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 Common Duiker

Suni 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 LowerIncisor 0 4Canine 0 0Premolar 3 3Molar 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 CalvesSucrose, xylose SelectFructose, glucose SelectLactose, maltose IndifferentSaccharin IndifferentAcid IntolerantAlkali TolerantSalt Tolerant Mature cattleSweet Strong preferenceSour Mild preferenceSalt 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, l___

__volume__ Cattle SheepRumen 80 60-100 9-18

Reticulum 5

Omasum 5-8 6-10 1-2

Abomasum 5-8 5-8 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

• Papillae– Finger-like structures

(10mm x 2 mm) covering the rumen wall

– Particularly well-developed 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.

Structures within the reticulorumen

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 n-butyrate

– 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

• 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

•Simple nerve endings•All afferent vagal fibers•Few receptors in the sacs of the rumen

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

Diameter of particles, mm >5 3-5 2-3 1-2 <1 % Rumen 2.5 9.6 20.1 35.1 32.7 Reticulum 2.4 8.5 17.9 34.0 37.3 Omasum .1 .8 5.9 25.4 68.0 Abomasum .7 2.5 7.4 24.6 64.7• VFA absorption

– 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

Regions of the abomasum

Epithelial

Cardiac

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+ Variable, up to 124Na+ Variable, 21 to 167Cl- 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 Length, ft Cattle Sheep Digesta pH FunctionsTotal 90-150 60-110Duodenum 3-4 2-3 2.7-4 Enzymes pH change Flow reg.Jejunum 60-100 - 4-7 Enzymes AbsorptionIleum 30-50 - 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 7.2-7.8 Ionic (mEq/l) Na+ 135-165 K+ 4-5 Cl- 110-126 HCO3

- 15-30• 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 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 mEq/l Na+ 154 K+ 7 Cl- 122 HCO3

- 23– Lipid

• 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) Upper jejunum Lower Ileum Na+ 136 135 K+ 8 9 Cl- 134 105 HCO3

- 12 42– pH 7.1 8– Enzymes

• Enzymes– Lactase (in young)– Maltase– Isomaltase– Dipeptidases

• Highest activity in upper to mid-jejunum • Activity may be breed-related.

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

Rumen volume:LI volume

Roughage selectors 15-30:1

Concentrate selectors 6-10:1• Structure

– 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

• 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

Large intestine functions (Cont.)