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Pathologyisthescienceorstudyofdisease.Initsbroadestsense,pathologyisliterallyabnormalbiology,thestudyofindividualswhoareillordisordered.Asabasicbiologicscience,pathologyincludesfieldssuchasplantpathology,insectpathology,comparativepathology,aswellashumanpathology.Pathology,inthecontextofhumanmedicine,isnotonlyabasicortheoreticalscience,butalsoaclinicalmedicalspecialty.Pathologistsspecializeinlaboratorymedicine;theyconsultwithotherphysicians,therebyassistinginthediagnosisandtreatmentofdisease.Thescopeoflaboratorymedicineincludesallofthestudiesperformedonpatientsamples,includingsamplesoftissue,blood,andotherbodyfluids.Laboratorystudiesinvolveanatomicpathologystudyandassessmorphologicalterationsincellsandtissues.Surgicalpathology,cytopathology,andautopsypathologyareincludedinthiscategory.Manystudiesareperformedusingothermeans.Theseareasofclinicalpathologyincludeclinicalchemistry,microbiology,hematology,immunology,andimmunohematology.Pathophysiologydealswiththedynamicaspectsofthediseaseprocess.Itisthestudyofdisorderedoralteredfunctions,forexample,thephysiologicchangescausedbydiseaseinalivingorganism.ConceptofNormalcyMostpeoplehavesomenotionofnormalandwoulddefinediseaseorillnessasadeviationfromoranabsenceofthatnormalstate.However,oncloserscrutiny,theconceptofnormalcyturnsouttobecomplexandcannotbedefinedsimply;correspondingly,theconceptofdiseaseisfarfromsimple.Anyparameterofmeasurementappliedtoanindividualorgroupofindividualshassomesortofaveragevaluethatisconsiderednormal.Averagevaluesforheight,weight,andbloodpressurearederivedfromobservationsonmanyindividualsandincludeacertainamountofvariation.Variationsinnormalvaluesoccurforseveralreasons.First,individualsdifferfromoneanotherintheirgeneticmakeup.Thus,notwoindividualsintheworld,exceptthosederivedfromthesamefertilizedovum,haveexactlythesamegenes.Second,individualsdifferintheirlifeexperiencesandintheirinteractionwiththeenvironment.Third,ineveryindividualtherearevariationsinphysiologicparametersbecauseofthewayinwhichthecontrolmechanismsofthebodyfunction.Forinstance,bloodglucoseconcentrationsinahealthypersonvarysignificantlyatdifferenttimesduringtheday,dependingonfoodintake,activitiesoftheindividual,andsoforth.Thesevariationsgenerallyoccurwithinacertainrange.Thesituationissomewhatanalogoustoathermostaticallycontrolledroom.Thetemperaturemaydipslightlybelowthedesiredlevelbeforesuchadropissensedbythethermostat.Thecorrectiveactiontriggeredbythethermostatmay,inturn,overshoottheidealslightlybeforetheheatinputishalted.Indeed,suchvariationsinbodytemperature,eveninthenormalstate,occurinallindividuals.Finally,forphysiologicparametersmeasuredbyfairlyintricatemeans,asignificantamountofvariationinobservedvaluesmayresultfromerrororimprecisioninherentinthemeasurementprocessitself.Becauseoftheseconsiderations,determininganormalrangeofvariationfromanaveragevalueisacomplexmatter.Thiscomplexityincludesknowingthedegreeofphysiologicoscillationofaparticularmeasurement,accountingforthedegreeofvariationamongnormalindividualsevenunderbaselineconditions,andfiguringtheprecisionofthemeasurementmethod.Finally,thebiologicsignificanceofthemeasurementmustbeestimated.Singlemeasurements,observations,orlaboratoryresultsthatseemtoindicateabnormalitymustalwaysbejudgedinthecontextoftheentireindividual.Asinglereadingofelevatedbloodpressuredoesnotmakeanindividualhypertensive;asingleslightlyelevatedbloodglucoseleveldoesnotmeanthattheindividualisdiabetic;andasinglehemoglobinvaluelowerthanaveragedoesnotnecessarilyindicateanemia.Toplacetheaboveconsiderationsinperspective,conceptsofnormalcyandevenofdiseaseare,toanextent,arbitraryandinfluencedbyculturalvaluesaswellasbybiologicrealities.Forexample,inourcultureadefectofacentralnervoussystemfunctionmayproduceasignificantreadingdisabilityandwouldbeanabnormality,whereasthesamedefectmightneverbenotedinaprimitiveculture.Furthermore,atraitthatmightbeaverageandthusnormalinonepopulationmightbeconsidereddistinctlyabnormalinanother.Consider,forinstance,howa¡°normal¡±personfromourpopulationwouldbeviewedbyagroupofcentralAfricanpygmies;orconversely,howaninfantfromaprimitiveculture,withthe¡°normal¡±chronicdiarrheaandpoorweightgain,mightbeviewedinoneofourwell-babyclinics.ConceptofDiseaseDiseasecanbedefinedaschangesinindividualsthatcausetheirhealthparameterstofalloutsidethenormalrange.Themostusefulbiologicyardstickfornormalcyrelatestotheindividual¡¯sabilitytomeetthedemandsplacedonthebodyandtoadapttothesedemandsorchangesintheexternalenvironmentsoastomaintainreasonableconstancyoftheinternalenvironment.Allcellsinthebodyneedacertainamountofoxygenandnutrientsfortheircontinuingsurvivalandfunction,andtheyalsorequireanenvironmentthatprovidesnarrowrangesoftemperature,watercontent,acidity,andsaltconcentration.Thus,themaintenanceofinternalconditionswithinfairlynarrowlimitsisanessentialfeatureofthenormalbody.Whensomeofthestructuresandfunctionsofthebodydeviatefromthenormtothepointwheretheabilitytomaintainhomeostasisisdestroyedorthreatenedorwheretheindividualcannolongermeetenvironmentalchalle