Unit 11 Normally, the rate of cell reproduction and the process of cell differentiation are precisely controlled so that both of these mechanisms cease once the appropriate numbers and types of cells are formed.2 Cell walls are very thin so that oxygen and carbon dioxide can easilyget through.3 Many cells, especially those of the central nervous system and the heart ,are extremely sensitive to low blood levels of oxygen ,so that lack of oxygen results in serious damage.4 The undifferentiated stem cells that can be triggered to enter the cell cycle are different from the cells that continue to divide and reproduce.5 In the blood there are many red blood cells that serve as vehicles for carrying oxygen.6 Besides neurons, there are other well-differentiated cells that are unable to divide. 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。Uint2Key to guided translation 1 Although all pathogens pose potential threats if they gain entry to the body, the chances of this occurring are actually quite small.2Although pain is often not considered an actual body defense mechanism, the role of pain in the bodys response to invasion cannot be ignored.3 Referred pain means that the pain can be present in a given place although the source may be elsewhere.4As the process continues, monocytes exit the blood and mature into macrophages in the tissue environment.5 As body temperature rises, the body is stimulated to produce more white blood cells.6 As your body produces its own antibodies, you will not have to worry about subsequent exposures to the disease.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。 Unit 3 1 It has been proved that most congenital cataracts are not progressive and are not denseenough to cause significant visual impairment.2It has been reported that overexposure to heat or to ionizing radiationmay causetraumaticcataracts.3It was thought by some that much use of the eyes for school work may have played a part in causing myopia; but it has been foundthat people who do not do any reading suffer from the problem to as great a degree.4 Pressure within the eye may increase, slowly destroyingthe optic nerve.5 Blinking your eyes more often at work spreads a tear film over the eyes, keeping them moist.6When the ciliarymuscle contracts, the ligament relaxes and the lens becomes more nearly spherical, enabling the eye to focus on objects that are nearer to the eye. 1 晶状体中心浑浊可能隔断视轴，造成可见到两个或多个模糊视象的视觉缺损。在进行眼底镜检查时，白内障可表现为肉眼可见的遮住瞳孔的混浊，或者由眼底红色背景衬映的混浊。2 美国眼科学会已就何时应进行手术制定了标准：即病眼的最佳矫正视觉敏感度在20/50或以下。然而，如果有妨碍正常活动的眩目或无法正常工作，或者白内障有可能导致其他眼病，如继发性青光眼和色素层炎，则最佳矫正视力在20/40甚至更好一些，仍然可能适宜进行手术