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Comparative Digestive Physiology

Comparative Digestive Physiology. 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.

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Comparative Digestive Physiology

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  1. Comparative Digestive Physiology

  2. 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.

  3. 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.

  4. 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.

  5. 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.

  6. 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

  7. 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)

  8. 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

  9. 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

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

  11. 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)

  12. 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

  13. 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

  14. 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

  15. Feldhammer Fig. 6.1

  16. Fig. 6.6

  17. 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

  18. Structural Adaptations of Teeth in Mammals

  19. Teeth Specializations • Carnivores • Canine teeth highly developed and used for tearing • Molars are pointed for bone crushing

  20. 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

  21. 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

  22. 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

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

  24. Adaptations to Feed Sources • Gastric capacity and structure • Capacity is greatest inpregastric 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

  25. 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)

  26. Digestive Tracts Stomach Small Intestine Cecum Large Intestine Rule Size = Function

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

  28. 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

  29. 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

  30. 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

  31. Pig _________________________________________ Stomach (2 gal) Large Intestine (16’, 2 gal) Esophagus Mouth Small intestine (60’, 2.5 gal) Cecum (10”, 0.5 gal)

  32. Human Digestive Tract

  33. 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

  34. 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)

  35. 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

  36. Cow _________________________________________ Large intestine (33’, 8 gal) Esophagus 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)

  37. 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

  38. Capacity of Digestive Tracts

  39. 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

  40. 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

  41. 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

  42. 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

  43. Pregastric Fermenters

  44. 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

  45. Postgastric Fermenters

  46. 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.

  47. 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

  48. 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 intestine’s cecum.

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