Your Digestive System
The story we are about to tell is of stormy seas, acid rains,
and dry, desert-like conditions. It’s an arduous journey
that traverses long distances and can take several days.
It’s one in which nothing comes through unchanged. It’s
the story of your digestive system whose purpose is to turn
the food you eat into something useful – for your body.
Down the Hatch
It all starts with that first bite of pizza. Your teeth tear off
that big piece of crust. Your saliva glands start spewing
out spit like fountains. Your molars grind your pizza crust,
pepperoni, and cheese into a big wet ball. Chemicals in
your saliva start chemical reactions. Seemingly like
magic, starch in your pizza crust begins to turn to sugar!
A couple more chews, and then your tongue pushes the
ball of chewed food to the back of your throat. A trap
door opens, and there it goes, down to your gullet!
Next, your muscles squeeze the wet mass of food down,
down, down a tube, or oesophagus, the way you would
squeeze a tube of toothpaste. It’s not something you tell
your muscles to do – they just do it – in a muscle action
called peristalsis. Then the valve to the stomach opens
up and pizza mush lands in your stomach.
Inside the Stomach
Imagine being inside a big, pink muscular bag –
sloshing back and forth in a sea of half digested mush
and being mixed with digestive chemicals. Acid rains
down from the pink walls, which drip with mucus to
deep them from being eroded.
Sounds a little like an amusement park ride gone
crazy? Every time you think you’ve got your
equilibrium back, the walls of muscles fold in on
themselves again. Over and over again, you get
crushed under another wave of slop. Every wave
mixes and churns the food and chemicals together
more – breaking food into even smaller and smaller
bits. Then another valve opens. Is the end in sight you
ask, as the slop gets pushed into the small intestine?
Inside the small intestine, chemicals and liquids from
places like the liver and pancreas break down and
mix up leftovers. The small intestine looks like a strange
underwater world filled with things that resemble small
finger like cactuses. But they’re not cactuses, they’re
villi. Like sponges, they’re able to absorb tremendous
amounts of nutrients from the food you eat. From the
villi, the nutrients will flow into your bloodstream.
But hold on!! The story is still not over yet – the
leftovers that your body cannot use still have
more travelling to do! Next, they’re pushed into
the large intestines. It’s much wider and much
drier. You find that the leftovers are getting
smaller, harder and drier as they are pushed
through the tube. After all, this is the place
where water is extracted and recycled back
into your body. In fact, the leftovers that leave
your body are about 1/3 the size of what first
arrived in your intestines.
Where food turns into poop
Finally, the end of the large intestine is in sight!
Now the drier leftovers are various handsome
shades of brown. They sit at the end of their
journey, waiting for you to expel them – out
your anus. Of course, you know the rest! A
glorious, if slightly stinky, journey, don’t you
• How long are your intestines?? At least 8
metres in an adult. Be glad you’re not a
full-grown horse – their coiled up intestines
are 29 metres long!!
• Chewing food takes from 5 – 30 seconds
• Swallowing takes about 10 seconds
• Food sloshing in the stomach can last 3 – 4
• It takes 3 hours to food to move through
• Food drying up and hanging out in the
large intestine can last 18 hours to 2 days.
• Americans eat about 333 million kgs of
• Americans eat over 1 billion kgs of
chocolate a year
• In your lifetime, your digestive system may
handle about 50 tons.
The digestive system is often referred to as a food
refinery – purifying the raw material foods to provide
fuels and building materials for the cells. It is the
process that breaks down food into substances that
can be absorbed by the body for energy, growth and
The digestive system prepares food for use by
hundreds of millions of body cells.
Two processes occur; ingestion, food is taken into the
digestive system and swallowed, and digestion, when
food enters the alimentary canal (full length of the
digestive pipe). There are two types of digestion;
mechanical, the breakdown of food by the action of
the teeth and alimentary canal (squeezing, turning
and tossing food), and chemical (breaking bonds of
large molecules), the breakdown of food by enzymes,
acid and bile in the mouth, stomach and small
The gut modifies food physically and chemically and
disposes of unusable waste.
Structures of the Digestive System
The alimentary canal consists of four layers of tissue:
• Mucosa – made up of epithelial cells (an
internal skin which absorbs molecules of
nutrients and produces digestive secretions –
liquids) and thin layers of connective tissue,
varies along the alimentary canal according to
what role it has to play eg one cell thick in the
small intestine where absorption of food
• Submucosa – a layer of connective tissue lying
beneath the mucosa (protein containing fluid).
Collects and supplies lymph (protein containing
• Muscle layer – found under the submucosa, is
responsible for mixing food and mechanical
• Outer membrane – outer most layer of the
canal and is a thin membrane (thick skin of
protective sheath), connects and anchors to
the wall of the abdominal cavity.
Organs of the Alimentary Canal -
Mouth, Pharynx and Salivary Glands
• Teeth cut and crush food and mix food with
saliva. Saliva travels to the mouth through ducts
from the salivary glands. Saliva contains an
enzyme called amylase, water and minerals and
some mucus, which helps to lubricate the food.
• Once swallowing begins, it is an automatic or
reflex event, over which we have no conscious
control. It occurs in four stages:
– The tongue pushes against the top of the mouth
(hard palate) and pushes the food to the back of
the mouth or pharynx.
– Soft palate is pushed upwards by the tongue,
closing the opening to the nasal cavity.
– Larynx is pulled upwards against the epiglottis, to
prevent food from entering the respiratory
– Food is then squeezed backwards and down and
guided through the pharynx to the oesophagus.
• It is a simple straight tube with very
• Solid foods are moved down the
oesophagus by waves of peristalsis and
reach the stomach in 6 seconds (liquids
travel much more rapidly)
• Sphincters are found in the alimentary
canal, where one organ joins with
another. They act like a rubber band
around the top of a bag.
– Demonstrate squeezing a plastic bag,
therefore touch sensitive
• By their contraction or relaxation they
control the passage of small quantities of
digested food from one organ to the next.
• Examples: between the oesophagus and
the stomach, between the stomach and
the small intestine.
Organs of the Alimentary Canal -
• The stomach is a large muscular sac.
• It has three mechanical tasks:
– It stores or acts as a reservoir for the
swallowed food and liquid. This requires the
muscles of the upper part of the stomach
to relax and accept large volumes of
swallowed material. Food may remain in
the stomach for 3 – 4 hours and may hold
over 2 litres of food and gastric juices.
– The lower part of the stomach mixes the
food, liquid and gastric juices (acid, pepsin
and mucus) together by waves of peristaltic
contractions to pound and churn the
contents. This is converted into a semi-fluid
mixture called chyme (slurry).
– Through the pyloric sphincter the chyme is
slowly released into the small intestine.
Organs of the Alimentary Canal -
Acid on meat activity – observation
Acid is neutralized in the duodenum by pancreatic bicarb to prevent small
Acid activates pepsin to break down protein
Mucous (secreted by goblet cells) protects lining of stomach from self
Ulcers – due to bacteria
• It is a very long tube – up to 8 metres in adults,
which is coiled up inside the abdominal cavity.
• It is divided into 3 parts
• The duodenum is the main site of chemical
digestion in the gut tube. Digestive secretions
enter the duodenum from:
– The liver, via the bile duct
– The pancreas, via the pancreatic duct.
– Gland cells in the wall of the intestines
• The sodium bicarbonate of pancreatic juice
soon neutralises the acid chyme from the
stomach. The digestive enzymes, assisted by
the bile salts from the liver, and then complete
the breakdown of food molecules to small
soluble compounds. These products of
digestion can pass through the intestinal lining
and enter the bloodstream,
• Digestive foods are absorbed into the ileum.
Organs of the Alimentary Canal –
Small Intestine Food intolerance (gluten,
lactose) gut gets upset, causing
inflammation, fatigue etc, can
cause at the brain level (mood
Coke angle can be helpful in
stabilizing – potassium
dialysis tube allowing one
way flow of some things
Into blood or lymph (fats)
Osmotic pull – no energy
used (passive process), high
to low concentration.
Show the rate of flow and
strong can you make
• Material that is not absorbed by the ileum is passed
through to the large intestine. These are vegetable
fibres and cellulose (roughage), bacteria and mucus
(Recovers H2O – solvent carrying medium – if not
diarrhoea. Vitamin manufacture bacteria – if CHO
gets too thick or roughage too high)
• The large intestine consists of three main areas:
– Caecum and appendix
• The caecum and appendix have no special features
• There is no secretion of enzymes in the large intestine
and it can absorb very little food. However, it does
absorb a great deal of water which then leaves the
undigested residue which passes on to the rectum in
a semi-solid state, known as faeces.
• Remaining food material may spend up to 24 hours in
the large intestine, before it is expelled through the
anus after temporary storage in the rectum.
Organs of the Alimentary Canal –
You tube –
Slow emptying of viscous meal –
Vomiting reverse giant contractions -
1. The Liver
It produces bile, cleans poisonous substances
such as alcohol and lactic acid from the blood
and stores nutrients such as glycogen. The liver
acts as a filter system for the body.
2. The Gall Bladder
Muscular pouch that stores bile from the liver,
then releases it into the small intestine. Bile
breaks down large droplets of fat.
3. The Pancreas
It produces several digestive enzymes and
hormones and releases them into the small
intestine. Insulin is one of these. It controls the
blood glucose levels.
– Exocrine part produces many enzymes,
which enter the duodenum via the
– Endocrine part produces insulin.
Organ Gland Secretion Function
Mouth Salivary Glands Saliva
Disaccharides (starch and glycogen broken down to
Stomach Gastric Glands Gastric juice
Acid environment for pepsin
Liver Liver Bile
• Bile salts
Emulsification of fats (green)
Pancreas Exocrine pancreas Pancreatic juice
Polysaccharides to disaccharides
Fats to fatty acids and glycerol
Proteins to polypeptides
Small Intestine Intestinal Glands Intestinal juice
Peptides to amino acids
Disaccharides to monosaccharides
Secretions of the Digestive System
• Absorption of small molecules takes place in the
• For absorption to be efficient and complete, a
large area of contact between the digested
food molecules and the intestinal lining is
• There are four ways in which the structure of the
small intestine provides a huge surface area for
– Great length
– Large circular folds
– Villi (finger like projections of mucosal lining)
– Microvilli (tiny folds on the surface of each cell)
• In addition the small intestine has a very large
supply of blood vessels to transport the
absorbed food molecules from the gut to the
tissues of the body.
• From the small intestine, absorbed molecules
are carried in the hepatic portal vein to the liver
• Here the food molecules may be stored by the
liver cells or changed before joining the general
circulation of the body.
Absorption and Surface Area
Surface Area model
Huge increase in contact
area but then must slow
down the flow, compared
protein in the first 2 hours
when most needed (post
Organ Action Process
• Mechanical Digestion mixes salivary juice and breaks food down in size
• Chemical Digestion of STARCH begins and salivary juice breaks down
to a simpler form
Oesophagus Swallowing • Mechanical movement is caused by muscular contraction called
PERISTALSIS which forces food into the STOMACH
Stomach Churning and
• Mechanical mixing of the DIGESTIVE juices, which enter the
stomach; HYDROCHLORIC ACID kills any dangerous bacteria and
ENZYMES begin to act on PROTEINS
• Mechanical peristalsis continues
• Chemical BILE from the GALL BLADDER begins to chemically change
FATS into smaller fat particles; PANCREATIC JUICE (from the
PANCREAS) containing ENZYMES begin to act on proteins, fats and
starch (each one has a particular enzyme, which causes the changes).
PANCREATIC JUICE also contains INSULIN, which controls the
amount of sugar in the blood by controlling sugar metabolism.
Absorption Absorption begins soon after food enters the small intestine PROTEINS
and CARBOHYDRATES enter the capillaries in the walls of the
FATS enter the LYMPH vessels, which are also in the walls of the intestine.
Proteins and carbohydrates are carried directly to the LIVER.
FATS will enter the bloodstream directly after being removed from the
intestines by the lymph vessels.
Large Intestine Movement WATER only is absorbed from what remains in the large intestine. The
remainder (mostly waste materials and some water) is passed along for
storage in the RECTUM, ready for elimination through the ANUS.
• If too much water is removed, CONSTIPATION results.
• If too little water is removed, DIARRHEA results.
Digestive System Summary