醫(yī)學(xué)英語(yǔ)原文翻譯1至5單元.doc

1.生理學(xué)是研究生物體正常功能的一門科學(xué)。它研究生物體如何進(jìn)行各種活動(dòng)，如何飲食，如何運(yùn)動(dòng)，如何適應(yīng)不斷改變的環(huán)境，如何繁殖后代。這門學(xué)科包羅萬(wàn)象，涵蓋了生物體整個(gè)生命過(guò)程。生理學(xué)成功地解釋了生物體如何進(jìn)行日?；顒?dòng)，基于的觀點(diǎn)是生物體好比是結(jié)構(gòu)復(fù)雜而靈巧的機(jī)器，其操作受物理和化學(xué)規(guī)律控制。盡管從生物學(xué)整個(gè)范疇看，生物體某些活動(dòng)過(guò)程是相似的，如基因編碼的復(fù)制，但許多過(guò)程還是某些生物體群組特有的。鑒于此，將這門學(xué)科分成不同部分研究如細(xì)菌生理學(xué)，植物生理學(xué)和動(dòng)物生理學(xué)是有必要的。Physiologyisthestudyofthefunctionsoflivingmatter.Itisconcernedwithhowanorganismperformsitsvariedactivities:howitfeeds,howitmoves,howitadaptstochangingcircumstances,howitspawnsnewgenerations.Thesubjectisvastandembracesthewholeoflife.Thesuccessofphysiologyinexplaininghoworganismsperformtheirdailytasksisbasedonthenotionthattheyareintricateandexquisitemachineswhoseoperationisgovernedbythelawsofphysicsandchemistry.Althoughsomeprocessesaresimilaracrossthewholespectrumofbiologythereplicationofthegeneticcodeforexamplemanyarespecifictoparticulargroupsoforganisms.Forthisreasonitisnecessarytodividethesubjectintovariouspartssuchasbacterialphysiology,plantphysiology,andanimalphysiology.2.正如要了解一個(gè)動(dòng)物如何活動(dòng)，首先需要了解它的構(gòu)成，要充分了解一個(gè)生物體的生理學(xué)活動(dòng)就必須掌握全面的解剖學(xué)知識(shí)。一個(gè)生物體的各部分起著什么作用可通過(guò)實(shí)驗(yàn)觀察得知。盡管我們對(duì)志愿者進(jìn)行了許多重要的生理調(diào)查，但是實(shí)驗(yàn)條件需要精確控制，所以我們當(dāng)前大多生理知識(shí)還是源于對(duì)其它動(dòng)物如青蛙，兔子，貓和狗等的研究。當(dāng)我們明確大多數(shù)動(dòng)物物種的特定生理過(guò)程存在共同之處時(shí)，相同的生理原理適用于人類也是合理的。通過(guò)這種方法，我們獲得了大量的知識(shí)，從而讓我們對(duì)人類生理學(xué)有了更深入的了解，為我們有效治療許多疾病提供了一個(gè)堅(jiān)實(shí)的基礎(chǔ)。Tostudyhowananimalworksitisfirstnecessarytoknowhowitisbuilt.Afullappreciationofthephysiologyofanorganismmustthereforebebasedonasoundknowledgeofitsanatomy.Experimentscanthenbecarriedouttoestablishhowparticularpartsperformtheirfunctions.Althoughtherehavebeenmanyimportantphysiologicalinvestigationsonhumanvolunteers,theneedforprecisecontrolovertheexperimentalconditionshasmeantthatmuchofourpresentphysiologicalknowledgehasbeenderivedfromstudiesonotheranimalssuchasfrogs,rabbits,cats,anddogs.Whenitisclearthataspecificphysiologicalprocesshasacommonbasisinawidevarietyofanimalspecies,itisreasonabletoassumethatthesameprincipleswillapplytohumans.Theknowledgegainedfromthisapproachhasgivenusagreatinsightintohumanphysiologyandendoweduswithasolidfoundationfortheeffectivetreatmentofmanydiseases.Theorganizationofthebody3.機(jī)體的基本組成物質(zhì)是細(xì)胞，細(xì)胞結(jié)合在一起形成組織。組織的基本類型有上皮組織，結(jié)締組織，神經(jīng)組織，肌組織，每類組織都有各自的特征。許多結(jié)締組織中細(xì)胞量相對(duì)較少，但是有大量的細(xì)胞外基質(zhì)。相比而言，光滑的肌組織由大量密密麻麻的肌細(xì)胞通過(guò)特定的細(xì)胞連接組成。各種器官如腦，心臟，肺，小腸和肝等由不同種類的組織聚集而成。這些器官是不同生理系統(tǒng)的組成部分。心臟和血管組成心血管系統(tǒng)；肺，器官，支氣管，胸壁和膈肌組成呼吸系統(tǒng)；骨骼和骨骼肌組成骨骼肌系統(tǒng)；大腦，脊髓，自主神經(jīng)和神經(jīng)中樞以及周圍軀體神經(jīng)組成神經(jīng)系統(tǒng)等等。3.Thebuildingblocksofthebodyarethecells,whicharegroupedtogethertoformtissues.Theprincipaltypesoftissueareepithelial,connective,nervous,andmuscular,eachwithitsowncharacteristics.Manyconnectivetissueshaverelativelyfewcellsbuthaveanextensiveextracellularmatrix.Incontrast,smoothmuscleconsistsofdenselypackedlayersofmusclecellslinkedtogetherviaspecificcelljunctions.Organssuchasthebrain,theheart,thelungs,theintestines,andtheliverareformedbytheaggregationofdifferentkindsoftissues.Theorgansarethemselvespartsofdistinctphysiologicalsystems.Theheartandbloodvesselsformthecardiovascularsystem;thelungs,trachea,andbronchitogetherwiththechestwallanddiaphragmformtherespiratorysystem;theskeletonandskeletalmusclesformthemusculoskeletalsystem;thebrain,spinalcord,autonomicnervesandganglia,andperipheralsomaticnervesformthenervoussystem,andsoon.4.細(xì)胞在形體和功能上差異很大，但是它們有某些共同的特征。第一，細(xì)胞由一層薄膜也稱細(xì)胞膜包被；第二，細(xì)胞能把大分子分解為小分子釋放能量供活動(dòng)所需；第三，在生命過(guò)程中某個(gè)階段，細(xì)胞體內(nèi)存在一個(gè)以脫氧核糖核酸(DNA)形式出現(xiàn)，包含基因信息的細(xì)胞核。4.Cellsdifferwidelyinformandfunctionbuttheyallhavecertaincommoncharacteristics.First,theyareboundedbyalimitingmembrane,theplasmamembrane.Secondly,theyhavetheabilitytobreakdownlargemoleculestosmalleronestoliberateenergyfortheiractivities.Thirdly,atsomepointintheirlifehistory,theypossessanucleuswhichcontainsgeneticinformationintheformofdeoxyribonucleicacid(DNA).5.活體細(xì)胞不斷轉(zhuǎn)化物質(zhì)。細(xì)胞分解葡萄糖和脂肪，為自身生長(zhǎng)和修復(fù)所需的蛋白質(zhì)合成和運(yùn)動(dòng)等其它活動(dòng)提供能量。這些化學(xué)變化統(tǒng)稱為新陳代謝。把大分子分解為小分子的過(guò)程稱為分解代謝，小分子合成大分子的過(guò)程稱為合成代謝。5.Livingcellscontinuallytransformmaterials.Theybreakdownglucoseandfatstoprovideenergyforotheractivitiessuchasmotilityandthesynthesisofproteinsforgrowthandrepair.Thesechemicalchangesarecollectivelycalledmetabolism.Thebreakdownoflargemoleculestosmalleronesiscalledcatabolismandthesynthesisoflargemoleculesfromsmalleronesanabolism.6.細(xì)胞在進(jìn)化過(guò)程中不斷分化進(jìn)行不同的功能活動(dòng)。有些細(xì)胞具有收縮能力（如肌細(xì)胞），有些可以傳導(dǎo)電信號(hào)（如神經(jīng)細(xì)胞）。進(jìn)一步進(jìn)化的細(xì)胞能夠分泌不同物質(zhì)如荷爾蒙（如內(nèi)分泌細(xì)胞）或酶。胚胎發(fā)育過(guò)程中，分化的過(guò)程由于很多不同細(xì)胞來(lái)源于受精卵而再次發(fā)生。6.Inthecourseofevolution,cellsbegantodifferentiatetoservedifferentfunctions.Somedevelopedtheabilitytocontract(musclecells),otherstoconductelectricalsignals(nervecells).Afurthergroupdevelopedtheabilitytosecretedifferentsubstancessuchashormonesorenzymes.Duringembryologicaldevelopment,thisprocessofdifferentiationisre-enactedasmanydifferenttypesofcellareformedfromthefertilizedegg.7.大多數(shù)組織包含有不同的細(xì)胞類型。比如，血液中含紅細(xì)胞，白細(xì)胞和血小板。紅細(xì)胞運(yùn)輸全身的氧氣。白細(xì)胞在抵御感染時(shí)起重要作用，血小板是血液凝集過(guò)程中重要的成分。結(jié)締組織有多種不同類型，但有一個(gè)共同特征，即細(xì)胞分布在豐富的細(xì)胞外基質(zhì)中。神經(jīng)組織含神經(jīng)細(xì)胞和神經(jīng)膠質(zhì)細(xì)胞。7.Mosttissuescontainamixtureofcelltypes.Forexample,bloodconsistsofredcells,whitecells,andplatelets.Redcellstransportoxygenaroundthebody.Thewhitecellsplayanimportantroleindefenseagainstinfectionandtheplateletsarevitalcomponentsintheprocessofbloodclotting.Thereareanumberofdifferenttypesofconnectivetissuebutallarecharacterizedbyhavingcellsdistributedwithinanextensivenoncellularmatrix.Nervetissuecontainsnervecellsandglialcells.8.大型多細(xì)胞動(dòng)物體的細(xì)胞不能產(chǎn)生氧氣和營(yíng)養(yǎng)物質(zhì)，需要直接從外界環(huán)境中獲取。這些氧氣和營(yíng)養(yǎng)物質(zhì)必須由血液運(yùn)送到細(xì)胞。這是血液的主要功能之一，血液憑借心臟的泵血作用在血管內(nèi)流動(dòng)循環(huán)。8.Thecellsoflargemulticellularanimalscannotderivetheoxygenandnutrientstheyneeddirectlyfromtheexternalenvironment.Thesemustbetransportedtothecells.Thisisoneoftheprincipalfunctionsoftheblood,whichcirculateswithinbloodvesselsbyvirtueofthepumpingactionoftheheart.Theheart,bloodvessels,andassociatedtissuesformthecardiovascularsystem.9.心臟有四個(gè)腔，其中有兩個(gè)心房，兩個(gè)心室構(gòu)成了一對(duì)并排存在的泵。右心室將去氧的血液泵至肺中，肺中的血液吸收空氣中的氧氣，而左心室把從肺回流來(lái)的有氧血液泵出至身體其它部位，供應(yīng)給各組織。生理學(xué)家研究促使心臟跳動(dòng)的因素，心臟如何泵送血液使其循環(huán)，心臟如何根據(jù)各組織所需分配血液。血漿和組織間的流動(dòng)液體交換流入淋巴系統(tǒng)，最終回流到血液中。9.Theheartconsistsoffourchambers,twoatriaandtwoventricles,whichformapairofpumpsarrangedsidebyside.Therightventriclepumpsdeoxygenatedbloodtothelungswhereitabsorbsoxygenfromtheair,whiletheleftventriclepumpsoxygenatedbloodreturningfromthelungstotherestofbodytosupplythetissues.Physiologistsareconcernedwithestablishingthefactorsresponsiblefortheheartbeat,howtheheartpumpsthebloodaroundthecirculation,andhowitisdistributedtoperfusethetissuesaccordingtotheirneeds.Fluidexchangedbetweenthebloodplasmaandthetissuespassesintothelymphaticsystem,whicheventuallydrainsbackintotheblood.10.機(jī)體進(jìn)行各項(xiàng)活動(dòng)所需的能量最終來(lái)源于呼吸。這一過(guò)程包括食物（主要是糖類和脂肪）的氧化，釋放它們所含的能量。這一過(guò)程中，氧氣來(lái)自于肺中的空氣，經(jīng)由血液到達(dá)全身各組織。組織呼吸活動(dòng)中釋放的二氧化碳由肺動(dòng)脈中的血液運(yùn)送至肺，然后呼氣排出體外。需回答的基本問(wèn)題如下：空氣是如何進(jìn)出肺的？呼吸的空氣量如何適應(yīng)機(jī)體所需？限制肺吸收氧氣頻率的因素是什么？10.Theenergyrequiredforperformingthevariousactivitiesofthebodyisultimatelyderivedfromrespiration.Thisprocessinvolvestheoxidationoffoodstuffstoreleasetheenergytheycontain.Theoxygenneededforthisprocessisabsorbedfromtheairinthelungsandcarriedtothetissuesbytheblood.Thecarbondioxideproducedbytherespiratoryactivityofthetissuesiscarriedtothelungsbythebloodinthepulmonaryarterywhereitisexcretedintheexpiredair.Thebasicquestionstobeansweredincludethefollowing:Howistheairmovedinandoutofthelungs?Howisthevolumeofairbreathedadjustedtomeettherequirementsofthebody?Whatlimitstherateofoxygenuptakeinthelungs?11.機(jī)體所需營(yíng)養(yǎng)物質(zhì)來(lái)源于飲食。食物經(jīng)口腔進(jìn)入體內(nèi)，在胃腸道內(nèi)經(jīng)酶將其分解成小分子物質(zhì)。這些消化物通過(guò)腸壁吸收入血液，通過(guò)門靜脈進(jìn)入肝臟。經(jīng)肝臟作用后，這些營(yíng)養(yǎng)物質(zhì)能夠滿足組織生長(zhǎng)修復(fù)及能量需求。在消化系統(tǒng)部分，重要的生理學(xué)問(wèn)題是：食物是如何消化的？食物如何被個(gè)體分解消化？個(gè)體營(yíng)養(yǎng)物質(zhì)如何吸收？食物如何在腸內(nèi)轉(zhuǎn)運(yùn)的？未消化的殘留如何從體內(nèi)排出？11.Thenutrientsneededbythebodyarederivedfromthediet.Foodistakeninbythemouthandbrokendownintoitscomponentpartsbyenzymesinthegastrointestinaltract.Thedigestiveproductsarethenabsorbedintothebloodacrossthewalloftheintestineandpasstotheliverviatheportalvein.Thelivermakesnutrientsavailabletothetissuesbothfortheirgrowthandrepairandfortheproductionofenergy.Inthecaseofthedigestivesystem,keyphysiologicalquestionsare:Howisfoodingested?Howisitbrokendownanddigested?Howaretheindividualnutrientsabsorbed?Howisthefoodmovedthroughthegut?Howaretheindigestibleremainseliminatedfromthebody?12.腎臟主要功能是控制細(xì)胞外液體的形成。在這一過(guò)程中，腎臟也會(huì)把不可揮發(fā)的廢物排出去。為行使這一功能，在排出之前，腎臟產(chǎn)生含有各種成分的尿液并將其暫時(shí)儲(chǔ)存在膀胱中。這一部分主要的生理學(xué)問(wèn)題是：腎臟如何調(diào)節(jié)血液中的成分？如何排出有毒廢物？如何應(yīng)對(duì)像脫水這樣的應(yīng)激反應(yīng)？以及尿液可以存儲(chǔ)和排出體外的機(jī)制是什么？12.Thechieffunctionofthekidneysistocontrolthecompositionoftheextracellularfluid.Inthecourseofthisprocess,theyalsoeliminatenon-volatilewasteproductsfromtheblood.Toperformthesefunctions,thekidneysproduceurineofvariablecompositionwhichistemporarilystoredinthebladderbeforevoiding.Thekeyphysiologicalquestionsinthiscaseare:howdothekidneysregulatethecompositionoftheblood?Howdotheyeliminatetoxicwaste?Howdotheyrespondtostressessuchasdehydration?Whatmechanismsallowthestorageandeliminationoftheurine?Thereproductivesystem13.生殖是活生物體的一個(gè)基本特征。生殖腺產(chǎn)生專門的性細(xì)胞，被稱為配子。性生殖的核心是雌雄配子即精子和卵子的產(chǎn)生和融合，因此兩個(gè)獨(dú)立個(gè)體的基因特征融合而產(chǎn)生一個(gè)基因上與雙親不同的后代。13.Reproductionisoneofthefundamentalcharacteristicsoflivingorganisms.Thegonadsproducespecializedsexcellsknownasgametes.Atthecoreofsexualreproductionisthecreationandfusionofthemaleandfemalegametes,thespermandova(eggs),withtheresultthatthegeneticcharacteristicsoftwoseparateindividualsaremixedtoproduceoffspringthatdiffergeneticallyfromtheirparents.14.這一系統(tǒng)由骨、骨骼肌、關(guān)節(jié)和它們的相關(guān)組織組成。其主要功能是提供運(yùn)動(dòng)需要，維持姿勢(shì)及呼吸運(yùn)動(dòng)。它也為內(nèi)臟器官提供物理支持。這一部分，肌肉收縮機(jī)制是主要問(wèn)題。內(nèi)分泌系統(tǒng)和神經(jīng)系統(tǒng)14.Thisconsistsofthebonesoftheskeleton,skeletalmuscles,joints,andtheirassociatedtissues.Itsprimaryfunctionistoprovideameansofmovement,whichisrequiredforlocomotion,forthemaintenanceofposture,andforbreathing.Italsoprovidesphysicalsupportfortheinternalorgans.Herethemechanismofmusclecontractionisacentralissue.Theendocrineandnervoussystems15.不同器官系統(tǒng)的活動(dòng)需要協(xié)作和調(diào)節(jié)，以便共同作用滿足機(jī)體需要。人體有兩大調(diào)節(jié)系統(tǒng)：神經(jīng)系統(tǒng)和內(nèi)分泌系統(tǒng)。神經(jīng)系統(tǒng)通過(guò)電信號(hào)迅速將信息傳導(dǎo)給特定細(xì)胞。這樣神經(jīng)將電信號(hào)傳遞給骨骼肌以控制收縮。內(nèi)分泌系統(tǒng)分泌化學(xué)物質(zhì)激素。激素通過(guò)血流到達(dá)施與調(diào)節(jié)作用的細(xì)胞。激素在許多不同器官中起著重要作用，在月經(jīng)期調(diào)節(jié)和其它生殖方面尤其重要。15.Theactivitiesofthedifferentorgansystemsneedtobecoordinatedandregulatedsothattheyacttogethertomeettheneedsofthebody.Twocoordinatingsystemshaveevolved:thenervoussystemandtheendocrinesystem.Thenervoussystemuseselectricalsignalstotransmitinformationveryrapidlytospecificcells.Thusthenervespasselectricalsignalstotheskeletalmusclestocontroltheircontraction.Theendocrinesystemsecreteschemicalagents,hormones,whichtravelinthebloodstreamtothecellsuponwhichtheyexertaregulatoryeffect.Hormonesplayamajorroleintheregulationofmanydifferentorgansandareparticularlyimportantintheregulationofthemenstrualcycleandotheraspectsofreproduction.16.免疫系統(tǒng)通過(guò)殺死入侵的有機(jī)體，清除致病或損傷細(xì)胞為機(jī)體提供防御功能。16.Theimmunesystemprovidesthebodysdefensesagainstinfectionbothbykillinginvadingorganismsandbyeliminatingdiseasedordamagedcells.17.雖然研究各器官如何行使功能很有益處，但我們必須認(rèn)識(shí)到機(jī)體作為一個(gè)整體所做的活動(dòng)依賴于各器官系統(tǒng)間錯(cuò)綜復(fù)雜的相互作用。如果一部分無(wú)法正常工作，全身其它器官系統(tǒng)也會(huì)受到影響。例如，如果腎臟出現(xiàn)問(wèn)題，內(nèi)部環(huán)境的調(diào)節(jié)受損，結(jié)果導(dǎo)致其它器官系統(tǒng)功能紊亂。17.Althoughitishelpfultostudyhoweachorganperformsitsfunctions,itisessentialtorecognizethattheactivityofthebodyasawholeisdependentontheintricateinteractionsbetweenthevariousorgansystems.Ifonepartfails,theconsequencesarefoundinotherorgansystemsthroughoutthewholebody.Forexample,ifthekidneysbegintofail,theregulationoftheinternalenvironmentisimpairedwhichinturnleadstodisordersoffunctionelsewhere.18.各種復(fù)雜機(jī)制共同作用調(diào)節(jié)細(xì)胞外液的形成，不同個(gè)體細(xì)胞有自身機(jī)制調(diào)節(jié)內(nèi)在組成成分。盡管外界環(huán)境和動(dòng)物活動(dòng)不停變化，調(diào)節(jié)機(jī)制維持著體內(nèi)環(huán)境的穩(wěn)定。內(nèi)部環(huán)境的穩(wěn)定被稱為穩(wěn)態(tài)，它是機(jī)體能夠正常發(fā)揮作用所必須的。18.Complexmechanismsareatworktoregulatethecompositionoftheextracellularfluidandindividualcellshavetheirownmechanismsforregulatingtheirinternalcomposition.Theregulatorymechanismsstabilizetheinternalenvironmentdespitevariationsinboththeexternalworldandtheactivityoftheanimal.Theprocessofstabilizationoftheinternalenvironmentiscalledhomeostasisandisessentialifthecellsofthebodyaretofunctionnormally.19.例如，心臟的跳動(dòng)依賴于心肌細(xì)胞有節(jié)律的收縮。這一活動(dòng)依賴于電信號(hào)，而電信號(hào)反過(guò)來(lái)依賴存在于細(xì)胞外和細(xì)胞內(nèi)液體中鈉和鉀離子的濃度。如果細(xì)胞外液中鉀離子過(guò)多，心肌細(xì)胞興奮性增強(qiáng)，可能出現(xiàn)不規(guī)律的收縮。因此，要維持心臟正常跳動(dòng)，細(xì)胞外液中鉀離子的濃度就必須控制在一定范圍內(nèi)。19.Totakeoneexample,thebeatingoftheheartdependsontherhythmicalcontractionsofcardiacmusclecells.Thisactivitydependsonelectricalsignalswhich,inturn,dependontheconcentrationofsodiumandpotassiumionsintheextracellularandintracellularfluids.Ifthereisanexcessofpotassiumintheextracellularfluid,thecardiacmusclecellsbecometooexcitableandmaycontractatinappropriatetimesratherthaninacoordinatedmanner.Consequently,theconcentrationofpotassiumintheextracellularfluidmustbekeptwithinanarrowrangeiftheheartistobeatnormally.20.一天中，一個(gè)成人需要消耗約1千克食物，2-3升液體。以一個(gè)月計(jì)算，這相當(dāng)于約30千克食物，60-90升液體。然而，一般來(lái)說(shuō)，機(jī)體體重是基本不變的。這類個(gè)體可以說(shuō)處于平衡狀態(tài)。食物和液體的攝入量相當(dāng)于正常機(jī)體活動(dòng)消耗的能量加上尿液和糞便中丟失的能量。在一些情況下，如饑餓狀態(tài)，攝入量與機(jī)體所需量并不相當(dāng)，肌組織斷裂，提供葡萄糖產(chǎn)生能量。蛋白質(zhì)的攝入低于肌組織斷裂的速度，機(jī)體處于負(fù)氮平衡。同樣地，如果機(jī)體組織正處于生長(zhǎng)期，如生長(zhǎng)期的兒童，孕婦和早期訓(xùn)練階段的運(yùn)動(dòng)員，那么蛋白質(zhì)的日常攝入量比正常機(jī)體所需要的多。相反，此時(shí)個(gè)體處于正氮平衡。20.Inthecourseofaday,anadultconsumesapproximately1kgoffoodanddrinks23litersoffluid.Inamonth,thisisequivalenttoaround30kgoffoodand6090litersoffluid.Yet,ingeneral,bodyweightremainsremarkablyconstant.Suchindividualsaresaidtobeinbalance;theintakeoffoodanddrinkmatchestheamountsusedtogenerateenergyfornormalbodilyactivitiesplusthelossesinurineandfeces.Insomecircumstances,suchasstarvation,intakedoesnotmatchtheneedsofthebodyandmuscletissueisbrokendowntoprovideglucoseforthegenerationofenergy.Here,theintakeofproteinislessthantherateofbreakdownandtheindividualissaidtohaveanegativenitrogenbalance.Equally,ifthebodytissuesarebeingbuiltup,asisthecaseforgrowingchildren,pregnantwomenandathletesintheearlystagesoftraining,thedailyintakeofproteinisgreaterthanthenormalbodyturnoverandtheindividualisinpositivenitrogenbalance.21.平衡的概念可以應(yīng)用到機(jī)體的任何構(gòu)成成分上，包括水和鹽，而且平衡在機(jī)體調(diào)節(jié)其自身成分上是非常重要的。攝入必須等于所需，為維持機(jī)體平衡，任何多余的能量都必須排出。此外，因?yàn)闄C(jī)體的每種化學(xué)成分都有一個(gè)可取的濃度范圍，控制機(jī)制維持這個(gè)范圍。例如，兩餐間血糖濃度大約為4-5mmol.l1.進(jìn)食后不久，血糖含量超過(guò)這一范圍，刺激胰腺分泌胰島素，降低濃度。隨著葡萄糖濃度的下降，胰島素分泌減少。在此情況下，循環(huán)胰島素水平的改變都是為了使血漿中的葡萄糖維持在一個(gè)合適的范圍內(nèi)。這種調(diào)節(jié)稱為負(fù)反饋機(jī)制。在胰島素分泌期間，葡萄糖像肝糖原或脂肪一樣被儲(chǔ)存。21.Thisconceptofbalancecanbeappliedtoanyofthebodyconstituentsincludingwaterandsaltandisimportantinconsideringhowthebodyregulatesitsowncomposition.Intakemustmatchrequirementsandanyexcessmustbeexcretedforbalancetobemaintained.Additionally,foreachchemicalconstituentofthebodythereisadesirableconcentrationrange,whichthecontrolmechanismsareadaptedtomaintain.Forexample,theconcentrationofglucoseintheplasmaisabout45mmol.l1betweenmeals.Shortlyafterameal,plasmaglucoserisesabovethislevelandthisstimulatesthesecretionofthehormoneinsulinbythepancreas,whichactstobringtheconcentrationdown.Astheconcentrationofglucosefalls,sodoesthesecretionofinsulin.Ineachcase,thechangesinthecirculatinglevelofinsulinacttomaintaintheplasmaglucoseatanappropriatelevel.Thistypeofregulationisknownasnegativefeedback.Duringtheperiodofinsulinsecretion,theglucoseisbeingstoredaseitherglycogenorfat.22.負(fù)反饋環(huán)是在機(jī)體出現(xiàn)紊亂時(shí)，將一些變量控制在限定范圍內(nèi)的一個(gè)控制系統(tǒng)。雖然上面的例子講到血糖，但這一基本原則可以應(yīng)用到其它生理變量中如體溫、血壓和血漿的滲透濃度。負(fù)反饋環(huán)需要一種能對(duì)不確定的變量做出反應(yīng)而對(duì)其它生理變量不應(yīng)答的傳感器。因此，滲透壓感受器應(yīng)該能對(duì)機(jī)體體液滲透的變化而不是體溫和血壓的變化產(chǎn)生應(yīng)答。感受器傳遞的信息必須和理想水平（系統(tǒng)的調(diào)定點(diǎn)）以比較者的身份，以某種方式進(jìn)行比較。如果兩者不相符，一個(gè)錯(cuò)誤信號(hào)就會(huì)傳遞給效應(yīng)器，效應(yīng)器是一種能使變量保持在理想水平的系統(tǒng)。負(fù)反饋的這些特點(diǎn)可以通過(guò)檢測(cè)一種簡(jiǎn)單的加熱系統(tǒng)來(lái)理解。被控制的變量是室溫，它可以由一個(gè)溫度計(jì)檢測(cè)到，效應(yīng)器是一種加熱器。當(dāng)室溫降低到調(diào)定點(diǎn)以下時(shí)，溫度計(jì)就可以監(jiān)測(cè)到溫度的變化而開(kāi)啟加熱器，對(duì)室內(nèi)進(jìn)行加溫，直到室溫升高到先前調(diào)好的調(diào)定點(diǎn)，加熱器關(guān)閉。23.anegativefeedbackloopisacontrolsystemthatactstomaintainthelevelofsomevariablewithinagivenrangefollowingadisturbance.Althoughtheexamplegivenabovereferstoplasmaglucose,thebasicprinciplecanbeappliedtootherphysiologicalvariablessuchasbodytemperature,bloodpressure,andtheosmolalityoftheplasma.anegativefeedbacklooprequiresasensorofsomekindthatrespondstothevariableinquestionbutnottootherphysiologicalbariables,thusanosmoreceptorshouldrespindtochangesinosmolalityofthebodyfluidsbutnottochangesinbodytemperatureorbloodpressure.theinformationfromthesensormustbecomparedinsomewaywiththedesiredlevelbysomeformofcomparator.ifthetwodonotmatch,anerrorsignalistransmittedtoaneffector,asystemthatcanacttorestorethevariabletoitsdesiredlevel.thesefeaturesofnegativefeedbackcanbeappreciatedbyexaminingasimpleheatingsystem.thecontrolledvariableisroomtemperature,whichissensedbyathermostat.theeffectorisaheaterofsomekind.whentheroomtemperaturefallsbelowthesetpoint,thetemperaturedifferenceisdetectedbythethermostatwhichswitchesontheheater.thisheatstheroomuntilthetemperaturereachesthepersetlevelwhereupontheheaterisswitchedof