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. • 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. 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.
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.
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.
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
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)
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
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
Mouth • Functions • Grasp food • Taste • Masticate food • Mix with saliva • Why would taste be necessary for digestion? Think, Pair, Share • What is saliva? TPS
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)
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
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
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
Feldhammer Fig. 6.1
Fig. 6.6
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
Structural Adaptations of Teeth in Mammals
Teeth Specializations • Carnivores • Canine teeth highly developed and used for tearing • Molars are pointed for bone crushing
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
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
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
Stomach • Monogastric • One compartment • Varies in size by species • Ruminant • 4 compartments • Reticulum • Rumen • Omasum • Abomasum
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
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)
Digestive Tracts Stomach Small Intestine Cecum Large Intestine Rule Size = Function
Digestive Enzymes • Young animals produce little sucrase, maltase, amylase • Ruminants produce no sucrase • Adult pigs lack lactase • Activity changes with age • Lactase • Sucrase, maltase
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
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 _________________________________________ Stomach (2 gal) Large Intestine (16’, 2 gal) Esophagus Mouth Small intestine (60’, 2.5 gal) Cecum (10”, 0.5 gal)
Human Digestive Tract
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 _________________________________________ 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)
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 _________________________________________ 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)
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
Capacity of Digestive Tracts
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 Fermenters
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
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.
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
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.
