HormonesintheBody Uptothebeginningofthetwentiethcentury,thenervoussystemwasthoughttocontrolallcommunicationwithinthebodyandtheresultingintegrationofbehavior.Scientistshaddeterminedthatnervesran,essentially,onelectricalimpulses.Theseimpulseswerethoughttobetheengineforthought,emotion,movement,andinternalprocessessuchasdigestion.However,experimentsbyWilliamBaylissandErnestStarlingonthechemicalsecretin,whichisproducedinthesmallintestinewhenfoodentersthestomach,eventuallychallengedthatview.Fromthesmallintestine,secretintravelsthroughthebloodstreamtothepancreas.There,itstimulatesthereleaseofdigestivechemicals.Inthisfashion,theintestinalcellsthatproducesecretinultimatelyregulatetheproductionofdifferentchemicalsinadifferentorgan,thepancreas. Suchacoordinationofprocesseshadbeenthoughttorequirecontrolbythenervoussystem;BaylissandStarlingshowedthatitcouldoccurthroughchemicalsalone.ThisdiscoveryspurredStartingtocointhetermhormonetorefertosecretin,takingitfromtheGreekwordhormon,meaning“toexcite”or“tosetinmotion.”Ahormoneisachemicalproducedbyonetissuetomakethingshappenelsewhere. Asmorehormoneswerediscovered,theywerecategorized,primarilyaccordingtotheprocessbywhichtheyoperatedonthebody.Someglands(whichmakeuptheendocrinesystem)secretehormonesdirectlyintothebloodstream.Suchglandsincludethethyroidandthepituitary.Theexocrinesystemconsistsoforgansandglandsthatproducesubstancesthatareusedoutsidethebloodstream,primarilyfordigestion.Thepancreasisonesuchorgan,althoughitsecretessomechemicalsintothebloodandthusisalsopartoftheendocrinesystem. Muchhasbeenlearnedabouthormonessincetheirdiscovery.Someplaysuchkeyrolesinregulatingbodilyprocessesorbehaviorthattheirabsencewouldcauseimmediatedeath.Themostabundanthormoneshaveeffectsthatarelessobviouslyurgentbutcanbemorefar-reachinganddifficulttotrack:Theymodifymoodsandaffecthumanbehavior,evensomebehaviorwenormallythinkofasvoluntary.Hormonalsystemsareveryintricate.Evenminuteamountsoftherightchemicalscansuppressappetite,calmaggression,andchangetheattitudeofaparenttowardachild.Certainhormonesacceleratethedevelopmentofthebody,regulatinggrowthandform;othersmayevendefineanindividual’spersonalitycharacteristics.Thequantitiesandproportionsofhormonesproducechangewithage,soscientistshavegivenagreatdealofstudytoshiftsintheendocrinesystemovertimeinthehopesofalleviatingailmentsassociatedwithaging. Infact,somehormonetherapiesarealreadyverycommon.Acombinationofestrogenandprogesteronehasbeenprescribedfordecadestowomenwhowanttoreducemoodswings,suddenchangesinbodytemperature,andotherdiscomfortscausedbylowernaturallevelsofthosehormonesastheyentermiddleage.Knownashormonereplacementtherapy(HRT),thetreatmentwasalsobelievedtopreventweakeningofthebones.AtleastonestudyhaslinkedHRTwithaheightenedriskofheartdiseaseandcertaintypesofcancer.HRTmayalsoincreasethelikelihoodthatbloodclots—dangerousbecausetheycouldtravelthroughthebloodstreamandblockmajorbloodvessels—willform.SomeproponentsofHRThavetemperedtheirenthusiasminthefaceofthisnewevidence,recommendingitonlytopatientswhosesymptomsinterferewiththeirabilitiestolivenormallives. Humangrowthhormonemayalsobegiventopatientswhoaresecretingabnormallylowamountsontheirown.Becauseofthecomplicatedeffectsgrowthhormonehasonthebody,suchtreatmentsaregenerallyrestrictedtochildrenwhowouldbepathologicallysmallinstaturewithoutit.Growthhormoneaffectsnotjustphysicalsizebutalsothedigestionoffoodandtheagingprocess.Researchersandfamilyphysicianstendtoagreethatitisfoolhardytodispenseitincasesinwhichtherisksarenotclearlyoutweighedbythebenefits. Whichofthefollowingsentencesexplainstheprimarygoalofhormonereplacementtherapy?
A.The quantities and proportions of hormones produce change with age, so scientists have given a great deal of study to shifts in the endocrine system over time in the hopes of alleviating ailments associated with aging.
B.A combination of estrogen and progesterone has been prescribed for decades to women who want to reduce mood swings, sudden changes in body temperature, and other discomforts caused by lower natural levels of those hormones as they enter middle age.
C.HRT may also increase the likelihood that blood clots—dangerous because they could travel through the bloodstream and block major blood vessels—will form.
D.Because of the complicated effects growth hormone has on the body, such treatments are generally restricted to children who would be pathologically small in stature without it.
正确答案是A