Digestive Systems
Chris Ellason
Digestion and Absorption:
 The process of digestion includes:
– The prehension of food or feed
– The mechanical chewing and grinding
– Mixing with digestive acids and enzymes to
chemically break down feedstuffs
 The process of absorption includes:
– Transport of the digested foods across the
intestinal mucosa to the blood or lymph system
General Terms
 Prehension: to take into the mouth
 Mastication: chewing
 Deglutition: swallowing
 Regurgitation: the backward flow of food
through the esophagus
Three Major Categories
 Carnivore: consume flesh of other animals,
examples are dogs and cats
 Omnivore: consume both plants and flesh,
examples are primates
 Herbivore: consume plant material,
examples are horses and cattle
Carnivore
 Very Well developed stomach
 Uncomplicated intestine
 Limited fiber digestion
Omnivore
 Combination of carnivores/herbivores
 More complicated GIT than carnivores
 Colonic digesters
– Pigs, humans
 Cecal digesters
– rat
Herbivore
 Cow, horse, rabbit
– Each has a different type of GIT
 Cow- ruminant
 Horse- simple stomach, large cecum, large
sacculated LI
 Rabbit- larger stomach, very large
sacculated cecum, unsacculated LI
Salivary Glands
 3 pair
– Parotid
– Mandibular
– Sublingual
 Water - moistens food aids in swallowing
 Mucin - lubrication for swallowing
 Bicarbonate salts
 Enzymes
Salivary Glands
Salivary glands
Different Digestive Tracts
 Farm animals have a variety of digestive
systems
– Ruminants: have 4 different compartments to
the stomach
• Examples include cattle, sheep, goats
– Nonruminants (also known as monogastrics)
• Hogs, dogs, and cats have a single, simple stomach
• Poultry have a two part stomach
• Horses have a large, functional cecum
Pregastric vs Postgastric
 Pregastric: Fermentation that occurs in the
rumen of ruminant animals. It occurs before
food passes into the portion of the digestive
tract in which digestion actually occurs.
 Postgastric: The fermentation of feed occurs
in the cecum, behind the area where
digestion has occurred.
Pregastric vs Postgastric
 Ruminants
– More efficient
– Less intake
 Non- ruminant herbivores
– Only postgastric
– Less efficient
– Greater intake
Monogastric Systems
 Mouth: prehension and chewing of food;
some carbohydrate enzyme activity
 Esophagus
 Stomach
– Storage
– Muscular movements (break down food)
– Secretes Digestive Juices (hydrochloric acid)
• pH about 2
Monogastric Systems
Small Intestine
 Duodenum
– Active Digestion Site
– Produce enzymes
• Pancreas
– Helps to neutralize ingesta entering the SI
• Liver
– Produces bile; breaks down fats
• Intestinal Walls
Small Intestine
 Jejunum
– Active in nutrient absorption
 Ileum
– Active in nutrient absorption
 Villi
 pH 6 to 7
Large Intestine
 3 Sections
– cecum
– colon
– rectum
 Active in water resorption
 Secretion of some minerals
 *Bacterial Fermentation*
Horses are Different
 Saliva
– contains no enzymes
– may secrete up to 10 gallons/day
– stimulated by scratching
 Esophagus
– only one way peristaltic movement
• Impossible for regurgitation
Horses are Different
 Stomach
– much smaller in comparison to other species
– not very extensive muscular contraction
– So how should we feed differently?
 Small Intestine
– same as pig but no gall bladder
• Can’t handle a high fat diet
Horses are Different
 Large Intestine
– over 60% of GIT
– 4 parts
•
•
•
•
cecum
large colon
small colon
rectum
Cecum and Large Colon
 Similar to Rumen
– bacterial cellulose breakdown
– bacterial protein breakdown
– VFA production
– Water Soluble Vitamin production
Small Colon and Rectum
 Primary site for water resorption
 Can become impacted with feed
Horse GI Tract
Avian Species
 Beak
– no teeth
– can be used to reduce particle size
 Esophagus
– ingesta holding and moistening
– Salivary Amylase
– Fermentation in some species
Avian Species
 Proventriculus
– Gastric juice production
– pH 4
– Rapid pass through of food
Avian Species
 Gizzard (ventriculus)
– thick muscular wall
– particle size reduction (similar to mastication)
– nonglandular
– normally contains grit
– no enzymatic secretion
Avian Small Intestine
 Functions in digestion and absorption of
feed and nutrients just as in other
monogastrics
 pH is slightly acidic
 Most enzymes found in mammals except?
Avian Large Intestine
 Contains 2 blind pouches instead of ?
 Mostly water absorption
 Some bacterial activity but less than in most
mammals
 very short in comparison
Avian Species
Ruminant Digestive System
 Mouth
– what is unique about the teeth?
– Can only chew on one side of mouth at a time
 Saliva production is continuous
 Production about 12 gallons/d
Rumen Stomach
 4 PARTS
– reticulum
– rumen
– omasum
– abomasum
Reticulum
 Honeycomb
 most cranial
 not truly separated from rumen
 no enzymatic secretion
 walls are tough, tend to catch heavy objects
Rumen
 Large compartment extends from
diaphragm to pelvis
 papillae
 Fermentation chamber
 Majority of absorption of byproducts and
conversion to volatile fatty acids
Rumen Digestion
 Reticulorumen provides a favorable
environment for bacterial fermentation
 Continuous turnover of digesta and removal
of fermented digesta
 Anaerobic fermentation
Omasum
 Manyplies (Stockmans Bible)
 short blunt papillae
 very muscular
 no enzymatic secretion
 reduction of particle size
 water resorption
Abomasum
 True Stomach
 First Glandular portion of the tract
 Very similar in structure and function to
nonruminant stomach
Other Unique Points
 Esophageal Groove
– cardia to omasum
– milk bypass
 Rumination: The process where rumen
contents are regurgitated, remasticated, and
reswallowed for further digestion
 Eructation: expulsion of accumulated
fermentation gases from rumen via esophagus
Ruminant GI Tract
Rumination
 Regurgitation of ingesta with mastication
– reticular contraction that concentrates ingesta at
the cardia
– increased inhalation of air at same time
– contraction of diaphragm
– ballooning of esophageal walls
– ingesta sucked into esophagus
– returned to mouth by reverse peristalsis
Rumination
 Excess liquids are swallowed
 Mastication commences
 More time spent masticating here than
initial intake
 Amount of time ruminating is a function of
diet composition
Rumen Contents and Motility
 Rumen contents are not uniform
 Occur in stratified layers
 Change from ventral to dorsal
 Rumen mat
– high concentrate diets eliminate mat
– more viscous fluid in high grain diets
– lowest dry matter in high forage diets
Rumen Contents and Motility
 Motility of rumen mixes contents
 Divided into
– primary contractions
– secondary contractions
 Contractions require up to 50 seconds to
complete
Rumen Contents and Motility
 Contractions begin with reticulum
 Progress dorsally
 Finish with ventral blind sac and ventral
pillars
Rumen and Acute Acidosis
 Optimal rumen pH is 6.7
 Variation in pH is normally +/- 0.5
 Introduction of high grains result in
– breakdown of rumen mat
– proliferation of facultative anaerobes
– these produce high levels of lactic acid
Rumen and Acute Acidosis
 Lactate is a much stronger acid that other
VFAs
 In severe cases lactate can make-up well
over 50% of total rumen acids
 Succinate and Formate can also appear in
high quantities
Rumen and Acute Acidosis
 Increased acids can reduce pH as far as 4.0
 Severe rumenitis occurs at these pH levels
 Absorption of lactic acid results in systemic
acidosis
 All chronic acidosis results in rumen
parakeratosis
Factors Affecting Digestibility
 Rate of passage: increased rate of passage
of digesta through the tract reduces
digestibility
– Factors increasing rate of passage include:
• Increased level of feeding/intake (ruminants)
• Finer processing (such as grinding) of feed
Note: grinding grain usually increases
digestibility but grinding hay decreases
digestibility