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THE DIGESTIVE
SYSTEM
AgriScience 332
Animal Science
#8646-E
TEKS: (c)(3)(A)
Introduction
The digestive system works to
convert food into simpler molecules
that can be absorbed and utilized
by the cells of the body.
Functions of the digestive system
include:
• Ingesting food,
• Grinding food,
• Digesting food,
• Absorbing nutrients, and
• Eliminating solid wastes.
The anatomy and physiology of the
digestive system differs among
species of animals, which have
digestive tracts that are adapted to
the most efficient use of the feed
they consume.
Herbivores are animals that depend
entirely on plants for food.
Examples of
herbivores are
cattle, sheep,
horses, and
rabbits.
Photo by Stephen Ausmus courtesy of USDA Agricultural Research Service.
Carnivores are animals that rely
almost entirely on meat for food.
Examples of carnivores include
cats and dogs.
Omnivores are animals that
consume both flesh and plants.
Examples of omnivores are swine,
chickens, and humans.
The length and complexity of the
digestive tract depends on the
species; carnivores have a
relatively short and simple tract,
while herbivores’ tracts are much
longer and more complex.
Among herbivores, there is a
difference between species based
on the stomach type, monogastric
or polygastric.
Ruminants are animals that have
a four-chambered stomach
(polygastric) that includes a large
rumen.
Non-ruminants are animals that
have a single stomach
compartment (monogastric).
Capacity of Total
Digestive System
The average
capacity of the
digestive system
and the types of
feed best suited to
the animal varies
among the species.
Animal:
Cattle
Capacity
(in liters)
356
Sheep &
Goat
Horse
Swine
44
211
27
Dog
7
Human
6
(Campbell, Kenealy, and Campbell, 2003)
Swine have larger digestive
capacity per pound of body
weight than dogs or humans.
Their digestive system is better
suited for concentrated feeds
with limited amounts of forage.
Horses have a much larger digestive
system than swine.
Even though they are non-ruminants,
horses can utilize large amounts of
roughage in their diets because they
have an enlarged cecum.
Cattle and sheep can utilize large
amounts of bulky feeds
(roughages) because they are
ruminants.
A 1200 pound cow may have a
stomach capacity for 300 pounds
of feed.
Some similarities occur in the
composition of the walls of the
digestive tract of various species.
The walls of the digestive tract,
which extend from the mouth
to
the anus, have four layers:
epithelium, lamina propria,
muscles, and visceral peritoneum.
Epithelium – mucous membrane
that lines the digestive tract from
mouth to anus and is continuous
with external skin of animals.
Lamina Propria – thin layer of
connective tissue that supports
the epithelium in the intestines.
Muscles – muscles of the esophagus
are striated, while the remainder of
the digestive tract is smooth muscle.
Visceral Peritonieum – covers the
digestive organs in the abdomen.
Anatomy of the
Digestive System
The digestive tract extends from
the lips to the anus.
As previously mentioned, the
length and complexity of the
digestive tract depends on the
species.
The digestive tract is made up of the
following parts:
• Mouth,
• Pharynx,
• Esophagus,
• Stomach,
• Small intestine, and
• Large intestine.
Accessory glands and organs
that assist in digesting food
include the following:
• Salivary glands,
• Liver,
• Gallbladder, and
• Pancreas.
Mouth – the primary functions of
the mouth are to grasp food, grind
food, and mix the food with saliva.
The mouth accomplishes these
tasks with the use of specialized
structures, including lips, tongue,
teeth, cheeks, jaw, and salivary
glands.
Lips of horses are designed to
grasp food.
The lips of swine and cattle are
used mostly for closing the mouth.
The tongue is used by most animals
to grasp food.
The tongue helps in the chewing
process and in the formation of
boluses.
Finger-like projections called
papillae cover the top surface of the
tongue and contain the taste buds.
Teeth are responsible for cutting
(incisors) and grinding (premolars
and molars) food.
After an animal is born, it develops
a set of milk or baby teeth.
As the animal ages, the milk teeth
are replaced by permanent teeth.
Animals’ cheeks consist mostly of
muscle that is lined with a mucous
membrane.
The cheeks line up food with teeth.
The movement of the jawbone is
controlled by powerful muscles that
open and close the jaw and move
it from side to side in chewing.
Salivary glands secrete saliva that
softens food, which aids in
swallowing.
Saliva contains mostly water, but
does contain some enzymes that
begin the chemical breakdown of
some starches.
Pharynx – a common pathway for
food and air.
Food passes from the mouth into
the pharynx, where the pharyngeal
muscles force food into the
esophagus.
Esophagus – a muscular tube that
connects the pharynx to the
stomach.
The esophagus passes through the
chest cavity and connects with the
stomach just after passing through
the diaphragm.
The cardia sphincter muscle
controls the movement of food
into the stomach.
The pyloric sphincter muscle
controls movement of food out
of the stomach.
As previously mentioned, ruminants
and non-ruminants differ in the
number of compartments that make
up the stomach.
Non-ruminants, or monogastrics,
have a single stomach compartment
that is sometimes called the “true”
stomach.
The stomach of non-ruminants is
located just beyond the diaphragm
on the left side of the body.
The “true” stomach has folds in the
epithelial lining that creates gastric pits.
Glands are located throughout the
stomach and secrete digestive fluids
into the pits, including hydrochloric
acid, pepsin, and rennin.
Ruminants, or polygastrics, have
stomachs with four compartments:
rumen, reticulum, omasum,
abomasum.
The relative sizes of the four stomach
compartments of the ruminant vary
with age of the animal.
In a calf at birth, the total capacity
of the non-glandular compartments
(rumen, reticulum, and omasum) is
about 30%, but by two months of
age, the total capacity of the
non-glandular compartments is 70%.
The rumen of calves becomes
functional at about six to eight
weeks of age and, by the time
the animal reaches maturity, the
rumen makes up 80% of the total
stomach capacity.
The rumen, reticulum, and omasum
contain no glands, but do contain
approximately one billion bacteria
and one million protozoa (per
milliliter).
These three compartments soak
food and allow microbial digestion
to take place.
Rumen – the first compartment of
the ruminant stomach, which fills
most of the left side of the abdomen.
The rumen has a very thick muscular
wall and consists of two sacs, dorsal
sac and ventral sac, that contain
many papillae.
Food first passes into the rumen,
where it can be regurgitated as cud.
Reticulum – the forward most
portion of the ruminant stomach.
The inner surface of the reticulum
has inward folds, resembling a
honeycomb shape.
The esophageal groove is a groove
that extends from the cardia
sphincter to the omasum.
It is capable of closing off the
rumen and reticulum, allowing food
to bypass these two parts and go
directly to the omasum.
Omasum – the third compartment
of the ruminant stomach that
contains muscular projections,
which are covered by mucous
membrane and contain many small
papillae.
The papillae in the omasum are
responsible for grinding roughage.
Abomasum – the only glandular
stomach of ruminants, the
abomasum is located under the
omasum.
The epithelial lining and glands
of the abomasum are the same
as those in the stomach of
non-ruminants.
Small intestine – a three-part tube
that is the site of some digestion
and the absorption of nutrients.
The small intestine is made up of
the duodenum, jejunum, and
ileum.
The small intestine is lined with
many villi, which absorb nutrients.
Large intestine – a larger tube of
the digestive tract that consists of
the cecum, colon, and rectum.
The size of the cecum is much
greater in horses and rabbits than
in other domestic animals.
The large intestine is the site of
water absorption and some mineral
and nutrient absorption, depending
on the species.
Wastes are eliminated from the
rectum through the anus, which is
controlled by sphincter muscles.
Poultry Digestive System
The anatomy of the poultry
digestive system differs from
other animals.
Poultry do not have teeth and the
prehensile structure is the beak.
Food passes from the mouth through
the esophagus to an enlargement of
the gullet called the crop.
The crop temporarily stores food and
softens it before it passes to the
proventriculus (glandular stomach).
From the proventriculus, food quickly
passes to the ventriculus, or gizzard.
The gizzard crushes and grinds
coarse feed aided by grit and
gravel that accumulated in the
gizzard during the bird’s life.
Food passes from the gizzard into
the small intestine where an
abundant supply of pancreatic
enzymes and bile are used to aid
in the bird’s digestion.
Anatomy and Physiology of
Accessory Digestive Organs
Several organs secrete enzymes
into the digestive tract that aid in
the digestive process.
These organs include the salivary
glands, pancreas, liver, and gall
bladder.
Salivary glands – paired glands,
including the parotid, mandibular, and
sublingual salivary glands.
The parotid and mandibular salivary
glands secrete serous, which is a
clear, watery fluid.
The sublingual salivary glands secrete
serous and mucous, a thick, cloudy
protective coating for the mucous
membranes of the digestive system.
Pancreas – an elongated, lobe-shaped
organ located at the beginning of the
small intestine, behind the liver.
The exocrine functions of the
pancreas are to secrete several
enzymes into the small intestine to
aid in digestion.
The endocrine function of the
pancreas is to produce insulin, which
lowers blood sugar.
Liver – a lobe-shaped organ located
just behind the diaphragm on the
right side of the body.
The liver purifies blood it receives
from the stomach, spleen, pancreas,
and intestines.
The liver also produces bile, which
is used in the digestion of fats.
Gall Bladder – a small, sac-like
organ attached to the liver that
collects bile produced by the liver
and secretes it into the duodenum.
Horses are the only domestic
animals that do not have a gall
bladder.
The Physiology of Digestion
An animal’s appetite is controlled
by the hypothalamus and is
influenced by the level of glucose
in the blood and fill in the stomach.
Environmental temperature and
the animal’s health also influence
appetite.
Digestion is the conversion of
feedstuffs into nutrients the body
can use.
Most feedstuffs are too complex
to be used without being broken
down into simpler molecules.
The digestive process includes
mechanical, chemical, and microbial
actions.
Mechanical actions include
mastication (chewing), deglutition
(swallowing), regurgitation, gastric
and intestinal motility, and
defecation.
Mastication reduces food particle
sizes to create more surface area
on which digestive juices can act.
Mastication mixes food with saliva.
In ruminants, large quantities of
ingested food are regurgitated as
boluses (cud) so that it can be
re-chewed.
As previously mentioned, poultry
have no teeth.
Mechanical digestion for poultry
takes place mainly in the gizzard,
where grinding reduces the size of
food.
Microorganisms aid digestion of
ruminants in the rumen, reticulum,
and omasum and aid digestion of
horses and rabbits in the cecum.
Microorganisms break down the
cellulose of plant cell walls, which
provides ruminants with 60% to
80% of their energy.
The rumen is the site of
approximately 60% to 90% of
digestion in ruminants.
In addition to breaking down
cellulose, microorganisms also
perform important functions in
the animal by synthesizing all of
the B-complex vitamins and all of
the essential amino acids needed
by their host.
Chemical digestion is mostly caused
by enzymes, which speed up the
biochemical reactions without being
used up in the process.
Various body cells make enzymes
that are used to break down
carbohydrate, protein, and fat
compounds into simpler molecules.
The following tables show the
enzymes that break down
compounds into simpler molecules.
Chemical Digestion of Carbohydrates
Enzyme:
Secreted by:
Action:
Ptylin
Salivary glands of Converts carbohydrates into
swine & horses.
maltose & dextrin.
Amylopsin
Pancreas into
duodenum.
Converts starches & dextrins into
simpler dextrins & maltose.
Sucrase
Small intestine
Converts sucrose into glucose
and fructose.
Maltase
Small intestine
Converts maltose into glucose.
Lactase
Small intestine
Converts lactose into glucose
and galactose.
Chemical Digestion of Proteins
Enzyme:
Secreted by:
Action:
Hydrochloric
Acid (chemical)
True stomach
Activates enzymes pepsin &
rennin.
Pepsin
Stomach
Rennin
Stomach
Breaks protein down into
proteoses and peptones.
In young nursing animals,
coagulates milk to aid
digestion.
Continue protein digestion by
breaking down more complex
substances into amino acids.
Trypsin
Pancreas into
Chymotrypsin
duodenum.
Carboxypeptidase
Chemical Digestion of Fats
Enzyme:
Secreted by:
Action:
Lipase
Stomach
Converts fats into higher
fatty acids & glycerol.
Bile (chemical)
Produced by liver,
stored and secreted by
gall bladder into
duodenum.
Emulsifies fats and
breaks them into smaller
globules.
Steapsin
Pancreas into
duodenum.
Completes conversion of
fats into higher fatty
acids and glycerol.
Hydrochloric acid in stomach helps
dissolve minerals in the diet.
Water and vitamins require no
digestion before being utilized
the body.
by
Absorption is the process by which
digested nutrients pass from the walls
of the digestive tract into the blood.
The small intestine is the site of most
absorption of nutrients for carnivores
and omnivores and is the site of a
significant amount of nutrient
absorption for herbivores.
The small intestine has numerous
finger-like projections, called villi,
that contain many blood vessels,
which are responsible for collecting
and absorbing nutrients.
Very little, if any, absorption of
nutrients occurs in the mouth,
esophogus, or stomach.
Some absorption of volatile fatty acids
does occur across the rumen wall.
Very few nutrients are absorbed in
the large intestine, except for a
substantial amount of volatile fatty
acids in herbivores.
The colon of the large intestine is
the absorption site of water.
The end products of fat digestion
are fatty acids and glycerol, which
are absorbed by the lymph ducts.
The blood absorbs the end products
of carbohydrate digestion
(monosaccharides and volatile fatty
acids), protein digestion (amino
acids and peptides), water, and
inorganic salts.
Digestion is complete after
absorption has made the nutrients
available for other parts of body.
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Reproduction or redistribution of all, or
part, of this presentation without
written permission is prohibited.
Instructional Materials Service
Texas A&M University
2588 TAMUS
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2007