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The
gut
breaks down plant and animal materials into nutrients that can be used by animals’ bodies.
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Plant material is more difficult to break down than animal tissue. The gut of
herbivores
is therefore longer and more complex than that of
carnivores
. Herbivores usually have a compartment (the
rumen
or
functional caecum
) housing micro-organisms to break down the
cellulose
wall of plants.
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Chewing by the teeth begins the food processing. There are 4 main types of teeth:
incisors, canines, premolars
and
molars
. In dogs and cats the premolars and molars are adapted to slice against each other and are called
carnassial
teeth.
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Saliva
is secreted in the mouth. It lubricates the food for swallowing and contains an enzyme to break down starch.
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Chewed food is swallowed and passes down the
oesophagus
by waves of contraction of the wall called
peristalsis.
The food passes to the stomach where it is churned and mixed with acidic
gastric juice
that begins the digestion of protein.
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The resulting
chyme
passes down the small intestine where enzymes that digest fats, proteins and carbohydrates are secreted.
Bile
produced by the liver is also secreted here. It helps in the breakdown of fats.
Villi
provide the large surface area necessary for the absorption of the products of digestion.
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In the
colon
and
caecum
water is absorbed and micro organisms produce some
vitamin B and K
. In rabbits, horses and rodents the caecum is enlarged as a
functional caecum
and micro-organisms break down cellulose cell walls to simpler carbohydrates. Waste products exit the body via the
rectum
and
anus
.
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The
pancreas
produces
pancreatic juice
that contains many of the enzymes secreted into the small intestine.
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In addition to producing bile the liver regulates blood sugar levels by converting glucose absorbed by the villi into glycogen and storing it. The liver also removes toxic substances from the blood, stores iron, makes vitamin A and produces heat.