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CellsandTemperature
Cellscannotremainaliveoutsidecertainlimitsoftemperatureandmuchnarrowerlimitsmarktheboundariesofeffectivefunctioning. Enzymesystemsofmammalsandbirdsaremostefficientonlywithinanarrowrangearound 37C;adepartureofafewdegreesfromthisvalueseriouslyimpairstheirfunctioning. Eventhoughcellscansurvivewiderfluctuationstheintegratedactionsofbodilysystemsareimpaired. Otheranimalshaveawidertoleranceforchangesofbodilytemperature.
Forcenturiesithasbeenrecognizedthatmammalsandbirdsdifferfromotheranimalsinthewaytheyregulatebodytemperature. Waysofcharacterizingthedifferencehavebecomemoreaccurateandmeaningfulovertime, butpopularterminologystillreflectstheolddivisioninto "warm-blooded" and "cold-blooded" species; warm-bloodedincludedmammalsandbirdswhereasallothercreatureswereconsideredcold- blooded. Asmorespecieswerestudied, itbecameevidentthatthisclassificationwasinadequate. Afencelizardoradesertiguana-eachcold-blooded----usuallyhasabodytemperatureonlyadegreeortwobelowthatofhumansandsoisnotcold. Thereforethenextdistinctionwasmadebetweenanimalsthatmaintainaconstantbodytemperature, calledhome0therms, andthosewhosebodytemperaturevarieswiththeirenvironments, calledpoikilotherms. Butthisclassificationalsoprovedinadequate, becauseamongmammalstherearemanythatvarytheirbodytemperaturesduringhibernation. Furthermore, manyinvertebratesthatliveinthedep`hsof`@eo#eafnever `Pperien` chang! intheepthsntheoa@anleveexper `ncech`@geInt@e #hil, oftheDeepwapdr, andhe)pbh@ytempaaturesemahnbnstant&