Chapter 41Digestion

Digestive SystemsAbsorb nutrients necessary to

maintain life, eliminate wastesIngestionDigestionAbsorptionElimination

Digestion—process by which food is chemically and mechanically broken down for use by the body

Digestive SystemsNo digestive system

Sponges, tapewormsAbsorb only small moleculesDigestion intracellular

Digestive SystemsIncomplete

Cnidarians, planariansSingle body opening

(Gastrovascular cavity)Limited enzymesDigestion begins

extracellularlyDigestion completed

intracellularlyNo regions w/ specialized

function

Digestive SystemsComplete

Annelids, mollusks, arthropods, vertebrates, etc. (most animals)

Two openings (mouth & anus)

Different regions w/ specialized function

Digestion primarily extracellular

Digestive SystemsContinuous feeders

Filter feeders (clams, mussels, barnacles, etc.)

Must complete digestion quickly

Discontinuous feedersMost animalsFeed periodicallyNeed storage area in gut

Complete Digestive SystemFunctions

Mechanical processing & motility

Secretion of digestive enzymesChemical digestionAbsorptionElimination of wastes

Anatomy determined by lifestyleCarnivoreHerbivore Omnivore

MouthInitial digestion beginsMechanical break-down of foodNo teeth

Worms, birds—swallow food whole Grinding done lower in digestive tract

Insects—some chew, some suckArachnids—no chewing

Suck existing fluids Inject digestive fluids into prey

MouthTeeth

Vertebrates onlyIncisors—shearingCanines—puncture,

tearing Capture prey Fighting predators or

within speciesPremolars—grindingMolars—crushing

MouthHerbivores

Plant material toughEmphasis on

grinding, crushingBroad, flat premolars

& molarsLimited or absent

canines

MouthCarnivores

Animal material easier to mechanically tear

Emphasis on piercing, cutting

Long, pointed canines

Molars & premolars modified for shearing or piercing

MouthOmnivores

Combination teethVaries based on amount of

each type of food

MouthSalivary glands

Secrete via ducts into mouth

Lubricate & moisten foodSalivary amylase

Begins to digest starches

TongueGrab foodManipulate foodTaste

EsophagusMuscular tubeLined by stratified

squamous epitheliumMoves food from

mouth to next stepPeristalsis—rhythmic

contractions of gastrointestinal tract

Upper GICrop

InsectsAnnelidsMost birdsTemporary storage

Upper GIProventriculus

Birds, many invertebratesSecretes digestive enzymes

GizzardBirds, many invertebrates,

some reptilesDinosaursThick, muscular wallsGrinding food

Chitinous plates in some insects & mollusks

Birds swallow “grit” to use for grinding

Upper GIStomach (monogastric)

Temporary storageMechanical churning Digestive enzymesCombined functions of

crop, proventriculus, gizzard

Pepsin—breaks down proteins

Hydrochloric acid—slows salivary amylase, activates pepsin

Upper GIStomach (ruminant)

Cattle, goats, llamas, camels, deer, sheep, etc.

Four-chambered stomach Rumen, reticulum, omasum,

abomasumFood swallowed, separated in

reticulorumenSolid (cud) regurgitated from

reticulumCud chewed for further

breakdown, then swallowedMoved to omasum, water &

inorganic minerals absorbedMoves to abomasum, like

monogastric stomach

Lower GISmall intestine

Short in carnivoresLong in herbivores

DuodenumJejunumIleum

Lower GISecretion of enzymes

Intestinal mucosaGall bladderPancreas

AbsorptionVilli—folds of intestinal

liningMicrovilli—folds of

intestinal cells

Lower GICecum

Single in mammalsDouble in most birds &

some reptilesLarge in herbivoresReduced in carnivoresRemove fluids & saltsIn herbivores has large

numbers of bacteria to break down plant material

Lower GIColon (large intestine)

Most food digested & absorbed by this point

Water & wasteFunctions to reabsorb waterDiarrhea—water not

absorbed properlyRectum

Storage of feces ready for elimination

AnusMuscular sphincter

controlling elimination of feces

Other OrgansLiver

Secretes bile (breaks down fats)Many other important functions

Gall bladderStores & concentrates bileNot present in all animals, even

if closely related Absent horse, deer; present cow,

goat, sheep Absent doves; present hawks &

owls Absent rats; present mice

PancreasMany enzymes to break down

foodBuffers against HCl from

stomach