医学英语chapter10 Endocrine System.docx

医学英语chapter10 Endocrine System.docx

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医学英语chapter10EndocrineSystem

Chapter10

EndocrineSystem

AnatomyandPhysiologyoftheEndocrineSystem

Theendocrinesystemisacollectionofglandsthatsecretehormonesdirectlyintothebloodstream.Hormonesarechemicalsthatactontheirtargetorganstoeitherincreaseordecreasethetarget’sactivitylevel.

Hormonesarereleasedintotheextracellularspacessurroundingsecretorycells.Paracrinehormonestravelonlyshortdistancesandaffectnearbycells.Otherhormonesarecarriedbythebloodtoallpartsofthebodyandmayproducegeneraleffects.Ineithercase,thephysiologicalactionofaparticularhormoneisrestrictedtoitstargetcells–thosecellsthatpossessspecificreceptorsforthehormonemolecules.Inotherwords,ahormone’stargetcellshavereceptorsthatothercellslack.

Endocrineglandsandtheirhormoneshelpregulatemetabolicprocesses.Theycontroltheratesofcertainchemicalreactions,aidintransportingsubstancesthroughmembranes,andhelpregulatewaterbalance,electrolytebalance,andbloodpressure.Inthiswaytheendocrinesystemisinstrumentalinmaintaininghomeostasis,thatis,adjustingtheactivitylevelofmostofthetissuesandorgansofthebodytomaintainastableinternalenvironment.

Thebodyactuallyhastwodistincttypesofglands:

exocrineglandsandendocrineglands.Exocrineglandsreleasetheirsecretionsintoaductthatcarriesthemtotheoutsideofthebody.Forexample,sweatglandsreleasesweatintoasweatductthattravelstothesurfaceofthebody.Endocrineglands,however,releasehormonesdirectlyintothebloodstream.Thesecretionofthethyroidglanddirectlyentersthebloodstream.Becausetheyhavenoducts,theyarereferredtoasductlessglands.

Theendocrinesystemconsistsofthefollowingglands:

thepituitarygland,thethyroidgland,parathyroidglands,theadrenalglands,thethymusgland,thepancreas,testesinthemale,andovariesinthefemale.Theendocrineglandsasawholeaffectthefunctionsoftheentirebody.

Somehormonesareproducedbyorgansotherthantheendocrineglandsalreadymentioned.Forexample,thekidneysecretesahormonecallederythropoietin,whichstimulatetheproductionofredbloodcellsbythebonemarrow.Thegastrointerstinaltractsecretesthreehormones,gastrin,secretin,andcholecystokinin.Thesehormonesstimulatethesecretionofgastricacidandenzymes（gastrin）,thesecretionofpancreaticenzymes（secretin）,andthecontractionofthegallbladder（cholecystokinin）.TheskinproducesvitaminD,whichisalsoconsideredahormone.VitaminDstimulatestheabsorptionofcalciumfromthegastrointestinaltractandisnecessaryforthemaintenanceofproperamountsofcalciuminthebonesandintheblood.

PituitaryGland

Thepituitarygland,alsocalledhypophysis,islocatedatthebaseofthebraininasmalldepressionoftheskullcalledthesellaturicica.Thesmallpea-sizedglandisdividedintoananteriorlobecalledtheadenophypophysis,whichisformedbyanupgrowthfromthepharynxandisgrandularinnature,andaposteriorlobecalledneurohypophysis,whichisderivedfromadowngrowthfromthebaseofthebrainandiscomposedofnervoustissue.Bothlobesarecontrolledbythehypothalamusinthebrain.

Theanteriorpituitarysecretesseveraldifferenthormones.Growthhormone（GH）,alsocalledsomatotropin,promotesgrowthofthebodybystimulatingcellstorapidlyincreaseinsizeanddivide.Thyroid-stimulatinghormone（TSH）regulatesthefunctionofthethyroidgland.Adrenocorticotropinhormone（ACTH）regulatesthefunctionoftheadrenalcortex.Prolactin（PRL）stimulatesmildproductioninthebreastfollowingpregnancyandbirth.Follicle-stimulatinghormone（FSH）andluteinizinghormone（LH）bothexerttheirinfluenceonthemaleandfemalegonads.FSHisresponsibleforthedevelopmentofovainovariesandspermintestes.Italsostimulatestheovarytosecreteestrogen.LHstimulatessecretionofsexhormonesinbothmalesandfemalesandplaysaroleinreleasingovainfemales.Melanocyte-stimulatinghormone（MSH）stimulatesmelanocytestoproducemoremelanin,therebydarkeningtheskin.

Theposteriorpituitaryreleasestwohormonesthatareactuallyproducedinthehypothalamus.Thesehormones,antidiuretichormoneandoxytocin,arestoredintheposteriorpituitaryuntilnervoussignalsarrivefromthehypothalamustotriggertheirrelease.Antidiuretichormone（ADH）,alsocalledvasopressin,promoteswaterreabsorptionbythekidneytubules.ADHcanalsoincreasebloodpressurebyconstrictingarterioles.Oxytocinstimulatesuterinecontractionsduringlaboranddelivery,andafterbirththereleaseofmildfromthemammaryglands.

Thepinealglandisasmallpinecone-shapedglandthatispartofthethalamusregionofthebrain.Thepinealglandsecretesmelatonin.Thishormoneisnotwellunderstood,butplaysaroleinregulatingthebody’scircadian.Thisisthe24-hourclockthatgovernsourperiodsofwakefulnessandsleepiness.

PinealGland

Thethyroidgland,whichresemblesabutterflyinshape,hasrightandleftlobes.Itislocatedoneithersideofthetracheaandlarynx.Thethyroidcartilage,orAdam’sapple,islocatedjustabovethethyroidgland.Thisglandproducesthehormonesthyroxine,whichisalsoknownasT4,andtriiodothyronine,whichiscalledT3.Thesehormonesareproducedinthethyroidglandfromthemineraliodine.T3andT4helptoregulatetheproductionofenergyandheatinthebodytoadjustthemetabolicrateofthebody.

Thethyroidglandalsosecretescalcitonininresponsetohypercalcemia.Itsactionistheoppositeofparathyroidhormoneandstimulatestheincreaseddepositionofcalciumintobone,therebyloweringbloodlevelsofcalcium

ThyroidGland

.

ParathyroidGlands

Thefourtinyparathyroidglandsarelocatedonthedorsalsurfaceofthethyroidgland.Theparathyroidhormonesecretedbytheseglandsregulatestheamountofcalciumintheblood.Ifcalciumlevelsinthebloodfalltoolow,parathyroidhormonelevelsinthebloodareincreasedandwillstimulatebonebreakdowntoreleasemorecalciumintotheblood.

ThymusGland

ChildAdult

Inadditiontoitsroleaspartofimmunesystem,thethymusisalsooneoftheendocrine,glandsbecauseitsecretesthehormonethymosin.Thymosin,liketherestofthethymusgland,isimportantforproperdevelopmentoftheimmunesystem.Thethymusglandislocatedinthemediastinalcavityanteriorandsuperiortotheheart.Thethymusispresentatbirthandgrowstoitslargestsizeduringpuberty.Atpubertyitbeginstoshrinkandeventuallyisreplacedwithconnectiveandadiposetissue.

Themostimportantfunctionofthethymusisthedevelopmentoftheimmunesysteminthenewborn.ItisessentialtothegrowthanddevelopmentofthymiclymphocytesorTcells,whicharecriticalforthebody’simmunesystem.Thecompletefunctionofthethymusglandisnotverywellunderstood.

AdrenalGlands

Thetwoadrenalglandsarelocatedaboveeachofthekidneys.Eachglandiscomposedoftwosections:

adrenalcortexandadrenalmedulla.

Theouteradrenalcortexmanufacturesseveraldifferentfamiliesofhormones:

mineralocorticoids,glucocorticoids,andsteroidsexhormones.However,becausetheyareallproducedbythecortextheyarecollectivelyreferredtoascorticosteroids.Themineralocorticoidshormone,aldosterone,regulatessodiumandpotassiumlevelsinthebody.Thesecretionofaldosteroneincreasesthereabsorptionintothebloodstreamofsodiumbythekidneytubules.Atthesametime,itstimulatestheexcretionofanothereletrocytecalledpotassium.Theglucocorticoidhormone,cortisol,haveanimportantinfluenceonthemetabolismofsugar,fat,andproteinswithinallbodycellsandhaveapowerfulanti-inflammatoryeffect.Cortisoneisahormoneverysimilartocortisolandcanbepreparedsynthetically.Cortisoneisusefulintreatinginflammatoryconditionssuchasrheumatoidarthritis.Theadrenalcortexofbothmenandwomensecretssteroidsexhormones:

androgens,estrogen,andprogesterone.Thesehormonesregulatesecondarysexualcharacteristics.Allhormonessecretedbytheadrenalcortexaresteroidhormones.

Theinnerportionoftheadrenalmedullaisresponsibleforsecretingthehormonesepinephrineandnorepinephrine.Epinephrineisalsocalledadrenaline.Epinephrinecanincreasecardiacrate,dilatesbronchialtubes,andstimulatetheproductionofglucosefromastoragesubstancecalledglycogenwhenglucoseisneedbythebody.Norepinephrinecanconstrictsvesselsandraisebloodpressure.Thesehormonesarecriticalduringemergencysituationsbecausetheyincreasebloodpressure,heartrate,andrespirationlevels.Thishelpsthebodyperformbetterduringemergenciesorotherwisestressfultimes.

Pancreas

Thepancreasislocatedalongthelowercurvatureofthestomach.Itistheonlyorganinthebodythathasbothendocrineandexocrinefunctions.Theexocrineportionofthepancreasreleasesdigestiveenzymesthroughaductintotheduodenumofthesmallintestines.Theendocrinesectionsofthepancreas,isletsofLangerhans,arenamedafterDr.PaulLangerhans,aGermananatomist.Theisletscellsproducedtwodifferenthormones:

insulinandglucagon.Insulin,producedbyBisletcells,stimulatesthecellsofthebodytotakeinglucosefromthebloodstream.Lossorimpairmentofthefunctionofinsulinresultsindiabetesmellitus（DM）andhyperglycemia,whichisahighbloodsugarlevel.Incontrast,overproductionofinsulinwillresultinhypoglycemia,oralowbloodsugarlevel.

Therearetwodistinctlydifferenttypesofdiabetesmellitus:

type1andtype2.Intype1thereisadestru