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DIGESTIVE SYSTEM
Components
1
...

 It runs from the mouth to anus
2
...

Functions
1
...
Propulsion
3
...
Digestion
5
...
Absorption
7
...
Immunity
1
...

2
...

 Major propulsive mechanism
moving food through the tract
...

Contract and relax (refers to the circular
and longitudinal smooth muscle)
- Mixing of contents
3
...

- Reduction in size of ingested material
increases its surface area facilitating
enzymatic digestion
...

- Longitudinal smooth muscle layer behind
the bolus relaxes while that in front of the
bolus contracts
...

-

4
...


Food materials are moved from the
stomach to the mouth
...

Food is also churned along the digestive
tract by segmental contractions
...

Digestion
Ingested nutrient are chemically broken
down into particles small enough for
absorption
(absorbable/bioavailability/digestible)
Macromolecules (protein, DNA,
polysaccharides, triglycerides) must first
be reduced into smaller molecules
...

Secretion

6
...

-

8
...

Water, mucous, acids, enzymes, buffers
and salts—
Released into the lumen of digestive tract
along its length
Absorption
Absorb ingested nutrients:
Organic substrates, electrolytes, vitamins
and water pass from the lumen into the
body
...

-

b
...

SALIVARY SECRETIONS

Salivary Glands and Saliva
-

Extramural glands (glands outside the wall of
digestive system) associated with the oral
cavity
...
Minor Salivary Glands – (labial, buccal,
palatial) have short ducts
...


b
...

Secretions can be serous, mucous or mixed
...
Serous cells – produced watery secretions
containing enzymes, ions and small amount
of mucin
...
Mucous cells – produced viscous, stringy
secretion called mucous
...
Mixed glands – from minor salivary glands
have both mucous and serous glands
Major Salivary Glands and Secretion
a
...

b
...

c
...


-

Saliva coat oral cavity and
esophagus
6
...
Lysozyme – (found in saliva) a
bacteriostatic enzyme that lyses bacteria,
protects the mouth
...
IgA – attaches to microbes decreasing
their ability to penetrate the epithelium
...
Cyanide – acts as a bacteriocide
d
...

e
...

7
...

- Used by cats that preen themselves
...
Nonruminants: As secretion of saliva
increases, concentration of sodium,
bicarbonate and chloride increase while
potassium decreases
...
Ruminants: As saliva production increases,
levels of sodium and phosphate in saliva
decrease while bicarbonate, chloride and
potassium increase
...
5%, thus hypo osmotic
Contains sodium, potassium, chloride,
bicarbonate and phosphate (all of these are
electrolytes)
Slightly acidic: pH 6
...

2
...

4
...


1
...
Provides alkaline buffering fluid
3
...
Lubricates and binds
5
...
Assist oral hygiene
7
...

Provides alkaline buffering and fluids –
bicarbonate and phosphate can neutralize
acidic feedstuff
...

Removes wastes – urea and uric acid are
excreted in the saliva
Initiates starch digestion – amylase in the
saliva begins starch digestion
...


Salivary glands and Salivation
-

Salivary glands continuously secrete saliva
keeping the oral cavity moist
...
s stimulation)
Sight, smell, sound or thought of food can
stimulate saliva production, evidenced by
Pavlov Experiment
 Pavlov trained dogs to salivate at
the sound of a bell
...


Ruminant Saliva
-

Isotonic, contain high concentrations of
bicarbonate and phosphate and high pH
Acts to buffer acids produced during
fermentation in the rumen
Adult cow produces as much as 100 to 200 L
of saliva/day

Tongue
-

Maneuvers food mass in the mouth
Assists movement of food in the esophagus
Prehension (in ruminants)

Chewing (Mastication)
-

A chemical process which includes:
a
...
Breaking of food
c
...
Moves food from the mouth;
b
...
Transport food to the stomach
Stages of Swallowing
1
...
Pharyngeal Stage – Bolus moves
involuntarily through the pharynx to the
esophagus
3
...


Esophagus
-

Collapsible muscular tube lying behind the
trachea
In many species of birds, upper portion is
expanded to form the crop
 Crop stores food in some species
(pigeons)
 Produces secretion (crop milk) that is
used to feed the young
...

 Sphincters prevent large volumes of
air from entering and regurgitation
 Excessive gas leads to
burping/belching (eructation)

Functions of Stomach
1
...
Mechanical breakdown of ingested food
3
...
Production of intrinsic factor required for
Vitamin B12 absorption from the small
intestine (IF deficiency leads to pernicious
anemia)
Stomach
- Regions:
a
...
the smallest region, located near
the heart
...

b
...
act as a blind-ended sac
...
Body
...

d
...
The pyloric region is the
caudalmost portion of the stomach
...
Mucous neck cells – produce a more
acidic mucus than goblet cells
...
Peptic or chief cells – produce pepsinogen
(the inactive form of pepsin, and an enzyme
that digests protein)
3
...

GASTRIC SECRETIONS
-

Pepsin is not produced within the chief cells
 Causes self-digestion of cells
Chief cells, instead, produce pepsinogen
(zymogen)
 Activated to pepsin as it enters the
stomach lumen and comes in contact
with HCl
...


-

Hydrogen ions are generated from
dissociation of carbonic acid
Hydrogen ions are transported into stomach
lumen in exchange for potassium
Chloride ions enter parietal cell in exchange
for bicarbonate ions
Chloride ions travel down the concentration
gradient and enter the stomach lumen
Once in the lumen, hydrogen and chloride
ions combine
Hydrochloric acid is produced

Alkaline Tide
1
...
Significant amount of bicarbonate enters the
blood
3
...
Stimulates secretion of both parietal and
chief cells;
2
...
Thus, mixing luminal contents
...
Released into interstitial fluid bathing the G
cells
2
...
Overridden by acetylcholine and histamine

GASTRIC MOTILITY
-

-

-

-

Arrival of food in the stomach
1
...
Stomach actively dilates (adaptive
relaxation)
3
...

Peristalsis in the stomach
 Begins near the cardiac sphincter
with gentle ripple-like movements
toward pyloric sphincter
Peristaltic Waves
 Strengthen as they move toward the
pylorus
Pyloric sphincter allows liquids and small
particles to pass
...

Chyme is squirted through the sphincter
before it closes
...


VOMITING (EMESIS)
1
...
Impulses sent to emetic center (medulla
oblongata) initiating a motor response
3
...
Cardiac sphincter relaxation
2
...
Stomach contents are forced upward
through the esophagus, pharynx and mouth
...
metabolic alkalosis
b
...
electrolyte imbalances
...


-

-

-

During egestion, nondigestible materials
such as bone, fur, or feathers are orally
eliminated from the digestive tract
...
prior to egestion
 Gizzard contractions increase
resulting in compaction of
undigestible material into pellet
...


GASTIC SECRETIONS AND EMPTYING

Gastric secretions are controlled by neural and
hormonal mechanisms at:
a
...
Gastric Phase (Stomach)
c
...
Cephalic phase
- Causes increase in gastric secretion prior to
arrival of food
...

b
...

- Stimulates gastric secretion and motility
- Accounts for about 2/3 of gastric secretions
...
Intestinal Phase
- Involves neural and hormonal signals
 Functions to decrease gastric motility
- Stimulation of chemoreceptors and stretch
receptors
 Triggers enterogastric reflex:
o Inhibits gastrin production and
gastric motility
...

o Slowing gastric emptying into the
duodenum
...
Inhibition of vagal nuclei in the
medulla
b
...
Sympathetic stimulation of
pyloric sphincter
...


-

-

Release of gastric inhibitory peptide (GIP)
 GIP: Inhibits gastric secretions and
gastric motility
...
5
...

Secretin inhibits gastric HCl and pepsinogen
release in the stomach
...


CARBOHYDRATE DIGESTION (SMALL
INTESTINE)

Salivary Amylase (Ptyaline/alpha amylase)
-

ACTIVATION OF PROTEIN DIGESTING
ENZYMES

PANCREATIC SECRETION

Initiates digestion of starch and glycogen in
the mouth
...

Most abundant in pigs but little in horses
...
Trypsinogen
- Activated to trypsin by enterokinase
(related by intestinal mucosa)
- Trypsin inhibitor prevents conversion of
trypsinogen to trypisin
b
...
Carboxypeptidase
- Activated by trypsin

Composition:
a
...

b
...
Pancreatic amylase
- Converts starch to maltose
- Maltase in pancreatic juice converts
maltose to glucose
b
...
Gas tends to collect at the top
2
...
Suspension of finer particles below
‘B’ (ERUCTATION) Sequence
-

Moves gas from rumen toward the oral cavity
Formation of gas bubble

Grazing
-

Feed moved to the rumen before completely
masticated
Feed returned to oral cavity through
rumination

RUMINANT DIGESTION

-

-

Negative pressure from the thorax is
transferred from the thorax to the esophagus,
thus bolus of reticular contents moves
through the cardia and through reverse
peristalsis, the bolus is moved into the oral
cavity
This is followed by swallowing

Swallowing
-

Carries liquid portion of bolus back to the
rumen
...

Time spent ruminating varies with the diet
...

1
...
Secondary bacteria (further break down of
end products of primary bacteria);
3
...

RUMENAL CARBOHYDRATE DIGESTION
Products of bacteria and protozoa carbohydrate
digestion include:
1
...
Carbon dioxide
3
...
Acetic acid (60 70%)
2
...
Butyric acid (10 15%)
Additional Details:
1
...
Percentage of propionic acid increases when
the animal is fed concentrates with soluble
sugars or starch
3
...
Hydrolysis of dietary proteins to peptides
and amino acids by rumen microorganisms
...
Microorganisms make amino acids from
nonprotein nitrogen sources (uric acid, urea
and ammonia)
3
...

4
...
Amino acids are then absorbed and used by
the animal
...
Triglycerides are hydrolyzed by ruminal
bacteria yielding glycerol and fatty acids
...
Glycerol is metabolized to propionic acid
...
Fatty acids pass to the duodenum where
they are absorbed

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