Reticulum

Honeycomb lining




No secretions
Formation of food
bolus
Regurgitation
initiated here
Collects hardware
(nails, wire)
Rumen

Digestion and
fermentation vat



Contains anaerobic
microbes (25-50 billion
bacteria/mL fluid)



Also protozoa, fungi
Produce VFA, protein
Papillae lining


40-50 gallons
No secretions
Increase surface area
Absorption of VFA

Passive diffusion
Omasum

Laminae/manyply lining




Reduces particle size
Absorption of water


Muscular folds
No secretions
~60% removed
Absorption of VFAs


~2/3 of VFAs entering or
10% of total produced
Prevents buffering of the
abomasum
Abomasum

True gastric stomach - four gallons in a cow


Three regions (cardiac, fundic, and pyloric)
Digestive secretions


Proteolytic enzymes and HCl
pH decreases
from 6 to 2.5




Denatures proteins
Kills bacteria
and pathogens
Dissolves minerals
Gastric digestion
Small Intestine

Composed of 3 segments (proximal to distal)

Duodenum



Jejunum


Releases bile and pancreatic secretions
Active site of digestion
Active site of nutrient absorption
Ileum


Active site of nutrient absorption
 Most water, vitamins & minerals
Some bacterial presence
 Fermentation
The pH of the small intestine increases towards 7.0 as
food moves from the duodenum to the ileum
Intestinal Epithelial Cell
Brush border
Specialized Cells Lining Villi
Nutrients
Mucus

Absorptive epithelial
cell


Contain brush border
on lumen/apical side
Brush border:



Enzymes
Nutrient transport
molecules
Goblet cell

Secretes mucus
Specialized Cells Lining Villi
Anti-microbial compounds

Endocrine cell


Paneth cell

CCK, Secretin, etc.
Secrete
hormones into
bloodstream or
local cells
Secretory
granules with
anti-microbial
properties
Small Intestine – Absorptive
Surface




Villi
Enterocyte
Brush border
Cell migration
from crypts to
tips of villus

2-3 days
Small Intestine - Structure







Lumen
Mucosa
Villi
Crypts
Lacteal
Enterocyte
Brush
border
Intestinal Wall
Villi
Mucosa
Enhanced Surface Area for
Increased Nutrient Absorption
Intestinal villi
Increased Surface Area in Small
Intestine for Absorption
Structure
Description
Increase in
surface area
Plicae circularis
Regular ridges in
small intestine
3x
Villi
Finger-like
projections on
mucosal (inner)
surface
1 um projections
on surface of
epithelium
10x
Microvilli
Brush Border
20x
Nutrient Absorption in the
Small Intestine

Principal site of absorption of amino acids,
vitamins, minerals and lipids


Generally, most absorption occurs in the
proximal (upper) part of the small intestine
but some absorption occurs in all segments


Glucose and other sugars in monogastrics
Duodenum, jejunum and ileum
Digestion and absorption within SI is rapid

Within 30 minutes of entering SI
Nutrient Absorption

Variety of mechanisms





Diffusion
Facilitated diffusion
Active transport
Pinocytosis or endocytosis
Dependent upon



Solubility of the nutrient (fat vs. water)
Concentration or electrical gradient
Size of the molecule to be absorbed
Diffusion


Water and small
lipid molecules pass
freely through
membrane
Move down
concentration
gradient to equalize
concentrations
Facilitated Diffusion
1) Carrier loads
particle on outside
of cell
2) Carrier releases
particle on inside of
cell
3) Reverse
Allows equalization of
concentrations
across membrane
Active Transport
1) Carrier loads
particle on outside
of cell
2) Carrier releases
particle on inside of
cell
3) Carrier returns to
outside to pick up
another particle
Active Transport


Unidirectional
movement
Transports nutrients
against concentration
gradient
Pinocytosis or Endocytosis



Substance contacts
cell membrane
Membrane wraps
around or engulfs
substance into sac
Sac formed
separates from the
membrane and
moves into cell
Transporters
Secretions Entering SI



Intestinal mucus
Brush border enzymes
Pancreatic juices


Produced & stored in pancreas
Bile



Secreted from
within SI
Produced in liver
Stored in gallbladder
Horse has no gallbladder


Enters
from
ducts into
SI
Direct bile secretion into duodenum
Cannot store bile—continuous intake of food
Intestinal Mucus

Secreted by glands in wall of duodenum


Brunner’s glands
Acts as lubricant and buffer to protect
duodenal wall
Primary Enzymes for Carbohydrates
Nutrient
Enzyme
Origin
Product
Starch, glycogen,
dextrin
Amylase
Saliva &
pancreas
Maltose &
Glucose
Maltose
Maltase
SI
Glucose
Lactose
Lactase
SI
Glucose &
galactose
Sucrose
Sucrase
SI
Glucose &
fructose
Primary Enzymes for Proteins
Nutrient
Enzyme
Origin
Product
Milk protein
Rennin
Gastric mucosa
Curd
Proteins
Pepsin
Gastric mucosa
Polypeptide
Trypsin
Chymotrypsin
Pancreas
Pancreas
Peptides
Peptides
Carboxypeptidase
Aminopeptidase
Pancreas
Small intestine
Peptides &
amino acids
Polypeptides
Peptides
Primary Enzymes for Lipids
Nutrient
Lipids
Enzyme
Origin
Product
Lipase & colipase
Pancreas
Monoglycerides
& free fatty acids
Bile

Green, viscous liquid


Secreted by liver via bile duct to duodenum






Stored in gall bladder (except in horses)
Functions to emulsify fats
Composition


Alkaline ph (neutralize acidic chyme)
Bile salts (glycocholic and taurocholic acids)
Bile pigments (bilirubin and biliverdin)
Cholesterol
95% reabsorbed and returned to liver
NOT AN ENZYME
Nutrient Digestion - Lipids
Large Lipid Droplet
Small
Action of bile salts
Lipid emulsion
Bile salts & pancreatic lipase
and colipase
Water soluble micelles
Pancreatic Juice



Clear, watery juice
Enters duodenum
via pancreatic duct
Aids in fat, starch,
and protein
digestion

Contains







HCO3Trypsinogen
ProChymotrypsinogen
enzymes
Procarboxypeptidase
Amylase
Lipase
Nuclease
Importance of Pancreas for
Digestion

Produces enzymes responsible for




50% of carbohydrate digestion
50% of protein digestion
90% of lipid digestion
Produces sodium bicarbonate for
neutralization of chyme in duodenum
Activation of Pancreatic Enzymes

Enterokinase



Secreted from crypts in duodenum
Trypsinogen
trypsin
Trypsin then converts:



Trypsinogen
trypsin
Chymotrypsinogen
chymotrypsin
Procarboxypeptidase
carboxypeptidase
Overview of Digestive Enzymes

Stomach



Pepsinogen
Chymosin (rennin)
Pancreas






Trypsinogen
Chymotrypsinogen
Procarboxypeptidase
Amylase
Lipase
Nuclease

Brush Border (SI)






Sucrase
Maltase
Lactase
Aminopeptidase
Dipeptidase
Enterokinase
Ruminant Small Intestine


Similar in structure and function to
monogastric
Differences are subtle but important

Limited ability to digest starches and
sugars

Little to none presented except in exceptional
circumstances (high-grain feeding)
Small Intestine
Digesta pH
Duodenum
2.7 - 4
Jejunum
4–7
Ileum
7-8

Functions
Enzymes
pH change
Flow rate regulation
Enzymes
Absorption
Absorption
Limited fermentation
Rate of pH increase through small intestine is slower than monogastrics


Better for peptic activity
May limit pancreatic protease and amylolytic activity
Pancreatic Secretions


Secretion pH is 7.2-7.8
Enzymes



Amylase
Lipase
Proteases




Trypsinogen converted to trypsin
Chymotrypsinogen converted to chymotrypsin
Procarboxypeptidase converted to carboxypeptidase
Nucleases
Activity of Pancreatic Enzymes


Concentration of enzymes in pancreatic juice
comparable to monogastrics
Activity is lower and may be affected by:




Less juice secreted/kg BW
Low digesta pH
High rate of passage
Limited activity particularly a problem for intestinal
digestion of starch escaping ruminal digestion

For ruminants fed high grain diets, less than 50% of starch
reaching small intestine is digested
Bile


Secreted with pancreatic juice in the
common bile duct of sheep
Secreted in the bile duct of cattle
Large Intestine

Composed of three segments




Cecum
Colon
Rectum
Function

Fermentative digestion




No enzyme secretion
Relies on microbes or secretions washed out of the SI
Absorption of remaining water, volatile fatty acids
(VFAs) from microbial fermentation and minerals
Digesta storage
Degree of development is species dependent
Monogastric Cecum


Located at junction of small and large
intestine
Function similar to rumen in ruminants

Microbial activity and digestion of feeds


Contains a microbial population similar to the rumen
 Cellulolytic & hemicelluloytic bacteria
Since cecum is located AFTER major site of
nutrient absorption (small intestine), then
microbial cell proteins are not available to the
animal

Fecal loss
Monogastric Large Intestine

Function:



Absorption of liquid
Mass movements move fecal matter to
anus
Usually only a few times a day

Associated with defecation
Bacteria



Cellulolytic – digest cellulose (forages)
Amylolytic – digest starches and sugars
(concentrates or grains)
Other types:

Proteolytic



Clostridium
Organic acid utilizers
Methanogens

Produce CO2, H2, formate, CH4
Ruminant Large Intestine

Fermentative digestion


Bacteria similar to rumen, but no protozoa
Digestion in colon may account for as much as:




Only important in conditions that increase the amount of
fermentative carbohydrate entering the large intestine




27% of cellulose digestion
40% of hemicellulose digestion
10% of starch digestion
Increased rate of passage of forages
High grain diets
May account for as much as 17% of total VFA absorption
VFAs are efficiently absorbed, but primarily used as energy
source for large intestinal mucosa cells
Ruminant Large Intestine

Absorption of ammonia-N


May account for as much as 30 to 40% of the net
transport of N into body fluid
Absorbed N may be used for:


Synthesis of nonessential amino acids
Recycling of N to the rumen


Regulated by:




Important on low protein diets
Increased by increasing N concentration of diet
Decreased by increasing the amount of carbohydrate
fermented in the large intestine
Mineral absorption
Water absorption

90% of water entering the LI is absorbed
Rectum

Muscular area of large intestine
used for storage of feces and
ultimately for defecation

Feces includes sloughed cells,
undigested food and microbial matter
Digestive Adaptations to
Varying Feed Sources

Gastric capacity and structure

Capacity is greatest in pregastric
fermentors



Stomach acts as reservoir
Small stomach in carnivores is related to
high nutrient density of the diet
Distribution and composition of epithelial
lining varies between species and dietary
adaptations
Digestive Adaptations to
Varying Feed Sources

Intestinal length and functions

Small intestine


Less variable among species than stomach and
hind gut, but generally shorter in carnivores
than in herbivores
Large intestine


Importance of hind gut fermentation dictates
variation in structure and size
Some hind gut fermentation occurs in most
species
Adaptations of the
Digestive Tracts
Stomach
Small Intestine
Cecum
Large Intestine
Rule Size
= Function
Adaptations of
Digestive Enzymes




Young animals produce little sucrase,
maltase, amylase
Ruminants produce no sucrase
Adult pigs lack lactase
Activity changes with age


Lactase
Sucrase, maltase
Utilizing Cellulose

Advantages




Ultra-abundant in the environment
Easily obtained – no need to “hunt” plants
Plant cell walls & fiber high in energy
Disadvantages


Indigestible by mammalian digestive enzymes
Cellulase is found only in bacteria & some
protozoans
Fermentative Digestion


All mammals have some fermentative
capacity that allows for utilization of
ingested fiber
The comparative importance of
fermentation is related to the fraction of
total digesta contained in fermentative
compartments of the gastrointestinal
(GI) tract
Advantages of Pregastric Fermentation

Make better use of alternative nutrients



Ability to detoxify some poisonous
compounds


Oxalates, cyanide, alkaloids
More effective use of fermentation endproducts


Cellulose
Nonprotein nitrogen
Volatile fatty acids, microbial protein, B vitamins
Allows wild animals to eat and run
Disadvantages of Pregastric
Fermentation

Fermentation is inefficient

Energy



Loss
% of total caloric value
Methane
5-8
Heat of fermentation
5-6
Relative efficiency is dependent on the diet NDF
Protein
 Some ammonia resulting from microbial
degradation will be absorbed and excreted
 20% of the nitrogen in microbes is in the form
of nucleic acids
Disadvantages of Pregastric
Fermentation


Ruminants are susceptible to ketosis
Ruminants are susceptible to toxins produced
by rumen microbes





Nitrates to nitrites
Urea to ammonia
Nonstructural carbohydrates to lactic acid
Tryptophan to 3-methyl indole
Isoflavonoid estrogens to estrogen coumestans
Pregastric Fermenters
Class
Ruminants
Nonruminants
Species
Dietary habit
Cattle, sheep
Grazing herbivores
Deer, antelope,
camel
Selective herbivores, including
folivores and frugivores
Colobine monkey,
hamster, vole
Selective herbivores
Kangaroo, hippo
Grazing and selective herbivores
Hoatzin
Folivore
Postgastric Fermentors

Cecal fermentors



Mainly rodents, rabbits and other small herbivores
Often associated with coprophagy
Colonic fermentors

Includes true herbivores (e.g., horse), omnivores
(e.g., pig and human), and carnivores (e.g., cat
and dog)


Horse has some expanded cecal fermentation in addition
to greatly expanded colonic fermentation
Degree of colonic sacculation is related to
importance of fiber digestion and fermentative
capacity
Postgastric Fermenters
Class
Cecal digesters
Species
Capybara
Rabbit
Rat, mouse
Colonic digesters
Sacculated
Elephant, horse, zebra
New World monkeys
Pig, human
Unsacculated
Panda
Dog, cat
Dietary habit
Grazer
Selective herbivores
Omnivores
Grazers
Folivores
Omnivores
Herbivores
Carnivore
Foregut vs. Hindgut
Fermentation

Foregut





More efficient per unit volume of food
Slower digestive process
Animal may starve with a full belly
Size restricted
Hindgut


More efficient relative to time
Faster turnover
Feeding Habits of Mammals
& Taxonomic Distribution
Feeding Habit






Number of Orders
Herbivory
Frugivory, Graniv., Nectivory
Carnivory
Planktonivory? (Krill feeders)
Insectivory
Omnivory
10
5
4
2
10
7
Percent of Species
40
4
12
<1
33
10
Classification of Animals by
Preferred Ingested Feedstuffs

Carnivore – consume animal products



Herbivore – consume plant products



dogs, cats
komodo dragon, tigers, eagles, sharks,
polar bear
cattle, sheep, goats, horses
giant panda, gorilla, elephant, ostrich,
green iguana, giraffe, American bison
Omnivore – plant and(or) animal
products


pigs
opossum, raccoon, blue jays, black bear,
human
Specialized Carnivores
Aphidivore - feeds on aphids
Apivore - feeds on bees
Erucivore - feeds on caterpillars
Insectivore - feeds on insects
Larvivore - feeds on larvae
Myrmevore - feeds on ants
Mucivore - feeds on flies
Piscivore - feeds on fish
Pupivore - feeds on pupae
Ranivore - feeds on frogs
Sanguivore - feeds on blood
Zoosuccivore - feeds on liquid
animal secretions of
decaying animal matter
Specialized Herbivores
Ambivore - feeds on grasses and broad leaf plants
Exudativore - feeds on gums, resins, and sap
Folivore - feeds on foliage (leaves or trees)
Forbivore - feeds on forbs (i.e. flowering plants)
Frugivore - feeds on fruit
Graminivore - feeds on grasses
Granivore - feeds on grains
Gumivore - feeds on gums secreted by some plants
Lignivore - feeds on wood
Mellivore - feeds on honey
Nectarivore - feeds on nectar
Nucivore - feeds on nuts (agouti)
Phytisuccivore - feeds on (tree) sap
Radicivore - feeds on roots
Classification by Type of
Digestion or Site of Digestion


Monogastrics or non-ruminants
Ruminants
OR


Pre-gastric fermentation (cranial)
Post-gastric fermentation (caudal)
Monogastric Animals


Single, simple stomach structure
Mostly carnivores and omnivores



Very simple - mink, cat and dog
Cecal digestion - horse, rabbit, elephant or
rat
Sacculated stomach - kangaroo
Ruminant Animals

Ruminant – herbivores possessing
multiple digestive tract compartments
for feed breakdown before feed reaches
the “true” stomach


True ruminants - cattle, sheep, goats
Pseudo-ruminants - camels, llamas, alpacas,
vicunas
A
GIT Capacity - Volume

Carnivores



Omnivores



stomach (70%) > SI = LI (15%)
GIT surface/body surface: 0.6:1
stomach = SI = LI (33%)
GIT surface/body surface: intermediate
Herbivores

Ruminants



stomach (70%) > SI (20%) > LI (10%)
GIT surface/body surface: 3:1
Non-ruminants


stomach (10%) < SI (30%) < LI (60%)
GIT surface/body surface: 2:1
GIT Classifications - Dog

Monogastric carnivore with
limited post-gastric
fermentation



Simple stomach, not capable
of effective utilization of
forage-based (high fiber)
diets
Unable to digest some of
the substances in grains,
fruits and vegetables
Similar to cat
GIT Classifications - Pig

Monogastric omnivore
with limited post-gastric
fermentation



Simple stomach, not
capable of effective
utilization of forage-based
(high fiber) diets
Unable to digest some of
the substances in grains,
fruits and vegetables
Similar to human
Pig
_________________________________________
Stomach
(2 gal)
Large Intestine
(16’, 2 gal)
Esophagus
Mouth
Cecum
(10”, 0.5 gal)
Small intestine
(60’, 2.5 gal)
Human
Digestive Tract
Rat
GIT Classifications - Kangaroo

Monogastric omnivore
with limited pre-gastric
fermentation


Sacculated
stomach,capable of
utilization of foragebased (high fiber) diets
Able to digest some of
the substances in
grains, fruits and
vegetables
GIT Classifications - Horse

Monogastric herbivore
with extensive postgastric fermentation


Simple stomach incapable
of utilization of foragebased (high fiber) diets
Extensive fermentation
after primary sites of
digestion and absorption
Horse
_________________________________________
Small Colon (12’, 3 gal)
Small intestine
(70’, 12 gal)
Esophagus
Large Colon
(12’, 19 gal)
Mouth
Cecum
(4’, 8 gal)
Stomach (3.5 gal)
GIT Classifications - Sheep

Ruminant herbivore with
extensive pre-gastric
fermentation



Highly developed sacculated
stomach capable of
extensive and effective
utilization of forage-based
(high fiber) diets
Extensive fermentation
before primary sites of
digestion and absorption
Similar to cattle and goats
Cow
_________________________________________
Large intestine
(33’, 8 gal)
Cecum
(3’, 3 gal)
Esophagus
Rumen (paunch)
(43 gal)
Mouth
Small intestine
(150’, 16 gal)
Abomasum
(glandular)
(5 gal)
Reticulum
(honeycomb)
(2.5 gal)
Omasum
(4 gal)
GIT Classifications

Avian is modified monogastric






Beaks replace lips and teeth
Crop (enlarged area of esophagus)
stores and softens feed prior to
entering stomach
Proventriculus – glandular stomach
Gizzard – muscular part of stomach
Branched cecum –postgastric
fermentation
Cloaca – both fecal and urinary
waste
 Uric acid rather than urea
(insoluble)
Avians (Poultry)
Mouth
No teeth, rigid tongue
Poorly developed salivary glands




Saliva contains amylase
Beak is adapted for prehension and
mastication
Avians (Poultry)
Esophagus

Enlarged area called crop



Ingesta holding and moistening
Location for breakdown of carbohydrate by
amylase
Fermentation
Proventriculus (stomach)


Release of HCl and pepsin (gastric juices)
Ingesta passes through very quickly (14
seconds)
Avians (Poultry)
Gizzard (ventriculus)

Muscular area with a hardened lining
reduces particle size



Muscular contractions every 20-30 seconds
Includes action of grit
HCl and pepsin secreted in proventriculus
Small intestine


Similar to other monogastrics
No Lacteals
Avians (Poultry)
Ceca and large intestine

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Contain two ceca instead of one as in other
monogastrics
Large intestine is very short (2-4 in) and
empties into cloaca where fecal material
will be voided via the vent
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Water resorption
Fiber fermentation by bacteria
H2O soluble vitamin synthesis by bacteria
Chicken
Beak
Esophagus
Crop (2”)
Small Intestine (55”)
Proventriculus
Gizzard (2”)
Pancreas
Ceca (7”)
Large Intestine (4”)
Cloaca
Feeding Behaviors
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Impact feed choices
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Neophobia (avoidance of new feed sources)
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Contact testing (based on taste and other sensory
information collected in mouth) prior to swallowing
Early introduction of a variety of feeds limits this problem
Chimpanzees select feeds based on easily
digestible carbohydrate content (sugars and
starches) rather than fat or protein content
Grazers and browser select early growth grasses
and plants vs. mature growth
In confinement feeding situations, grazers
consume concentrates first and then forages in
ration based on particle size (basis for creating
“total mixed rations”)
Feeding Behaviors
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Impact feed intake (avoiding
under- and over-consumption of
feed)
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Hiding feed in bear exhibit encourages
search and gather behaviors, limiting
intake and reducing the stress of
captivity
Predator behavior towards “prey”
meals vs. “bowl” meals
Grazing animals prefer to eat forage at
ground level rather than in elevated
feed bunks