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 Digestion  – peristaltic contractions Mechanical breakdown  Chemical breakdown Absorption  Passive diffusion and active transport Synthesis Excretion  - true protein, FA, starch, vitamins – 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

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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

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Mastication

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

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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:

Zygomatic Parotid Sublingual Mandibular

Functions of Saliva

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

Gland

Salivary Glands

Type of secretion Main constituents

Parotid Submaxillary Sublingual Serous Mucous or mixed Mucous or mixed Water, enzymes, ions Mucin (mucous), mucin plus enzymes (mixed), water Mucin (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

ruminants mature

 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  Dogs minimal (lubrication, no enzymes)    Sheep Horse Cattle 3-10 liters/d 10-12 liters/d 130-180 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

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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

Gland

Cardia Pylorus or Antrum Fundus Chief cells Parietal cells

Type of secretion

Mucous Mucous Enzyme Acid

Main constituents

Mucin Mucin 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 appear as a single continuous compartment, functionally different they they are distinctly

Reticulum

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