Unit11Normally,therateofcellreproductionandtheprocessofcelldifferentiationarepreciselycontrolledsothatbothofthesemechanismsceaseoncetheappropriatenumbersandtypesofcellsareformed.2Cellwallsareverythinsothatoxygenandcarbondioxidecaneasilygetthrough.3Manycells,especiallythoseofthecentralnervoussystemandtheheart,areextremelysensitivetolowbloodlevelsofoxygen,sothatlackofoxygenresultsinseriousdamage.4Theundifferentiatedstemcellsthatcanbetriggeredtoenterthecellcyclearedifferentfromthecellsthatcontinuetodivideandreproduce.5Inthebloodtherearemanyredbloodcellsthatserveasvehiclesforcarryingoxygen.6Besidesneurons,thereareotherwell-differentiatedcellsthatareunabletodivide.1细胞的专化程度越高，失去有丝分裂能力的可能性越大。神经元是体内分化程度最高的，一旦神经系统的发育得以完成，它们就失去了分裂和繁殖的能力。2在此情况下，体内有属于同一谱系的母细胞，这些细胞尚未分化到失去分裂能力的程度。它们已经得到充分的分化，因而它们产生的子细胞限定于同一细胞体系；但尚未分化到丧失积极增生潜力的程度。Uint2Keytoguidedtranslation1Althoughallpathogensposepotentialthreatsiftheygainentrytothebody,thechancesofthisoccurringareactuallyquitesmall.2Althoughpainisoftennotconsideredanactualbodydefensemechanism,theroleofpaininthebody'sresponsetoinvasioncannotbeignored.3Referredpainmeansthatthepaincanbepresentinagivenplacealthoughthesourcemaybeelsewhere.4Astheprocesscontinues,monocytesexitthebloodandmatureintomacrophagesinthetissueenvironment.5Asbodytemperaturerises,thebodyisstimulatedtoproducemorewhitebloodcells.6Asyourbodyproducesitsownantibodies,youwillnothavetoworryaboutsubsequentexposurestothedisease.1虽然发炎的影响因其令人不快和造成不适而常被认为是不好的，但发炎过程就其实质而言对人体却是有利的，它使人能应付日常压力的影响。2小小一块局部红肿就足以防止蚊叮产生伤害，而严重的情况，如阑尾炎，则可引起发烧、白细胞增多，乃至大量体液和蛋白质聚集到炎症部位。Unit31Ithasbeenprovedthatmostcongenitalcataractsarenotprogressiveandarenotdenseenoughtocausesignificantvisualimpairment.2Ithasbeenreportedthatoverexposuretoheatortoionizingradiationmaycausetraumaticcataracts.3Itwasthoughtbysomethatmuchuseoftheeyesforschoolworkmayhaveplayedapartincausingmyopia;butithasbeenfoundthatpeoplewhodonotdoanyreadingsufferfromtheproblemtoasgreatadegree.4Pressurewithintheeyemayincrease,slowly