ANS 331: Digestive System
I. Digestion
1. The process by which food is converted into substances that
can be absorbed and assimilated by the body.
2. It is accomplished in the alimentary canal by the
mechanical and enzymatic breakdown of foods into simpler
chemical compounds.
II. The Digestive Tract
1. General
A. Animals classified by their diet in a natural state
a. Carnivores
i. Meat-eaters
ii. Dog, Cat
b. Herbivores
i. Plant-eaters
ii. Cattle, Horses, Sheep
c. Omnivores
i. Meat- and Plant-eaters
ii. Pigs, Humans
B. Because of diversity in diet, digestive systems
have developed in different ways
--Cecum, Rumen, Colon, etc.
2. Mouth
A. Oral Cavity
B. Food is received and reduced in size
C. Mixed with saliva and swallowed as bolus
D. Teeth and Tongue assist with function
3. Teeth
A. Functions
a. Reduction in size of food by grinding
--Increase surface area for chemical and/or
microbial degradation
b. Cutting of foodstuffs to be presented to
mouth
c. Protective function
--Inflict wounds
d. Kill Prey
--Food gathering for carnivores
B. Four types of teeth
a. Incisors (I)
i. Most forward
ii. Used for cutting
b. Canines ©
i. Also fangs, eye teeth, tusks
ii. Used for tearing and separation
of food mass
c. Premolars (P)
i. Just caudal to canines
ii. Used for grinding and chewing
d. Molars (M)
i. Larger than premolars
ii. Used for grinding and chewing
C. Dental Formulas
a. Represent one side of mouth
b. Numerator = top teeth
c. Denominator = bottom teeth
d. Species:
Cow & Sheep I 0/4 C 0/0 P 3/3 M 3/3 = 32 teeth
Pig & Horse I 3/3 C 1/1 P 4/4 M 3/3 = 44 teeth (Horse can vary)
Humans I 2/2 C 1/1 P 2/2 M 2/2 = 32 teeth
D. Animals can be aged by teeth eruption and wear
4. Tongue
A. Muscular organ used to maneuver the food mass
B. Muscle fiber orientated in three directions
C. Functions:
a. Seizes and brings food to mouth
b. Brings food to molars
c. Assists in swallowing
D. Papillae
a. Projections on tongue
b. Traction for moving food
c. Used in grooming
d. Contain taste buds
5. Pharynx
A. Common Pathway for air and food
B. Food is prevented from entering larynx
and nasal cavity by reflexes and the mechanics
of swallowing
6. Esophagus
A. Muscular tube that extends from Pharynx to Stomach
a. Combination of Skeletal and Smooth Muscle
B. Food and water moved via contraction waves of
the muscular wall
C. Whole esophagus, including cranioesophageal
sphincter are closed when not transporting food
or water
D. Opening just above glottis (opening to larynx)
E. Usually lies on left side of trachea
7. Stomach
A. Functions
a. Storage of Food
b. Early digestion
B. Subdivided into parts (continuous) as viewed
from outside
a. Cardia
--Entrance
b. Fundus
c. Corpus
d. Antrum
--Enters small intestine (duodenum) through
pylorus
C. Inner aspects based on cell type
a. Esophageal
i. Esophageal-like Epithelium
--Stratified Squamous
b. Cardiac gland
i. Secrete mucous
ii. Columnar Epithelium
c. Fundic gland
i. Contain Gastric Glands
ii. Two types
1. Parietal Cells
--Hydrochloric Acid (HCl)
2. Neck Chief Cells
--Pepsinogen
iii. Also secrete mucous
d. Pyloric gland
i. Secrete mucous
ii. Secret gastrin
8. Small Intestine
A. Three sections
a. Duodenum
b. Jejunum
c. Ileum
B. Receives secretions from Pancreas and Liver (Bile)
C. Most digestion and absorption occurs in
small intestines
D. Histology
a. Mucosa
i. Epithelial Lining
b. Submucosa
i. Connective Tissue
ii. Also contains:
-- blood vessels
-- lymph vessels
--nerve fibers (Meissner's Plexus)
--Smooth muscle (Muscularis Mucosae)
iii. Lamina propria
--Submucosa inside of Muscularis
Mucosae
c. Muscularis Externa
--Smooth Muscle
i. Circular
ii. Longitudinal
d. Serosa
i. Outer epithelial and C.T. layer
E. Small Intestine contains large surface area for
digestion
and absorption.
a. Considerable length
--Looping and Coiling
Horse 73 feet
Sheep 85 feet
Pig 60 feet
Cow 151 feet
b. Folding of intestinal surface
i. Villi
ii. Microvilli
--Brush Border
iii. Provides 600X more surface area
than smooth cylinder
iv. Crypts of Lieberkühn
--Lie deep between Villi
--Only cells that undergo cell
division
--Cells migrate out and replace
older cells
--Replacement time:
2-4 d young animals
7-10 d older animals
F. Nerve Networks
a. Meissner's Plexus
i. Control Secretions
ii. Controls Blood Flow
iii. Sensory: Reflexes
b. Auerbach's Plexus
i. Between Circular
and Longitudinal Muscularis
ii. Control Movement
c. Two combine to form Enteric Nervous System
i. Contains Pacemakers and Conduction
fibers
ii. Under Autonomic Nervous System
Control
G. Blood and Lymph Supply
a. Capillaries supply each villi
i. Veins go to liver
--Hepatic Portal System
b. Central Lacteals
i. Lymph vessels
ii. Transport large molecules
iii. Bypasses liver to thoracic duct
9. Large Intestine
A. Enter from ileum
B. Site of Fermentation
C. Two Parts
a. Cecum
b. Colon
D. Cecum
a. Different degrees of development
b. Horse is very well developed
--Lies on right side
E. Colon
a. Three parts
i. Ascending
ii. Transverse
iii. Descending
b. All animals have Transverse and Descending
c. Pig, Ruminants, and Horses have counterpart
of Ascending
i. Pig and Ruminants
-- Ansa Spiralis (Coiled Colon)
ii. Horse
--Large colon
--Divided into Dorsal and Ventral
--Double Horse Shoe
d. Cecum and colon sacculated in pigs and horses
i. Due to bands of muscle
ii. Sacculations called Haustra
iii. Allow more time for microbial
digestion
e. Descending Colon
i. Part within Pelvis is called rectum
ii. Anus is the terminal opening
--Sphincter with smooth
and skeletal muscle
III. Digestive Accessory Organs
1. Salivary Glands
A. Three Primary Glands
a. Parotid
b. Mandibular
c. Sublingual
d. Also some less defined, scattered salivary
tissue
B. Secretions
a. Saliva
b. Functions
i. Lubricates chewed food
ii. Moistens the oral walls
iii Digestive enzymes
--Ptyalin (amylase)
c. Form from glands
i. Serous
ii. Mucous
iii. Mixed
2. Pancreas
A. Exocrine and Endocrine Gland
a. Located along duodenum
B. Exocrine Function
a. Digestive Secretions
b. Primary duct enter duodenum near common
bile duct
--In sheep and goats the duct enters the
bile duct before entering
the intestine
c. Accessory duct is also present a short
distance downstream
C. Endocrine Function
a. Hormones
b. No ducts
c. Endocrine cells located in
Islets of Langerhans
i. Alpha Cells--Secrete Glucagon
ii. Beta Cells--Secrete Insulin
iii. Delta Cells--Secrete Somatostatin
3. Liver
A. Multipurpose organ
a. Over 500 functions
b. Production of bile and bile salts
c. Protein Synthesis
i. Enzymes
ii. Hormones
d. Storage
e. Metabolic Conversion
f. Detoxification
B. Hepatic Portal System
a. Blood is received from stomach, spleen,
pancreas, and intestine from portal system
b. Circulated through sinusoids
(second capillary bed of portal system)
c. Detoxified before entering central vein
(second venous drainage of portal system)
i. Kupffer cells
--Phagocytose Foreign particles
and old RBCs
ii. Other material is removed by
membrane transport through the
liver epithelial cells to be
metabolized in a usable form
or to be excreted
d. Hepatic artery, portal vein and bile duct
form a triad in the liver lobules
--Bile flows opposite direction of blood
flow
IV. Foodstuffs
1. Carbohydrates
2. Proteins
3. Fats
4. Water
5. Inorganic Salts
6. Vitamins
V. Physical and Mechanical Factors
1. Prehension
A. The seizing and conveying of food into the mouth.
B. Prehensile Structures
a. Lips
b. Teeth
c. Tongue
C. Horse
a. Highly mobile upper lip (Eating grain)
b. Incisor teeth (Grazing)
D. Cow & Sheep
a. Immobile upper lip
b. Tongue is primary prehensile organ
c. Brings grass to incisors and
upper dental pad
--Sheared by upper movement of head
d. Sheep cleft upper lip allows for close
to the ground grazing.
D. Pig
a. Heavy snout and pointed lower jaw adapted
for rooting
2. Mastication
A. Mechanical breakdown of food in the mouth
a. Chewing
B. Varies among species
a. Herbivores need to chew more than Carnivores
C. A bolus is formed from chewing and mixed with
saliva for swallowing
3. Deglutition
A. Act of swallowing
B. Three stages
a. Through the mouth (voluntary)
b. Through the pharynx (reflex)
c. Through the esophagus (reflex)
C. Sequence of Swallowing Reflexes
a. Respiration is inhibited
b. Glottis (opening of larynx) is closed
c. Larynx pulled upward and forward
d. Base of tongue fold the epiglottis
(forward projection of glottis) over the
glottis as the tongue plunges the bolus
from the mouth into the pharynx
e. Soft palate is elevated, closing the nasal
passage
f. Pharynx contracts to direct food into the
esophagus
g. Reflex peristaltic wave in the esophagus
is initiated transporting bolus to stomach
4. Smooth Muscle Activity
A. Food is moved from stomach through intestine
by smooth muscle
B. Muscle Activity is Spontaneous
a. Modulated by ANS
C. Very excitable tissue
a. Membrane potential is a slow wave
b. At threshold, spike potential
(action potential) is observed
c. Increase frequency of spike potentials
= the longer the sustained muscle
contraction
d. Duration longer than nerve impulses
because Ca++ channels also involved
in Depolarization
i. Influx slower than Na+
-- Ca++ Channels slower to open
and close
ii. Ca++ also involved in myosin/actin
interaction
D. Stimulators (cause depolarization)
a. Stretch
b. Acetylcholine
c. Parasympathetics
E. Inhibitors (cause hyperpolarization)
a. Norepinephrine
b. Sympathetics
5. Segmentation and Peristalsis
A. Segmentation
a. Contractile waves that travel short
distances
B. Peristalsis
a. Contractile waves that travel longer
distances
C. Both waves conducted toward anus (aboral)
D. Pacemakers
a. Stomach
--Located at greater curvature
b. Intestine
--Located in longitudinal muscle near
entrance of bile duct
E. Peristaltic Reflex
--Intrinsic Reflex
a. Initiated by distention (stretch) of the
bowel
b. Cranial contraction, Caudal relaxation
c. Moves food aborally
d. Also stimulated by gastrin
and cholecystokinin
e. Inhibited by secretin
F. Extrinsic reflex
a. Peritoneal Irritation
6. Physical Functions of the Stomach
A. Functions
a. Storage of ingested food
i. Fundus
ii. Adapts volume so excessive pressure
doesn't occur
b. Mixing of the food with secretions
i. Corpus
ii. Mixes Saliva, Food,
and Gastric Secretions
c. Control the emptying of its contents
i. Antrum
ii. Pump acting with pyloric sphincter
iii. Helps with Corpus mixing when
sphincter is closed
d. Stretch receptors cause increase tone
and contractions
B. Delay of Gastric Emptying
a. Enterogastric Reflex (Neural Mechanisms)
i. Osmoreceptors in duodenum respond
to hypertonic content
--From electrolytes, or products
of CHO and Protein Digestion
--Inhibits rapid loss of water
from blood
ii. H+ receptors respond to high
H+ concentration
--pH balance
iii. Allows more time for equilibration
(homeostasis)
b. Entergastrone Reflex (Endocrine Mechanisms)
i. Cholecystokinin released from
duodenal mucosa
in response to lipids
ii. Gastric inhibitory polypeptide (GIP)
released from jejunal mucosa
in response to lipids and CHOs
iii. Allows more time for digestion
(homeostasis)
C. Emesis (Vomiting)
a. Emptying of cranial duodenum and stomach
toward mouth (orad direction)
b. Protective mechanism
i. Prevents absorption of Noxious
chemicals
c. Reflex
i. Antiperistalsis
ii. Closure of glottis and nasal cavity
iii. Controlled by vomiting center
in brain
d. Species differences
i. Pigs, Dogs, & Cats
--Vomit easily
ii. Ruminants
--Ejection from abomasum to
forestomachs
--No ejection from mouth
--Not the same as rumination
iii. Horses
--Vomiting is rare
--Cardia doesn't open in reverse
direction
7. Physical Functions of Small Intestine
A. Functions
a. Mix contents
b. Propel contents
B. Flow must be controlled
a. To provide proper mixing of luminal contents
with pancreatic enzymes and bile
b. To provide time for digestion of CHOs, Fat,
and Protein
C. Mechanisms
a. Segmentation of the Ileum (Delays Transport)
i. Segmentation is a mixing mechanism
where Peristalsis is more of a
propelling mechanism
b. Autonomic Nervous System
i. Parasympathetic increases transport
ii. Sympathetic decreases transport
c. Hormones
i. Secretin inhibits
ii. Cholecystokinin stimulates
8. Physical Functions of Large Intestine
A. Functions
a. Microbial Digestion
b. Reabsorption of Electrolytes and Water
B. Motor activity of cecum and colon is directed
toward delay in transit and filling of parts
(reservoir function)
a. Retrograde flow plays a big role
b. Increased colonic activity associated with
constipation
c. Decrease colonic activity associated with
diarrhea
9. Intestinal Transport of Electrolytes and Water
A. Electrolytes and Water must be reabsorbed to
maintain water balance
B. Diarrhea greatly decreases reabsorption
a. If uncontrolled or if water/electrolytes
replenished animal could die
--Blood volume loss and circulatory
collapse
10. Defecation
A. Evacuation of feces from the terminal colon
and rectum
B. Complex Reflex Act
a. Also voluntary control
C. Frequency
a. Horses 5-10 times/day
b. Cattle 10-20 times/day
c. Carnivores 2-3 times/day
D. Passage time: Mouth to Anus
a. Pigs 48 hrs
b. Horses 24-48 hrs
c. Cattle 12-24 hrs
VI. Digestive Secretions and their Functions
1. Saliva
A. Functions
a. Lubricates chewed food
b. Moistens the oral walls
c. Amylase
--Most abundant in pig
d. Ruminants: Buffer for microbial
fermentation
e. Antifoam agent
2. Gastric Secretions
A. Mucous
a. Secreted throughout digestive tract
b. Function: Lubrication
B. HCl
a. Secreted by Fundic Parietal Cells
b. Initiate Protein Digestion
C. Pepsinogen
a. Secreted by Fundic Neck Chief Cells
b. Converted to Pepsin by HCl
c. Initiates Protein Digestion
D. Gastrin
a. Secreted by Pyloric G Cells
b. Stimulate HCl and Pepsinogen secretion
c. Stimulate digestive tract motility
E. Intrinsic Factor
a. Secreted by mucous cells
b. Facilitates Vit. B12 reabsorption in ileum
F. Rennin
a. Secreted by young ruminants
b. Milk coagulating enzyme
--allows for more digestion in stomach
G. Factors regulating gastric secretion
a. Stimulation
i. Acetylcholine (Parasympathetic)
ii. Gastrin
iii. Histamine
iv. Secretin (Pepsinogen only)
b. Inhibition
i. Within stomach
-- Decrease in pH to 2
ii. From Duodenum
--presence of acidic, fatty,
and hypertonic solutions
iii. Mechanisms
1. Neuronal
--Inhibition of
parasympathetic
neurons that
stimulate G cells
2. Hormonal
--Secretin
--Cholecystokinin
--Gastric inhibitory peptide
(GIP)
3. Pancreatic Digestive Secretions
A. Bicarbonate
a. Neutralizes HCl
b. Buffer microbes in cecum and colon
B. Proteases
a. Secreted as proenzymes
--Activated in intestine
b. Types
i. Trypsinogen
--Activated by enterokinase
to trypsin
--Trypsin then activates others
ii. Chymotrypsinogen
iii. Elastase
iv. Carboxypeptidases A & B
c. Breakdown proteins
C. Pancreatic Lipase
a. Hydrolyzes Dietary Triglycerides
b. Activated by Bile Salts
D. Pancreatic Amylase
a. Hydrolyzes starch to maltose
E. Control of Secretion
a. Autonomic Nervous System
i. Parasympathetic stimulates
ii. Sympathetic inhibits
b. Hormones
i. Gastrin
--Stimulates secretion of
all enzymes
ii. Secretin
--Stimulates Bicarbonate secretion
iii. Cholecystokinin
--Stimulates protease and lipase
secretion
4. Biliary Secretions
A. Bile is a greenish-yellow solution
a. Bile Salts
b. Bilirubin
c. Cholesterol
d. Lecithin
e. Electrolytes
B. Continuously synthesized by liver
C. Bile salts are recycled
a. Quantity needed exceeds synthesis
b. Absorbed by intestine after being used
c. Resecreted by hepatic cells
(enterohepatic circulation)
D. Bile salts are formed from cholesterol
a. Combine with lecithin and cholesterol to
form micelles
b. Formation of micelles prevent cholesterol
precipitation and therefore gall stones
--Also depends on alkalinity
(bicarbonate buffers)
E. Bile is stored in the gall bladder
a. Can be concentrated depending
on storage time
b. Infrequent eaters are more concentrated
c. Frequent eaters are dilute
--Farm animals except horse
d. Horse has no gall bladder
F. Sphincter of Oddi controls release into duodenum
a. Controlled by cholecystokinin (CCK)
i. Released in response to lipids
and amino acids
ii. Causes gall bladder contraction
and sphincter relaxation
G. HCO3- secretion
a. Stimulated by CCK, Gastrin, and Secretin
b. More secreted from liver than pancreas
F. Functions of Bile
a. Buffer
b. Fat emulsification
c. Removal of products of lipid digestion
(micelle)
5. Breakdown and Absorption of Carbohydrates, Proteins,
and Fats
A. Most of digestion and absorption of CHOs, Proteins
and Fats
occurs in the small intestine
B. Read Book for more details
6. Microbial Digestion in the Large Intestines
A. No enzymatic digestion occurs in large intestine
B. Digestion that occurs results from microbial
digestion
a. Nonruminant herbivores and omnivores
C. End products of microbial digestion
are Volatile Fatty Acids
a. Important energy source
--75% of Horses Energy Requirement
b. Also assist in water reabsorption
--Water follows absorbed VFAs
V. The Ruminant Stomach
1. Ruminants are animals that regurgitate and remasticate
their food (Rumination)
A. Two Suborders
a. Ruminantia
--Deer, Moose, Elk, Reindeer, Caribou,
Antelope, Giraffe, Musk Ox, Bison,
Cow, Sheep, and Goat
b. Tylopoda
--Camel, Llama, and Alpaca
c. Differences
i. Tylopoda don't have a Omasum
ii. Tylapoda have sacculated surface
in Reticulum and Rumen that contain
cardiac glands
B. Ruminants stomach has adapted for fermentation of
ingested food
by bacteria and protozoa
a. Energy is obtained from plant cellulose
b. Fermentation requires controlled conditions
i. Secretions
ii. Motility
iii. Temperature
c. Rumination assist fermentation by grinding
foodstuffs up more finely and increasing
surface area for microbes to attack
2. Four Compartments of Ruminant Stomach
A. Rumen
B. Reticulum
C. Omasum
D. Abomasum
E. Rumen, Reticulum, Omasum are considered to
Forestomach
a. Lined with Stratified Squamous Epithelium
(Esophageal region of stomach)
F. Rumen is on left side, Abomasum on Right
3. Functions of the Compartments
A. Rumen
a. Allows for soaking, fermentation,
and mixing of bulk fibrous feedstuffs
B. Reticulum
a. Pump that causes liquid to flow into
and out of rumen
b. Directs ingesta to Rumen or from Rumen
to Omasum
c. Floods cardia prior to regurgitation
d. Reticular groove active in young ruminants
--Conveys milk towards omasum and abomasum
--Reflex loses responsiveness with age
C. Omasum
a. Grinds and triturates (squeeze out fluid)
rumen contents that is deposited within
folds
D. Abomasum
a. True Stomach
--Functions same as nonruminant
b. Starts digestion of degraded concentrates
and roughages
c. Starts digestion of microbes of fermentation
4. Rumination
A. The process of bringing food material back from
the ruminant stomach
a. Occurs in a cycle
B. Four Phases of cycle
a. Regurgitation
b. Remastication
c. Reinsalivation
d. Redeglutition
C. Regurgitation
a. Breath taken with closed glottis
b. Intrapleural and mediastinal pressure
decrease
without lungs inflating
c. Esophagus relaxes and cardia opens
d. Reticulum contracts
e. Rumen contents are aspirated into esophagus
because of negative pressure
f. Reverse peristalsis carries bolus to mouth
D. Remastication
a. Liquid is squeezed out of bolus
and swallowed
b. Number of chews varies depending on diet
--100 chews for all roughage diet
E. Reinsalivation
a. Occurs simultaneous to remastication
b. Saliva swallowed 2 or 3 times before
reglutition
--Used for microbial fermentation buffering
F. Redeglutition
a. Reswallowing
b. Next cycle occurs 5 seconds after swallowing
G. Time spent Ruminating
a. Diet dependent
b. Cows on hay diet = 8 h/d
c. 14 period/day
d. Happens during times of quiescence
5. Gas Production and Eructation
A. Gases produced are products of fermentation
a. Mainly CO2 and Methane
b. Trace amounts of N2, O2, and H2
c. Produced at rate of 0.5-1 L/min
B. CO2
a. Fermentation of CHOs
b. Deamination of amino acids
c. Salivary bicarbonate from lipid fermentation
d. 60-70% of gas production
C. Methane
a. Reduction of CO2 by methane-producing
bacteria
b. 30-40% of gas production
E. Eructation
a. Process by which gas from the forestomach
is removed by way of the esophagus to the
pharynx (Burp, Belch, etc.)
b. Occurs about once a minute
--Doesn't make a sound
c. Mechanoreceptors in dorsal sac of rumen
and cardia sense gas pressure
(Controlled by reflex center in medulla)
i. Rumen contracts to move gas bubble
forward
ii. Reticulum relaxes
iii. Cardia opens
d. Much of gases eructated are inspired
by lungs
i. May provide carbon source
for metabolism
ii. Also cause of off-flavors
e. Bloat (Tympanism)
i. Eructation mechanism fails
ii. Mechanoreceptors covered with foam
(tiny bubbles)
--Don't recognize gas build-up
iii. Types: Feedlot or grain bloat
--High concentrate
diet
Legume bloat
--Lush green alfalfa
or clover
iv. Animals die if not treated from
Asphyxia
--Pressure against diaphragm
prevents thoracic
enlargement
6. Chemistry and Microbiology of the Rumen
A. Bacteria
a. 80% of microbes
B. Protozoa
a. 20% or microbes
C. Both are anaerobic
D. Produce VFAs, gases, and B-complex vitamins
--From CHOs
E. Hydrolyze Proteins and Fats
~~~~~Revised 11/7/96~~~~~ TAW