
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Recall. . .
What are the main organs of the alimentary canal?






Mouth
Oesophagus
Stomach
Small intestine
Large intestine
Anus
Although not part of the alimentary canal, the liver,
gall bladder and pancreas are closely associated with
it. They play an important role in digestion by
secreting digestive enzymes.
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Digestion
– Mechanical / physical digestion physically
breaks down the food in the mouth (chewing).
Smaller pieces of food increase surface area
for digestion. It also takes place in the
stomach (churning of food by the muscular
stomach walls)
– Chemical digestion uses enzymes to
chemically break down complex food
substances into their simplest form. e.g.
amylase
Starch
maltose
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Chemical digestion:
• Starch (carbohydrate) digestion: in mouth and
small intestine.
• Protein digestion: in stomach and small
intestine
• Fat digestion: only in small intestine
Why must food be digested???
Large molecules of food are unable to pass
through cell membranes, thus must be broken
down into small molecules so that they can
diffuse through cell membranes into the blood
stream
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Mouth (Ahhh....)
• Mouth ingests food
• Teeth masticates food into small pieces to
increase surface area for digestion
• Saliva (pH 7) moisten and soften food
• Starch
maltose
• Tongue mixes food with saliva and rolls
food into a bolus before swallowing
• Saliva - water, mucus, salivary amylase
Salivary amylase
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7
https://vimeo.com/
Swallowing
76306735
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What Happens During Breathing
and Swallowing?
Normally, air passes into
the trachea (windpipe)
while food passes into
the oesophagus.
pharynx
During breathing, the
larynx is lowered and the
glottis is open.
air
glottis
larynx
(voice-box)
oesophagu
s
trachea
(windpipe)
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What Happens During Breathing
and Swallowing?
During swallowing, the
larynx is raised and the
glottis is covered by the
epiglottis. This prevents
food particles from
entering the trachea.
pharynx
food
particles
epiglottis
glottis
larynx
(voice-box)
trachea
(windpipe)
oesophagu
s
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What Happens During Breathing
and Swallowing?
Occasionally, small
particles of food or water
may get into the larynx
or trachea.
food
particles
larynx
(voice-box)
trachea
(windpipe)
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What Happens During Breathing
and Swallowing?
This automatically induces
violent coughing to force
the food particles or water
out and to prevent
choking.
food
particles
larynx
(voice-box)
trachea
(windpipe)
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Oesophagus
• Minimal digestion
• Carries food from mouth to stomach by
peristalsis
• Oesophagus has circular and longitudinal
muscles which are antagonistic.
• When circular muscles contract, longitudinal
muscles relax and vice-versa.
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Peristalsis
Part of the gut wall
The two layers of muscles
cause rhythmic, wave-like
contractions of the gut walls.
Such movements are known as
peristalsis.
circular
Peristalsis:
muscles
• enables food to be mixed
with the digestive juices; and
• moves the food along the
gut.
longitudinal
muscles
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Wall here dilates
Direction of movement of food
Circular muscles relax
Wall here constricts.
Circular muslces
contract; longitudinal
muscles relax
Longitudinal muscles
contract
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Peristalsis
– Move the food down!
• When circular muscles
contract, longitudinal
muscles relax. Gut wall
constricts i.e. gut becomes
narrower and longer. Food
is squeezed or pushed
forward.
• Gravity and slippery
mucous lining helps push
food down too.
•
http://arbl.cvmbs.colostate.edu/hbooks/pathphys/digesti
on/basics/peristalsis.html
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Stomach
• Stores food temporarily
• Stomach muscles churns and mixes food (also
by peristalsis) with gastric juice to form chyme.
• Gastric juice contains hydrochloric acid (HCl)
and enzymes like rennin and pepsin
– HCl is very acidic (pH2), thus it kills bacteria
and other microorganisms, as well as
stopping the action of salivary amylase
– Provides acidic medium for gastric enzymes
to work
• Only protein digestion here
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• The stomach is
“guarded” at the
entrance and exit
points by sphincter
muscles which
control the amount
of food entering
and leaving the
stomach.
https://vimeo.com/76
306734
Click stomach or video link
to see video
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Small Intestine
•
•
Subdivided into duodenum, jejunum and
ileum
In the small intestine, chyme stimulates
1. Pancreas to secrete pancreatic juice
2. Gall bladder to secrete bile
3. Intestinal glands to secrete intestinal juice
•
All three juices secreted are alkaline,
pH 8.5
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bile duct
•Pancreatic and
intestinal juice contain
many digestive enzymes.
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bile
1
pancreatic juice
pancreatic
duct
•Bile does not contain
enzymes. Bile emulsifies
fats, increasing the
surface area for lipase
action
3
intestinal juice
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Villi – absorption by diffusion
Diffusion
From intestine
To liver
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T
Large Intestine (colon)
• Large inverted U
shaped tube.
• No digestion takes
place here
• Absorbs water and
minerals salts
• Stores the faeces
(dead cells, mucus,
germs, undigested
food)
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Is the colon the main region for water
absorption?
No! About 94% of the total amount of
water passing through the alimentary canal
is absorbed by the small intestine! The
large intestine absorbs most of the
remaining 6% of water.
Rectum – temporarily stores faeces
Anus – egests (= removal of undigested
matter) faeces
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Organs associated with the
alimentary canal
These organs do
not digest food but
aid in digestion
• Gall bladder
• Pancreas
• Liver
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Blood capillaries –
transport sugars
and amino acids
away from the
small
intestine
One cell thick epithelium –
for efficient absorption of
food particles
Lacteal – fatty acids
and glycerol recombine
in the epithelium to
form fat which then
enters the lacteal as
fine fat droplets
This continual transport of digested food substances
maintains the concentration gradient for the absorption
of digested food substances.
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Gall bladder
• Temporarily stores bile (smelly green
substance) secreted by liver.
• Secretes bile in the presence of chyme.
• Bile breaks up large fat droplets into very
small fat droplets to increase surface area
for lipase action (Emulsification)
Bile
emulsifies fats
• *Bile is not an enzyme, so it is not affected
by temperature
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Pancreas
• Connects to small intestine by pancreatic
duct
• Produces pancreatic juice
• Secretes hormones like insulin (controls
blood glucose concentration) and glucagon
(controls carbohydrate metabolism)
Liver
• Produces bile, which is stored in the gall
bladder
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Absorption
Adaptations of the small intestine
• Small intestine is very long (~5 m)
• Internal surface of the small intestine has
many folds.
• On these folds, there are many finger-like
projections called villi
• These 3 adaptations increase surface
area for absorption
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Assimilation
• After travelling through the blood stream to
the rest of the body, cells can now make
use of
– glucose as source of energy
– amino acids to build new cytoplasm and tissue
cells
– fatty acids to build new cell membranes
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