Anatomy of the Ruminant Digestive
Tract
Taxonomy of Ruminants
• Subclass – Ungulata
• Order – Artiodactyla
• Suborders
– Ruminantia
• Families
– Tragulidae
» Chevrotain, mouse deer
– Giraffidae
» Giraffes
– Cervidae
» Deer, moose
– Bovidae
» Largest family (120 species)
» Pronghorn, african antelope, bison, buffalo, cattle, goats,
sheep
– Tylopoda
• Family
– Camelidae
» Camels, Llamas
Domesticated ruminant species
• 3.5 billion animals
– 95% are cattle, sheep, or goats
• Major domesticated ruminant species
–
–
–
–
–
–
Cattle
Sheep
Goats
Buffalo
Reindeer
Yak
• 75 million wild ruminants
Classification of ruminants by feeding
preference
• Classes of ruminants
– Concentrate selectors
– Intermediate feeders
– Roughage grazers
Concentrate selecting species
• Properties
–
–
–
–
–
Evolved early
Small rumens
Poorly developed omasums
Large livers
Limited ability to digest fiber
• Classes
– Fruit and forage selectors
• Very selective feeders
• Duikers, sunis
– Tree and shrub browsers
• Eat highly lignified plant tissues to extract cell solubles
• Deer, giraffes, kudus
Chevrotain
Suni
Common Duiker
Kudu
Intermediate feeding species
• Properties
– Seasonally adaptive
• Feeding preference
– Prefer browsing
• Moose, goats, elands
– Prefer grazing
• Sheep, impalas
Roughage grazing species
• Properties
– Late evolved
– Generally larger rumens
• Slows retention times
– Less selective
– Digests fermentable cell wall carbohydrates
• Classes
– Fresh grass grazers
• Buffalo, cattle, gnus
– Roughage grazers
• Hartebeests, topis
– Dry region grazers
• Camels, antelope, oryxes
Advantages of pregastric fermentation
• Make better use of alternative nutrients
– Cellulose
– Nonprotein nitrogen
• Ability to detoxify some poisonous compounds
– Oxalates, cyanide, alkaloids
• More effective use of fermentation end-products
including:
– Volatile fatty acids, microbial protein, B vitamins
• Decrease in handling undigested residues
• In wild animals, it allows animals to eat and run
Disadvantages of pregastric fermentation
• Fermentation is inefficient
– Energy
• Loss
Amount (% of total caloric value)
Methane
5-8
Heat of fermentation
5-6
• Relative efficiency is dependent on the diet NDF.
– Protein
• 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
• Ruminants are susceptible to ketosis
• Ruminants are susceptible to toxins produced by rumen
microbes
– Nitrates
Nitrites
– Urea
Ammonia
– Nonstructural carbohydrates
Lactic acid
– Tryptophan
3-methyl indole
– Isoflavonoid estrogens
estrogen
Coumestans
The ruminant digestive tract
• Lips
– Range from short, relatively immobile in
nonselective grazing species to very mobile
(prehensile) in selective grazing or concentrate
selecting species
Ruminant teeth
• Dental formula for cattle and
sheep
Jaw
Upper Lower
Incisor
0
4
Canine
0
0
Premolar
3
3
Molar
3
3
• Outstanding characteristic
– Upper jaw is wider than lower jaw
• Ruminants animals chew in a
lateral(grinding) motion on
one side of mouth at a time
• Needed to increase surface
area of feed particles
• Chewing primarily done
during rumination in grazing
species
• Results in premolar and molar
teeth becoming concave and
beveled
• Upper jaw fits lower jaw.
Tongue
• Structure
– Muscle covered with a mucus membrane
– Shape varies from being short and piston-like to long
and slender
• Uses
– Aid in chewing and forming boluses
– Aid in drinking
– Prehension of feed
• Covered with rough, hook-like papillae that assist in
grasping feed
• Important in nonselective grazing species
– Taste
• Taste buds located on:
– Fungiform papillae distributed across tongue
– Vallate papillae found on the back of the tongue
• More numerous than monogastric species
• More numerous on nonselective grazing species
Taste in ruminants
Calves
Sucrose, xylose
Select
Fructose, glucose
Select
Lactose, maltose
Indifferent
Saccharin
Indifferent
Acid
Intolerant
Alkali
Tolerant
Salt
Tolerant
Mature cattle
Sweet
Strong preference
Sour
Mild preference
Salt
Preference (?)
Bitter (alkaloids)
Strong rejection
• Believed that taste is primarily used for food avoidance by
grazing species while concentrate selecting species select
on the basis of smell.
Pharynx
• Structure similar to
monogastrics
• Involved in
rumination and
eructation
Esophagus
• Involved in rumination
• Differences from monogastric esophagus
– Circular and longitudinal muscles are striated muscle
along the entire length
• Provides greater strength
• Allows some voluntary control
– Funnel shaped
– Positioned between lungs
• Mucous membrane is comprised of stratified
squamous epithelial tissue
• Contains 3 sphincters
– Pharyngo-esophageal
– Diaphragmatic
– Cardiac
• Slit-like
• 1” long
• Sphincters active in rumination and eructation
Regions of the ruminant stomach
% of mature
__volume__
Rumen
Reticulum
Omasum
Abomasum
80
5
5-8
5-8
___Volume, l___
Cattle
Sheep
60-100
9-18
6-10
5-8
1-2
2
• Full capacity of the reticulorumen is only used in
animals fed low quality roughages. Only 60 to 70%
of the total capacity is used in animals fed high
quality roughages.
The ruminant stomach
Structures within the reticulorumen
• Folds
– Areas of tissue dividing the
reticulum and rumen into
different compartments
– Functions
• Mixing and particle sorting
• Prevent fluid from reaching
the cardia during eructation
• Pillars
– Highly muscled areas of the
rumen that form grooves on
the outside of the rumen
– Contains blood vessels,
lymph, and nerves
– Functions
• Contractions
Structures within the reticulorumen
• Papillae
– Finger-like structures
(10mm x 2 mm) covering
the rumen wall
– Particularly welldeveloped in ventral
portion of the rumen
– Few present on pillars in
roughage-selecting
species, but more evenly
distributed across rumen
in concentrate-selecting
species
– Function
• Increase absorptive
surface for VFAs.
Structure of the rumen wall
• Non-secretory stratified epithelium
covers was of reticulum, rumen, and
omasum
• Layers
– Stratum basale
• Columnar epithelium with .5 um between
cells
• Hemidesmosomes connect to basement
membrane
• Nuclei
• Numerous mitochondria
– Stratum spinosum
• Oval-shaped
• Desmosomes connect cells with
tonofibrils for nutrient transport
• Nuclei
• Fewer mitochondria
– Stratum granulosum
• Flat cells
• Connected by tight junctions
• Nuclei and mitochondria degenerate
– Stratum corneum
• No nuclei or mitochondria
• Tough layer of horny
mucopolysaccharide
• .1 um between cells
• Sloughs into digesta
Parakeratosis (Incomplete keratinization of
the rumen wall)
• Occurs in ruminants fed high grain diets for relatively long
periods
• Symptoms
–
–
–
–
–
Thickening of the horny layer (Hyperkeratosis)
Incomplete keratinization
Absence of the stratum granulosum
Retained nuclei in keratinized cells
Particularly bad on papillae
• Papillae shorten and thicken
• Increased number of branched papillae
• Debris collects in interpapillary spaces causing lesions
– Bacteria (Sphaeromonas necrophorus and Cornybacterium
pyrogenes) enter blood stream and form liver abscesses
• Cause
– High concentrations on VFAs, particularly butyrate
Effects of butyrate concentration on
structure of rumen epithelium
• Low concentrations
– Metabolized to ketones
in the epithelium
causing anoxia
• Stimulates blood flow
• May be a detoxifying
mechanism against nbutyrate
– Increases insulin and
IGF-1 concentrations
• Stimulate cell
proliferation
– Increases normal
epithelial growth
• High concentrations
– Levels overcome
capacity for
metabolism to ketones
– N-butyrate inhibits DNA
synthesis
• Mitosis inhibited
– Increases insulin and
IGF-1 concentrations
• Stimulate cell
proliferation
– Incomplete maturation
of epithelium
Blood circulation to and from the ruminant
stomach
• Supply
– Abdominal aorta
– Celiac-mesenteric trunk
– Arteries
•
•
•
•
Common hepatic
Right ruminal
Left ruminal
Left gastric
• Drainage
– Veins
•
•
•
•
Reticular
Right ruminal
Left ruminal
Omasal-abomasal
– Portal vein
– Liver
Factors increasing blood flow to the rumen
•
•
•
•
Carbon dioxide in epithelium
Volatile fatty acids
Feeding
Sight or smell of feed (slight)
Innervation of the rumen stomach
• Vagus nerve
– Involved in both sensory and motor pathways
– Controls contractions of reticulum and rumen,
rumination, and reflex of the reticular groove
• Sphlanchic nerve
– Minor role in contractions
• Intrinsic nerve
– A rich ganglionic plexus is present in the subepithelial
region of all compartments of the stomach ranging in
cells 18 to 40 um in diameter
– Small cells innervate reticular groove, reticulum and
pillars of the rumen
– Larger cells innervate abomasum
– Functions
• Vasodilatory
• Some control on muscle contractions
Sensory receptors of the reticulorumen
•Simple nerve endings
•All afferent vagal fibers
•Few receptors in the sacs of the rumen
• Tension receptors
– Most common
– Found in muscle layers
in reticulum,
reticuloruminal fold,
cranial sac, and lips of
reticular groove
– Excited by low
thresholds of
distension or
contractions
– Slow acting response
– Action
• Increase contractions
and salivation
• Epithelial receptors
– Found near basement
membrane in reticulum,
cranial sac, and
longitudinal pillars
– Stimuli
• High threshold distension
• Liquid movement
• Volatile fatty acids
– Rapid acting
(mechanoreceptors) or
Slow acting
(chemoreceptors)
– Action
• Inhibit contractions
Omasum
• Lies to right of rumen
• Entrance is the reticulo-omasal orifice
– 1 inch slit in cattle
– Lined with small papillae
– Controls particle of digesta leaving reticulorumen
• Round organ containing 100-150 flat parallel
sheets (laminae) covered with conical papilae
– Because of sheets, omasum has 1/3 of the surface area
of the stomach in cattle and 10% of the stomach surface
area in sheep
• Sulcus omasi
– Direct route for digesta from the reticulo-omasal orifice
and omaso-abomasal opening
• Fills by reticular contractions and aspiration
Functions of omasum
• Filter large particles
Rumen
Reticulum
Omasum
Abomasum
• VFA absorption
Diameter of particles, mm
>5
3-5
2-3
1-2
%
2.5
9.6
20.1
35.1
2.4
8.5
17.9
34.0
.1
.8
5.9
25.4
.7
2.5
7.4
24.6
– 43 to 77% of VFAs entering or 10% of total produced
• 85% of VFAs produced are absorbed before abomasum
• Exchanged for Cl-
– Improves efficiency of absorption
– Prevents buffering of the abomasum
• VFAs are strong buffers with a pK of 4.6
• Abomasum functions at pH 2
• Water absorption
– 30 to 60% of water entering
• Magnesium absorption
<1
32.7
37.3
68.0
64.7
Regions of the abomasum
Cardiac
Epithelial
Fundic
Pyloric
Lining of the different regions of the
abomasum
• Fundic region
– Contains gastric glands lined with secretory
columnar epithelium
• Chief cells
– Secrete pepsinogen
» Continuous
• Parietal cells
– Secrete hydrochloric acid
» Variable
• Pyloric region
– Stratified squamous epithelium
– Secrete mucus
Surface of the abomasum
• Arranged in 10 to 17 folds
– Functions
• Increase surface area
– Increase approximately 7 times
– Allows greater acid and pepsin secretion
• Prevents stratification of feeds
– Allows for mixing of digesta and enzymes
• Regulate flow
Secretion of the fundic region
• Composition
– pH 1.05 to 1.32
– Ionic (mEq/l)
H+
Na+
Cl-
Variable, up to 124
Variable, 21 to 167
138 to 172
– Enzymes
• Pepsinogen
– Converted to pepsin by HCl
• Lysozyme
– Lyses bacteria
• Secretion
–
–
–
–
30 to 35 l/day in cattle
Amount of HCL secreted to keep pH < 3
Pepsinogen secretion is constant
Factors that increase fundic secretion
• Presence of digesta
– Factors that stimulate rumen motility stimulate gastric secretion
– Peak secretion of occurs 45 to 90 minutes after feeding
• Presence of volatile fatty acids or lactic acid
– Acts through the hormone, gastrin, secreted by the fundic and pyloric mucosa
– Inhibition of secretion
• Abomasal pH < 2
• Distension or presence of HCL in duodenum
– Acts through the hormone, secretin
Motility and digesta flow in the abomasum
• Filled by contractions of the reticulorumen
• Empties by contractions confined to the pyloric region
• Flow from the abomasum
– Greatest prior to and during feeding
– Lowest flow occurs immediately after feeding
• Ingesta remains in abomasum for 1 to 2 hours
– As much as 10% backflows into abomasum
• Factors affecting abomasal motility
– Isotonic HCO3- in duodenum increases flow, while hypertonic
HCO3- in duodenum decreases flow
– Duodenal distension decreases flow
– Long chain fatty acids inhibit abomasal motility
• Acts through duodenal hormone, cholecystokinin
– Lactic acid in duodenum inhibits ruminal motility and,
therefore, abomasal filling
• Acts through duodenal hormone, secretin
– Increased dietary fiber decrease abomasal flow
– Hyperglycemia decreases abomasal flow
• May contribute to left abomasal displacement
Regions of the small intestine
Total
Duodenum
Length, ft
Cattle Sheep
90-150 60-110
3-4
2-3
Jejunum
60-100
-
Ileum
30-50
-
Digesta pH
Functions
2.7-4
Enzymes
pH change
Flow reg.
4-7
Enzymes
Absorption
7-8
Absorption
Variable Fermentation
• Rate of pH increase slower than monogastrics
– Better for peptic activity
– May limit pancreatic protease and amylolytic activity
Pancreatic secretion
• Secreted with bile in the common bile duct of sheep
– 1 ft from pylorus
• Secreted in the pancreatic duct of cattle
– 3 ft from pylorus
• Amounts secreted
– 2.2 – 4.8 l/day (Cattle)
– .3 to .4 l/day (Sheep)
• Composition
pH
Ionic (mEq/l)
Na+
K+
ClHCO3• Enzymes
7.2-7.8
135-165
4-5
110-126
15-30
– 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 may be limited 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.
– On high grain diets, intestinal starch digestion may be
as low as 48% (1kg).
Factors affecting pancreatic secretion
• H+ in duodenal digesta (HCL or Lactic acid)
– Increases volume and HCO3- concentration
– No effect on enzyme secretion
– Acts through duodenal hormone, secretin
• Fat in duodenal digesta
–
–
–
–
Slight increase in volume
Large increase in enzyme activity
No effect on HCO3- concentration
Acts through duodenal hormone, cholecystokinin
Bile
• Secreted with pancreatic juice in the common bile duct of
sheep
• Secreted in the bile duct of cattle about 2 ft from pylorus
• Amount
– 500-1500 ml/day in sheep
• Composition
– Ionic
Na+
K+
ClHCO3– Lipid
mEq/l
154
7
122
23
• Bile salts (Sodium taurocholate)
• Phospholipids (Phosphotidylcholine)
• Factors increasing secretion
– Bile salts in the duodenum
– HCl in the duodenum
• Through secretin
– Fat in the duodentum
• Through cholecystokinin
Small intestinal secretions
• Duodenum
– Secretion from the Brunner’s glands
– pH 6.7
– Contains small amounts of amylase and ribonuclease
• Jejunum and Ileum
– Composition
– Ionic (mEq/l)
Na+
K+
ClHCO3– pH
– Enzymes
Upper jejunum
136
8
134
12
7.1
• Enzymes
–
–
–
–
Lactase (in young)
Maltase
Isomaltase
Dipeptidases
• Highest activity in upper to mid-jejunum
• Activity may be breed-related.
Lower Ileum
135
9
105
42
8
Digesta flow in the small intestine
• Controlled by:
– Duodenal hormones
– Enteric nervous system in sub-mucosal and muscle
layers
• Flow by peristaltic contractions
– Go both directions
• May result in more rapid passage of large particles than
small particles and liquids
Large intestine structure
• Size
Roughage selectors
Concentrate selectors
• Structure
Rumen volume:LI volume
15-30:1
6-10:1
– Cecum
• Blind sac
– 25-25 cm long x 5-7 cm diameter (Sheep)
– Colon and Rectum
• Large tube with longitudinal folds
– No villi
Large intestine functions
• Fermentative digestion
– Bacteria similar to rumen, but no protozoa
– LI digestion may account for as much as:
• 27% of cellulose digestion
• 40% of hemicellulose digestion
• 10% of starch digestion
– Only important in conditions that increase the amount of
fermentative carbohydrate entering the LI
• 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 mucosa cells
Large intestine functions (Cont.)
• 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
– Important on low protein diets
– Regulated by:
• Increased by increasing N concentration of diet
• Decreased by increasing the amount of carbohydrate
fermented in the large intestine.
• Mineral absorption
– Na, K, Ca, P, Co, Mn, Mg, Cu, Zn, Cl
• Water absorption
– 90% of water entering the LI