Comparative Digestive Physiology

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Comparative Digestive Physiology
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Back in the day

• Digestion is simply breaking our food into its
  most basic components so that it can be utilized
  by the cell. (Legos)
• In a simple, one-celled paramecium, digestion is
  pretty straightforward.
• 1. A small groove on the surface of the cell
  pulls in water using cilia (tiny hairs)
• 2. food is pushed into a bubble called a
  vacuole.

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• 3. As the cell pushes the food inside, enzymes
  are secreted by the vacuole bubble which breaks
  carbohydrates into simple sugars and proteins
  into amino acids.
• 4. When all of the available nutrients have been
  absorbed, the vacuole is released.

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• Intake food is absorbed into the mouth
• Formation of vacuole (food bubble)
• The vacuole is moved through the cell
• Simple sugars, amino acids, and nutrients are
  absorbed from the vacuole
• Vacuole is expelled.

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Problem

• As our bodies became more complex with trillions
  of cells, digestion had to do the same.
• Early multi-celled organisms could no longer
  simply absorb and diffuse nutrients from their
  environment.
• If you are surrounded on all sides by cells, you
  cannot just absorb your food.
• This created the need for the Alimentary Canal, a
  long hollow tube that runs through our bodies.
• This enabled our bodies to grow much larger than
  a few cells.

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Regions of Alimentary Canal

• Foregut functions
• Ingestion and storage of feeds
• Midgut functions
• mechanical, chemical enzymatic digestion of
  feed
• nutrient absorption
• Hindgut functions
• water ion re-absorption
• formation, storage, excretion of feces

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Primary Functions of the Digestive Tract

• Transport food peristaltic contractions
• Digestion - reduce feed particles to molecules
  that can be absorbed into the blood
• mechanical breakdown by chewing
• chemical breakdown by HCl and digestive enzymes
• Absorption - allows nutrients to pass through
  membranes of GIT to the blood stream
• passive diffusion and active transport
• Synthesis - true protein, FA, starch, vitamins
• Excretion elimination of waste products
• via bile (toxins, microbes etc)
• via rectum (Ca, Mg, P)

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Primitive Gastrointestinal Tract

• Found in monotremes (egg-laying mammals),
  insectivores (bats, shrews, moles)
• Simple stomach, little or no division between
  small intestines and large intestines, large
  intestine simple, presence of caecum,
  non-sacculated colon

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Species-dependent Nutritional Adaptations

• Includes involvement of
• Teeth
• Jaws and jaw musculature
• Alimentary canal
• Stomach - May be simple or become sacculated to
  compartmentalize functions for prolonged storage
  of feed and utilization of bacterial fermentation
  (langures and ruminants)
• May also become voluminous for storage of large
  amounts of feed (vampire bats)
• Large intestine - varies substantially in length,
  compartmentalization, and complexity among species

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Mouth

• Functions
• Grasp food
• Taste
• Masticate food
• Mix with saliva
• Why would taste be necessary for digestion?
  Think, Pair, Share
• What is saliva? TPS

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Digestion in the Mouth

• Prehension
• Bringing the food to the mouth
• Upper limbs, head, beak, claws, mouth, teeth and
  lips
• Mastication or chewing
• To crush the food, increase surface area and
  allow enzymes to act on molecules
• Carnivores need only to reduce the size of the
  particle
• Herbivores must chew continuously (40-50,000
  times a day)

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Prehension

• Domestic mammals use lips, teeth and tongue
• Relative importance varies by species
• Horses
• lips when eating from manger
• teeth when grazing
• Cows and sheep have limited use of lips
• Use long rough tongue to grasp forage
• Pigs use snout to root in ground and pointed
  lower lip to convey feed into mouth
• Birds use beak and tongue
• Drinking varies as well
• Most mammals use suction
• Dogs and cats use tongue to form ladle

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Mastication

• Physical reduction of feed
• Especially important in non-ruminant herbivores
• Adaptations
• Carnivores
• Large canines and incisors, tearing but little to
  no chewing activity
• Herbivores
• Specialized molars, lots of chewing and grinding
• Edentates (sloths, armadilloes, anteater)
• Relative toothlessness

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Teeth and Mastication

• Teeth are essential for proper chewing
• Distinguishing difference between carnivores and
  herbivores
• May regulate the amount of forage an herbivore is
  able to consume
• Problems with older animals
• E.g. Usually Elephants die from starvation at old
  age, not old age itself

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Feldhammer Fig. 6.1
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Fig. 6.6
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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

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Structural Adaptations of Teeth in Mammals
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Teeth Specializations

• Carnivores
• Canine teeth highly developed and used for
  tearing
• Molars are pointed for bone crushing

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Teeth Specializations

• Omnivores
• grinding teeth patterns on posterior teeth
  (molars)
• piercing and ripping cusps on anterior teeth
  (incisors)
• Tongue - used to move feed to teeth

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Teeth Specializations

• Non-ruminant herbivores (horse)
• incisors for nipping, molars slightly angled,
  jaws move circularly (vertical and lateral)
• Ruminants
• no upper incisors, have dental pad, molars allow
  only lateral movements
• Different classes - roughage eaters, transition
  types, selective eaters all differ in tongue
  mobility and cleft palate

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Salivation

• Quantity and composition of saliva varies
  considerably between species
• Quantity related to level of chewing activity
• Amount of secretion
• Dogs minimal (lubrication, no enzymes)
• Sheep 3-10 liters/d
• Horse 10-12 liters/d
• Cattle 130-180 liters/d

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Stomach

• Monogastric
• One compartment
• Varies in size by species
• Ruminant
• 4 compartments
• Reticulum
• Rumen
• Omasum
• Abomasum

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Adaptations to Feed Sources

• Gastric capacity and structure
• Capacity is greatest in pregastric fermentors
• Stomachs act 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

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Adaptations to 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 (why? Nutrient density)
• Large intestine
• Importance of hind gut fermentation dictates
  variation in structure and size
• Some hind gut fermentation occurs in most species
  (i.e. second stomach)

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Digestive Tracts

• Stomach

Small Intestine
Cecum
Large Intestine
Rule Size Function
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Digestive Enzymes

• Young animals produce little sucrase, maltase,
  amylase
• Ruminants produce no sucrase
• Adult pigs lack lactase
• Activity changes with age
• Lactase
• Sucrase, maltase

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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

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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

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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

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Pig _________________________________________
Stomach (2 gal)
Large Intestine (16, 2 gal)
Esophagus
Mouth
Small intestine (60, 2.5 gal)
Cecum (10, 0.5 gal)
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Human Digestive Tract
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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

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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)
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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

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Cow _________________________________________
Esophagus
Large intestine (33, 8 gal)
Cecum (3, 3 gal)
Rumen (paunch) (43 gal)
Mouth
Abomasum (glandular) (5 gal)
Reticulum (honeycomb) (2.5 gal)
Small intestine (150, 16 gal)
Omasum (4 gal)
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GIT Capacity - Volume

• Carnivores
• stomach (70) gt SI LI (15)
• GIT surface/body surface 0.61
• Omnivores
• stomach SI LI (33)
• GIT surface/body surface intermediate
• Herbivores
• Ruminants
• stomach (70) gt SI (20) gt LI (10)
• GIT surface/body surface 31
• Non-ruminants
• stomach (10) lt SI (30) lt LI (60)
• GIT surface/body surface 21

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Capacity of Digestive Tracts
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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

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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

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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

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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

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Pregastric Fermenters
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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

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Postgastric Fermenters
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Summary

• If an animal eats a more nutrient dense diet,
  they will have a less developed digestive tract.
  
• Animals that eat hard-to-digest diets have well
  developed, compartmentalized digestive tracts.
• Imagine a spectrum with cows and other ruminants
  on one end and earthworms on the other. Most
  multi-celled animals fall somewhere in between
  these extremes.

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Summary

• The following factors affect what an animal can
  eat in its diet
• Teeth shape and prehensive strategies
• Saliva production
• Complexity of digestive tract (e.g. presence of
  compartments)
• Production of enzymes
• Presence and amount of anaerobic bacteria
• Length of small intestine
• Presence of a cecum

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Summary

• Examples of digestive strategies
• Carnivore e.g. dog or cat simple, short GIT.
  Few compartments. Production of enzymes for
  protein breakdown and vitamin synthesis.
• Omnivore humans and pigs slightly more complex
  and longer GIT. More post-gastric fermentation.
  Longer small intestine.
• Pregastric Fermenter e.g. cow.
  Compartmentalized stomach. Fermentative
  capability. Highly advanced GIT. Large
  capacity.
• Post-gastric Fermenter e.g. horse or rabbit.
  Little digestion until large intestines cecum.