Comparative Digestive Physiology

Why Do Animals Digest? Food not ingested in suitable state Physical nature of food determined by: gathering apparatus for uptake type of digestive system

Primary Functions of the Digestive Tract Transport food – peristaltic contractions Digestion Mechanical breakdown Chemical breakdown Absorption 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)

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

Associated Structures Pancreas Liver Gallbladder Salivary glands Contribute to small intestinal digestion

Primitive Gastrointestinal Tract Found in monotremes (egg-laying mammals), insectivores (bats, shrews, moles), and dermopterans (colugos) Simple stomach, little or no division between small intestines and large intestines, large intestine simple, presence of cecum, 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 (langurs 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

Ruminants 2.8 billion domesticated ruminants Cattle, sheep, deer, elk, bison Pregastric fermentation Ability to chew cud at frequent intervals distinguishes true ruminant from other foregut fermenters Kangaroo, colobine monkey are not true ruminants Four compartment stomach Reticulum Rumen Omasum Abomasum

Ruminants vary in size and habitat

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

Intermediate Feeding Species Properties Seasonally adaptive Feeding preference Prefer browsing Moose, goats, elands Prefer grazing Sheep, impalas

Roughage Grazing Species Properties Most recently evolved Larger rumens and longer 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

Structures in Mouth Lips Teeth Tongue Salivary glands

Mouth Functions Grasp food Taste Masticate food Mix with saliva

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 only to reduce the size of the particle to a size small enough to swallow Herbivores must chew continuously (40- 50,000 times a day) to increase surface area

Prehension Seizing and conveying feed to mouth Mechanisms vary with behavior and diet Forelimbs Primates, raccoon Snout Elephant, tapir Tongue Anteater, cow, sheep Lips Horse, sheep, rhinoceros

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

The Importance of Prehension in Diet Formulation White Rhino (“wijd” = wide) Squared off upper lip used to “crop” grass Grazes on savannah Black Rhino Prehensile upper lip for browsing Consumes bushes and shrubs in forest

Mastication Physical reduction of feed Especially important in non-ruminant herbivores Adaptations with teeth Carnivores Herbivores Edentates (sloths, armadilloes, anteater) Relative toothlessness

Morphological Adaptations for Herbivory  All related to finding, ingesting, masticating, and digesting plant cell walls  Dental adaptations for herbivory include changes to incisors, molar occlusal surfaces, & masseter  Solution for digestive problems is to provide a place in digestive tract for anaerobic bacteria & protozoans (microflora) to colonize

Monogastric Teeth Function: Mechanically reduce particle size Increase surface area Four types: Incisors are used for cutting Canine (fangs, eye teeth, tusks) are tearing teeth Premolars and molars (cheek teeth) grind the food

Ruminant Mouth - Teeth Function: Reduce particle size Anatomy: Upper dental pad Lower incisors Premolars Molars

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

Jaw & 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

Ruminant Mouth Lips range from short, relatively immobile in nonselective grazing species to very mobile (prehensile) in selective grazing or concentrate selecting species Chew in a lateral (grinding) motion on one side of mouth at a time Needed to increase surface area of feed particles Feed chewed primarily during rumination in grazing species

Jaw Muscles and Mastication Temporalis muscle - develops maximum force on anterior portion of jaw (largest muscle in carnivores and smallest muscle in herbivores) Masseter and medial pterogoid - maximum force for crushing and grinding Lateral pterogoid - allows lateral movement which is important for grinding (highly important in herbivores, but carnivores and many omnivores have almost no lateral movement of jaws)

Monogastric Tongue Function: Comprised of three muscles Maneuvers food in the mouth Moves feed to teeth for grinding and to the back of the mouth for swallowing Can distinguish between feed and toxins by papillae or taste buds

Ruminant Mouth - Tongue Drinking, chewing and forming boluses Prehension of feed Covered with rough, hook-like papillae that assist in grasping feed Important in nonselective grazing species Taste buds More numerous than monogastric species More numerous on nonselective grazing species Believed that taste is primarily used for food avoidance by grazing species while concentrate selecting species select on the basis of smell

Monogastric Salivary Glands Types of Glands: Parotid Sublingual Mandibular Zygomatic

Functions of Saliva Moisten feed (salt and water) Lubrication (aids swallowing) Starch and(or) lipid digestion (amylase and(or) lipase)

Salivary Glands GlandType of secretionMain constituents ParotidSerousWater, enzymes, ions SubmaxillaryMucous or mixedMucin (mucous), mucin plus enzymes (mixed), water SublingualMucous or mixedMucin (mucous), mucin plus enzymes (mixed), water

Monogastric Salivary Glands Flow rate affected by: Parasympathetic nervous system Increased tone = Increased flow Increased flow = Increased dilution Sympathetic nervous system Increased tone = Decreased flow Decreased flow = Increased concentration

Ruminant Mouth - Saliva From at least three paired glands Submaxillary, sublingual, parotid (50% of secretions) Aids in mastication, swallowing, forming bolus No digestive enzymes in the saliva of mature ruminants Provides N, P, S and Na for rumen microoganisms Buffering compounds to maintain rumen pH and mucin to prevent bloat

Salivation Quantity and composition of saliva varies considerably between species Quantity related to level of chewing activity Amount of secretion Dogsminimal (lubrication, no enzymes) Sheep3-10 liters/d Horse10-12 liters/d Cattle liters/d

Deglutition (Swallowing) Reflex initiated by presence of food in pharnyx Propulsion of food to stomach by esophageal peristalsis

Monogastric Esophagus Horse/Pig: Striated muscles for first 2/3 Smooth muscles for last 1/3 In horse, esophagus joins stomach at an oblique angle and cardiac sphincter (the valve between the stomach and esophagus) only allows one-way flow MOST horses cannot belch out gas or vomit Dog: Striated muscles throughout allow GREAT control of digesta movement both directions

Ruminant Esophagus Involved in rumination Different from monogastric esophagus Striated muscle along the entire length Provides greater strength Allows some voluntary control Funnel shaped Contains three sphincters active in rumination and eructation

Esophagus Species adaptations Ability to control peristaltic contractions ◆ Reverse peristalsis ◆ Amount and location of skeletal muscle ◆ Regurgitation vs. vomiting

Foregut in Birds Crop Bottom of the esophagus forms a sac called crop ◆ Stores undigested food. ◆ Birds with crop gorge when food is available, store it in crop, and slowly digest it later

Stomach Monogastric One compartment Varies in size by species Ruminant Four compartments Reticulum Rumen Omasum Abomasum

Gastric Digestion Functions Reservoir for controlled release of digesta to small intestine Horse has small capacity – requires increased number of smaller sized meals Mixing food Mechanical breakdown of feed Hydrolytic digestion by acid and enzymes Mainly protein Kill bacteria Secrete intrinsic factor: needed for vitamin B 12 absorption Hormone production

Stomach Regions Esophageal Non-glandular Cardiac Secretes mucus Fundic Parietal cells Chief cells Pyloric Mucus

Gastric Pits Formed by numerous folds in the epithelium Glands empty into the gastric pit Many types of glands may empty into one gastric pit

Gastric Glands GlandType of secretion Main constituents CardiaMucousMucin Pylorus or AntrumMucousMucin Fundus Chief cells Parietal cells Enzyme Acid acid Pepsinogen Pepsin HCl, intrinsic factor

Stomach Secretions HCl Decreases pH (~2-3) Denatures protein Kills bacteria Activates pepsinogen Mucus  Protects lining from acid and enzymes  No “autodigestion”  Lubricant Pepsinogen Activated form is pepsin Hydrolyzes protein Rennin (abomasum) Clots milk Lipase Some species

Gastric Motility and Emptying Motility aids mixing, mechanical and hydrolytic reduction of feed to chyme acid pulp Emptying is stimulated by distension of antral wall and presence of liquid chyme

Control of Gastric Secretions and Gastric Motility Cephalic phase Gastric phase Intestinal phase

Cephalic Phase Vagal reflex Parasympathetic innervation Increases gastric motility, enzyme secretion Small increase in HCl secretion

Gastric Phase Local reflex, depends on presence of feed in stomach Mainly mediated by gastrin Increases HCl secretion

Intestinal Phase Stimulated by duodenal distension, pH, osmolarity, nutrients (fat) Cholecystokinin (CCK) is released by the small intestine Decreases HCl secretion and gastric motility

Gastrointestinal Hormones Gastrin Origin: Stomach, Abomasum Stimulus: Food in stomach Function: Stimulates HCl & pepsinogen secretion, increases stomach motility Secretin Origin: Duodenum Stimulus: Acid Function: Stimulates pancreatic secretions. Slows stomach motility and acid production

Gastrointestinal Hormones Cholecystokinin (CCK) Origin: Duodenum Stimulus: Fat & protein in duodenum Function: Stimulates bile and pancreatic secretions Also regulates appetite and feed intake Gastric Inhibitory Protein (GIP) Origin: Duodenum Stimulus: Fats and bile Function: Inhibit stomach motility and secretion of acid and enzymes

Ruminant Stomach Anatomy: Reticulum Rumen Omasum Abomasum

Reticulo- rumen Although structurally they appear as a single continuous compartment, functionally they are distinctly different

Reticulum Honeycomb lining No secretions Formation of food bolus Regurgitation initiated here Collects hardware (nails, wire)