Reticulum

Honeycomb lining No secretions

Formation of food bolus

Regurgitation initiated here

Collects hardware (nails, wire)

Rumen Digestion and

fermentation vat 40-50 gallons No secretions

Contains anaerobic microbes (25-50 billion bacteria/mL fluid)

Also protozoa, fungi Produce VFA, protein

Papillae lining Increase surface area

Absorption of VFA Passive diffusion

Omasum Laminae/manyply lining

Muscular folds No secretions

Reduces particle size Absorption of water

~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 Releases bile and pancreatic secretions Active site of digestion

Jejunum 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 CellBrush border

Specialized Cells Lining Villi

Absorptive epithelial cell Contain brush

border on lumen/apical side

Brush border: Enzymes Nutrient transport

molecules

Goblet cell Secretes mucus

Nutrients Mucus

Specialized Cells Lining Villi

Endocrine cell Secrete

hormones into bloodstream or local cells

Paneth cell Secretory

granules with anti-microbial properties

Anti-microbial compounds

CCK, Secretin, etc.

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 Enterocyt

e 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

10x

MicrovilliBrush Border

1 um projections on surface of epithelium

20x

Nutrient Absorption in the Small Intestine Principal site of absorption of amino

acids, vitamins, minerals and lipids Glucose and other sugars in monogastrics

Generally, most absorption occurs in the proximal (upper) part of the small intestine but some absorption occurs in all segments 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 Diffusion1) Carrier loads

particle on outside of cell

2) Carrier releases particle on inside of cell

3) Reverse

Allows equalization of concentrations across membrane

Active Transport1) 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

Produced in liver Stored in gallbladder Horse has no gallbladder

Direct bile secretion into duodenum Cannot store bile—continuous intake of food

Secreted from within SI

Enters from ducts into SI

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

Maltose

Lactose

Sucrose

Amylase

Maltase

Lactase

Sucrase

Saliva & pancreas

SI

SI

SI

Maltose & Glucose

Glucose

Glucose & galactose

Glucose & fructose

Primary Enzymes for Proteins

Nutrient Enzyme Origin Product

Milk protein

Proteins

Polypeptides

Peptides

Rennin

Pepsin

TrypsinChymotrypsin

Carboxypeptidase

Aminopeptidase

Gastric mucosa

Gastric mucosa

PancreasPancreas

PancreasSmall intestine

Curd

Polypeptide

PeptidesPeptides

Peptides & amino acids

Primary Enzymes for Lipids

Nutrient Enzyme Origin Product

Lipids Lipase & colipase

Pancreas Monoglycerides & free fatty

acids

Bile Green, viscous liquid

Alkaline ph (neutralize acidic chyme) Secreted by liver via bile duct to duodenum

Stored in gall bladder (except in horses) Functions to emulsify fats Composition

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 lipaseand colipase

Water soluble micelles

Pancreatic Juice Clear, watery

juice Enters

duodenum via pancreatic duct

Aids in fat, starch, and protein digestion

Contains HCO3

-

Trypsinogen Chymotrypsinogen Procarboxypeptidas

e Amylase Lipase Nuclease

Pro-enzymes

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 Functions

Duodenum 2.7 - 4 Enzymes pH change Flow rate regulationJejunum 4 – 7 Enzymes AbsorptionIleum 7 - 8 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:

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 large intestine

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

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 Water absorption

90% of water entering the LI is absorbed

Ruminant Large Intestine

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

Rule Size = Function

Stomach

Small Intestine

Cecum

Large Intestine

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 Cellulose Nonprotein nitrogen

Ability to detoxify some poisonous compounds Oxalates, cyanide, alkaloids

More effective use of fermentation end-products 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 FermentersClass Species Dietary habit

Ruminants Cattle, sheep

Deer, antelope, camel

Grazing herbivores

Selective herbivores, including folivores and frugivores

Nonruminants Colobine monkey, hamster, vole

Kangaroo, hippo

Hoatzin

Selective herbivores

Grazing and selective herbivores

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 Species Dietary habit

Cecal digesters

CapybaraRabbitRat, mouse

GrazerSelective herbivoresOmnivores

Colonic digesters Sacculated

Unsacculated

Elephant, horse, zebraNew World monkeysPig, humanPandaDog, cat

GrazersFolivoresOmnivoresHerbivoresCarnivore

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 Percent of Species

Herbivory 10 40 Frugivory, Graniv., Nectivory 5 4 Carnivory 4 12 Planktonivory? (Krill feeders) 2 <1 Insectivory 10 33 Omnivory 7 10

Classification of Animals by Preferred Ingested Feedstuffs

Carnivore – consume animal products

dogs, cats komodo dragon, tigers, eagles, sharks,

polar bear Herbivore – consume plant products

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 CarnivoresAphidivore - feeds on aphidsApivore - feeds on beesErucivore - feeds on caterpillarsInsectivore - feeds on insectsLarvivore - feeds on larvaeMyrmevore - feeds on antsMucivore - feeds on fliesPiscivore - feeds on fishPupivore - feeds on pupaeRanivore - feeds on frogsSanguivore - feeds on bloodZoosuccivore - feeds on liquid animal secretions of decaying animal matter

Specialized HerbivoresAmbivore - feeds on grasses and broad leaf plantsExudativore - feeds on gums, resins, and sapFolivore - feeds on foliage (leaves or trees)Forbivore - feeds on forbs (i.e. flowering plants)Frugivore - feeds on fruitGraminivore - feeds on grassesGranivore - feeds on grainsGumivore - feeds on gums secreted by some plantsLignivore - feeds on woodMellivore - feeds on honeyNectarivore - feeds on nectarNucivore - feeds on nuts (agouti)Phytisuccivore - feeds on (tree) sapRadicivore - 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

stomach (70%) > SI = LI (15%) GIT surface/body surface: 0.6:1

Omnivores 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_________________________________________

Large Intestine (16’, 2 gal)

Cecum (10”, 0.5 gal)

Small intestine (60’, 2.5 gal)

Stomach(2 gal)

Esophagus

Mouth

Human Digestive Tract

Rat

GIT Classifications - Kangaroo

Monogastric omnivore with limited pre-gastric fermentation Sacculated

stomach,capable of utilization of forage-based (high fiber) diets

Able to digest some of the substances in grains, fruits and vegetables

GIT Classifications - Horse Monogastric

herbivore with extensive post-gastric fermentation Simple stomach

incapable of utilization of forage-based (high fiber) diets

Extensive fermentation after primary sites of digestion and absorption

Horse_________________________________________

Large Colon(12’, 19 gal)

Cecum (4’, 8 gal)

Small intestine (70’, 12 gal)

Stomach (3.5 gal)

Esophagus

Mouth

Small Colon (12’, 3 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_________________________________________

Small intestine(150’, 16 gal)

Abomasum(glandular)

(5 gal)

Omasum (4 gal)

Cecum(3’, 3 gal)

Esophagus

Mouth

Rumen (paunch)(43 gal)

Reticulum(honeycomb)

(2.5 gal)

Large intestine (33’, 8 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

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

Water resorption Fiber fermentation by bacteria H2O soluble vitamin synthesis by bacteria

Chicken Beak

EsophagusCrop (2”)

Small Intestine (55”)

Proventriculus

Gizzard (2”)Pancreas

Ceca (7”)

Large Intestine (4”)Cloaca

Feeding Behaviors Impact feed choices

Neophobia (avoidance of new feed sources) 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 Impact feed intake (avoiding

under- and over-consumption of feed) 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