医学英语综合教程课文和翻译

Unit2.CellsandAgingAgingisanormalprocessaccompaniedbyaprogressivealterationofthebody’shomeostaticadaptiveresponses;thespecializedbranchofmedicinethatdealswiththemedicalproblemsandcareofelderlypersoniscalledgeriatrics.衰老是一种正常的生理过程，伴有机体内环境稳定适应性应答的进行性改变。研究老年人的健康问题和保健的一门特殊医学分支被称作老年病学Theobviouscharacteristicsofagingarewellknown;grayingandlossofhair,lossofteeth,wrinklingofskin,decreasedmusclemass,andincreasedfatdeposits.Thephysiologicalsignsofagingaregradualdeteriorationinfunctionandcapacitytomaintainaconstantinternalenvironment(homeostasis)inresponsetochangesintemperature,diet,andoxygensupply.Thesesingsofagingarerelatedtoanetdecreaseinthenumberofcellsinthebodyandtothedysfunctioningofthecellsthatremain.衰老的明显特征众所周知：头发花白和脱落，牙齿脱落，皮肤起皱，肌肉减少，脂肪积存增加。衰老的生理征兆是机体对环境刺激反应的功能和能力的逐渐退化如同维持机体内环境稳定来应对温度，饮食和氧气供应改变的能力减退一样，机体新陈代谢也减慢了。衰老的这些迹象与机体中细胞数目的净减少以及存余细胞的功能紊乱有关。Theextracellularcomponentsoftissuesalsochangewithage.Collagenfibers,responsibleforthestrengthintendons,increaseinnumberandchangeinqualitywithaging.Thesechangesinthecollagenofarterialwallsareasmuchresponsiblefortheirlossofextensibilityasarethedepositsassociatedwithatherosclerosis,thedepositionoffattymaterialsinthearterialwalls.Elastin,anotherextracellularcomponent,isresponsiblefortheelasticityofbloodvesselsandskin.Itthickens,fragments,andacquiresagreateraffinityforcalciumwithage–changesthatmayalsobeassociatedwiththedevelopmentofatherosclerosis.组织的细胞外成分也随着年龄的变化而变化。负责肌腱力量的胶原纤维的数量增加，而质量却随着衰老降低。动脉壁上的胶原质的变化与动脉壁伸展性丧失有关，正如动脉粥样硬化，即动脉壁脂肪物质堆积造成动脉壁伸展性缺失一样。弹性蛋白是另一种细胞外成分，与血管和皮肤的弹性有关，随着年龄的变化，它变粗，易断裂并且与钙有很大的亲和力，这些变化可能也与动脉粥样硬化的发生有关。Glucose,themostabundantsugarinthebody,mayalsoplayaroleintheagingprocess.Accordingtoonehypothesis,glucoseisadded,haphazardly,toproteinsinsideandoutsidecells,formingirreversiblecross-linksbetweenadjacentproteinmolecules.Asapersonages,morecross-linksareformed,andthisprobablycontributestothestiffeningandlossofelasticityinagingtissues.葡萄糖是机体中含量最多的糖类，它在衰老的过程中可能也起作用。根据一个假说，随机地在细胞内外蛋白质上添加葡萄糖，结果会在相邻蛋白质分子之间形成不可逆的交联。随着人的衰老，会形成更多的交联，这很可能就是造成衰老的组织僵硬及弹性丧失的原因。Althoughmanymillionsofnewcellsnormallyareproducedeachminute,severalkindsofcells–heartcells,skeletalmusclefibers,nervecells-cannotbereplaced,Experimentshaveshownthatmanyothercelltypeshaveonlyalimitedcapabilitytodivide.Cellsgrownoutsidethebodydivideonlyacertainnumberoftimesandthenstop.Thenumberofdivisionscorrelateswiththedonor/sageandwiththenormallifespanofthedifferentspeciesfromwhichthecellsareobtained.Theseobservationsprovidesstrongevidenceforthehypothesisthatcessationofmitosisisanormal,geneticallyprogrammedevent.Accordingtothisview,an“aging”geneispartofthegeneticblueprintatbirth,anditturnsonatapreprogrammedtime,slowingdownorhaltingprocessesvitaltolife.虽然正常情况下，每分钟会有好几百万的新细胞产生，但有几种细胞：心脏细胞，骨骼肌纤维细胞，神经细胞是无法替代的。实验显示，许多种类的细胞分裂能力有限。在机体外生长的细胞仅仅分裂几代就停止了。细胞分裂数与捐献者的年龄和这些细胞来自的不同物种的正常寿命有关。这些发现为这种假说提供了有力的证据，即（细胞的）有丝分裂的终止是正常的，是由基因决定的。根据这个观点，衰老基因是出生时就存在的基因蓝图的一部分，在预定的时间里开启，减慢或停止对生命至关重要的生理过程。Anothertheoryofagingisthefreeradicaltheory.Freeradicalsareelectricallychargedmoleculesthathaveanunpairedelectron.Suchmoleculesareunstableandhighlyactiveandcaneasilydamageproteins.Someeffectsarewrinkledskin,stiffjoints,andhardenedarteries.FreeradicalmayalsodamageDNA.Amongthefactorsthatproducefreeradicalsareairpollution,radiation,andcertainfoodsweeat.OthersubstancesinthedietsuchasvitaminE,vitaminC,beta-carotene,andseleniumareantioxidantsandinhibitfreeradicalformation.Thefreeradicaltheoryofagingisbolsteredbytworecentdiscoveries.Strainsoffruittfliesbredforlongevityproducelarger-than-normalamoutsofanenzymecalledsuperoxidedismutase,whichfunctionstoneutralizefreeradicals.Also,injectionofgenesthatleadtoproductionofsuperoxidedismutaseintofruitflyembryosprolongstheiraveragelifetime.衰老的另一个理论即自由基理论，自由基是含有未配对电子的带电荷分子。这种分子性质不稳定，具有高反应活性，容易损害蛋白质。自由基的影响有：皮肤起皱，关节僵直，动脉硬化。自由基也可以损害DNA。产生自由基的因素有：空气污染，放射线，以及我们吃的某些食物。饮食中的其他物质如维生素E，维生素C，β-胡萝卜素以及硒都是抗氧化剂，可以抑制自由基的形成。最近的两个研究支持了衰老的自由基理论。孕育长寿的果蝇株从而产生超正常量的酶：过氧化物歧化酶，它可以中和自由基。同样，把产生过氧化物歧化酶生成的基因注入果蝇胚胎中会延长其平均寿命。Whereassometheoriesofagingexplaintheprocessatthecellularlever,othersconcentratesonregulatorymechanismsoperatingwithintheentireorganism.Forexample,theimmunesystem,whichmanufacturesantibodiesagainstforeigninvaders,maystarttoattackthebody’sowncells.Thisautoimmuneresponsemightbecausedbychangesinthesurfacesofcells,causingantibodiestoattachandmarkthecellfordestruction.Assurfacechangesincellsincrease,theautoimmuneresponseintensifies,producingthewell-knownsignsofaging.然而，关于衰老的理论，有些是在细胞水平上解释其过程，有的则强调整个机体内运作的调节机制，比如免疫系统产生各种抗抗外来异物的抗体，也可能会对自体细胞发起攻击。这种自身免疫应答可能是细胞表面的变化造成，引起抗体附着并标记细胞进行破坏。当细胞表面变化增加，自身免疫应答加强，产生众所周知的衰老的标志。