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 GlandsGland Type of

secretionMain constituents

Parotid Serous Water, enzymes, ions

Submaxillary Mucous or mixed Mucin (mucous), mucin plus enzymes (mixed), water

Sublingual Mucous or mixed 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

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 Dogs minimal (lubrication, no

enzymes) Sheep 3-10 liters/d Horse 10-12 liters/d Cattle 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

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 B12

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 Type of secretion

Main constituents

Cardia Mucous Mucin

Pylorus or Antrum

Mucous Mucin

Fundus Chief cells Parietal cells

EnzymeAcid

acid

Pepsinogen PepsinHCl, 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)