WJEC生物 稳态 负反馈 肾单位 ADH 渗透调节
WJEC 的 A Level Biology 把稳态安排在 A2 Unit 3 Energy, Homeostasis and the Environment 里，第 7 个主题的名字就是 Homeostasis and the kidney。WJEC GCE Biology 考纲（2015 年起施教）列出的考查范围从动态平衡、负反馈一路排到肾单位结构、ADH、肾衰竭治疗，最后落在不同环境下的髓袢适应。AQA 与 Edexcel 把同类内容拆进人体生理的各个章节，WJEC 则把它们集中在同一份卷子上考。下面按考纲条目的顺序走一遍，每个条目给出机制、常考问法和最容易丢分的位置。
Homeostasis occupies A2 Unit 3 of the WJEC A Level Biology specification, written for first teaching in 2015, and topic 7 there is called Homeostasis and the kidney; the listed requirements run from dynamic equilibrium and feedback through nephron structure, ADH and the treatment of kidney failure, ending with loop of Henle adaptations in different environments. Other boards scatter the same material across separate physiology chapters, whereas WJEC keeps it in one paper. The sections below follow the specification order, pairing each mechanism with the wording examiners use and the steps where marks usually disappear.
动态平衡与负反馈回路
稳态不是把身体参数钉死成一个数字，而是让它们在设定点附近小幅摆动，WJEC 考纲用 dynamic equilibrium 这个词描述这种状态。体温、血糖、血液水势、二氧化碳浓度都按同一套回路运转：感受器检测变化，协调中枢把实测值和设定值比较，效应器把偏差拉回来，拉过头时同一个回路再把它拽回去几次。负反馈的特征是结果反过来抑制起因，偏差越大，纠正信号越强。正反馈做相反的事，它放大原来的刺激，考纲的原话是 enhancing the size of a stimulus，分娩时不断加剧的宫缩、神经轴突去极化都是这类例子。很多学生以为稳态就是数值保持不变，答题时只写一句「维持稳定」，没有交代感受器、协调中枢、效应器分别是谁，这样的答案通常停在最低档。
Dynamic equilibrium is the specification's own phrase, and it means internal conditions are held inside a narrow band rather than pinned to one value; each loop contains the same three components: a receptor that detects the deviation, a coordination centre that compares it with the set point, and an effector that reverses it. Negative feedback works because the outcome switches off its own cause, while positive feedback amplifies the stimulus that started it, as in labour contractions and the depolarisation of an axon. Sentences such as the body keeps things stable are worth nothing on their own, because the mark scheme is checking for the three named components.
肾单位结构与超滤作用
肾单位从肾小体起步，肾小球是一团盘绕的毛细血管，血液由入球小动脉进入、出球小动脉离开，出球小动脉管径更细，于是肾小球内部的血压被顶得很高。血流经过时，水、葡萄糖、氨基酸、无机盐和尿素被这个压力压过三层结构：毛细血管内皮、基膜、足细胞之间的裂孔，进入肾小囊形成滤液。血细胞和血浆蛋白体积太大，被挡在血液里，所以滤液和血浆相比只少了蛋白质。这道滤过屏障只按分子大小筛选，不做主动选择。成年人两侧肾脏每分钟产生约 125 立方厘米滤液，一天下来接近 180 升，而最终排出的尿液只有 1 到 2 升，中间差出来的体积全部由肾小管重吸收。考题常要求先写压力来源、再解释蛋白质为何不出现，两问各占一半分值。
Ultrafiltration runs on a pressure gradient, not on selective transport; because the afferent arteriole is wider than the efferent one, glomerular blood pressure stays high enough to force water, glucose, amino acids, salts and urea through the capillary endothelium, the basement membrane and the gaps between podocytes, while blood cells and plasma proteins are held back by size. The filtrate therefore differs from plasma only in its protein content; roughly 125 cm3 forms each minute, close to 180 dm3 a day, and since only one or two dm3 leaves the body as urine, the tubules must reclaim nearly all of it. A frequent half-answer gives the structure but never names the pressure gradient that drives the process.
近曲小管的选择性重吸收
近曲小管承担了重吸收的主要工作量，管壁细胞朝管腔的一面长出密集的微绒毛，把可用表面积放大许多倍，细胞质里线粒体排得密，因为这里的搬运要消耗 ATP。管腔内的钠离子顺着浓度梯度进入上皮细胞，同时把葡萄糖或氨基酸一起带进来，这叫协同运输；进入细胞的钠随后被基侧膜上的钠钾泵打进组织液，细胞内保持低钠，管腔里的钠才能继续往里走。葡萄糖和氨基酸从组织液回到血液，水顺着渗透梯度跟随移动，滤液在近曲小管末端基本恢复等渗。常见的错误是把重吸收写成「靠扩散自动完成」，把协同运输和被动扩散混在一起。考纲要求你指出两种细胞适应特征并说明作用，答案必须落在微绒毛和线粒体上，只写「细胞很薄」拿不到分。
The proximal tubule does most of the reclaiming; its cells carry a dense brush border of microvilli that multiplies the surface area, and they are packed with mitochondria because the transport work burns ATP. Sodium in the lumen moves down its concentration gradient into the cell and drags glucose or amino acids with it, a process called co-transport, after which the sodium-potassium pump in the basal membrane pushes sodium into tissue fluid and keeps the intracellular concentration low; glucose and amino acids then pass into the blood, and water follows osmotically, so the filtrate leaves the tubule close to isotonic. Candidates who describe reabsorption as simple diffusion lose the explanation marks, since the mark scheme wants co-transport named alongside active transport.
髓袢 逆流倍增与髓质渗透梯度
髓袢的形状像发夹，降支对水通透但对溶质不通透，滤液往下走时水不断渗出，管内液体越来越浓；升支对水不通透，却把钠离子和氯离子主动泵进组织液，管内液体越走越稀。两支方向相反、作用相反，配合起来就成了逆流倍增器，把髓质组织液的渗透浓度从外层的低值一路抬到内层的高值。直小血管紧贴髓袢走，同样呈发夹形，血液在里面先降后升，溶质被交换回髓质，梯度不会被血流冲走。集合管最后穿过这片高渗区域，管内的水才有机会被抽出去。沙漠动物的髓袢特别长，髓质梯度更陡，尿液浓度可以远高于血浆；淡水鱼的情况反过来，髓袢短，水不断从体表渗入，肾脏要排出大量稀薄的尿液。
The loop of Henle works as a counter-current multiplier; its descending limb is permeable to water but not to solute, so the filtrate becomes more concentrated as it travels down, while the ascending limb is impermeable to water and pumps sodium and chloride ions out into the medulla, leaving the fluid inside progressively more dilute. Because the two limbs run in opposite directions and do opposite jobs, they build a steep osmotic gradient through the medulla; the vasa recta hairpin exchanges solute back into the tissue fluid so the gradient is not washed away, and the collecting duct then passes through that region, which is the only reason water can be drawn out of it. Desert species carry very long loops and produce urine far more concentrated than plasma, while freshwater fish face the opposite problem, taking in water across the body surface and needing to excrete a large volume of dilute urine.
抗利尿激素与集合管
下丘脑里的渗透压感受器监测血液水势，血浆浓度升高时信号传到垂体后叶，垂体后叶释放抗利尿激素进入血液。WJEC 考纲把垂体后叶分泌 ADH 作为内分泌腺参与稳态平衡的指定例子，所以这条链路要背到能默写：血浆浓度升高、渗透压感受器、垂体后叶、ADH、集合管、水通道蛋白插入细胞膜、水被重吸收、尿量减少而尿液变浓。集合管的重吸收量增加以后，血液水势回到设定点，ADH 分泌跟着下降，回路自动收口。学生最常把 ADH 说成「促进排尿」，方向整个反了。考纲还要求区分尿崩症与糖尿病：前者是 ADH 分泌不足或肾脏不响应，排出大量稀尿液；后者是血糖过高超出肾小管重吸收能力，尿里出现葡萄糖。饮酒后尿量增多，也是因为酒精抑制了 ADH 的分泌。
Osmoreceptors in the hypothalamus monitor the water potential of the blood, and when plasma becomes too concentrated they trigger the posterior pituitary to release antidiuretic hormone; WJEC names that posterior pituitary secretion of ADH as its set example of endocrine involvement in homeostasis, so the chain has to be reproduced exactly: raised plasma concentration, osmoreceptors, posterior pituitary, ADH, aquaporins inserted into collecting duct membranes, more water reabsorbed, smaller volume of more concentrated urine. Once water potential returns to the set point, ADH release falls again and the loop closes; learners regularly reverse the hormone and claim it promotes urine production. Diabetes insipidus involves too little ADH or a kidney that does not respond, producing large volumes of dilute urine, while diabetes mellitus spills glucose because the filtrate load exceeds what the tubules can reabsorb.
血糖调节 胰岛素与胰高血糖素
在 WJEC 的 Unit 3 里，考纲点名的内分泌例子只有 ADH，胰岛素与胰高血糖素属于人体生理的延伸部分，AQA 7402 的 3.6.4 与 Edexcel Biology A 9BN0 则把血糖调节写成独立条目，跨考试局复习时值得一并理清。胰岛的贝塔细胞感知血糖升高后分泌胰岛素，让肝细胞把葡萄糖转成糖原储存，同时加快细胞对葡萄糖的摄取与呼吸消耗；阿尔法细胞在血糖下降时分泌胰高血糖素，把糖原分解回葡萄糖，还能用氨基酸和甘油合成新葡萄糖。两条通路互为反向，血糖高时胰岛素压制起因，血糖低时胰高血糖素补上缺口，都靠负反馈收尾。一型糖尿病是胰岛贝塔细胞被自身免疫破坏，患者需要外源胰岛素；二型糖尿病多与靶细胞对胰岛素响应变弱有关，与体重和活动量关系密切。答题时把两类糖尿病的成因写混，是这一节最常见的失分点。
Inside WJEC Unit 3 the only endocrine example named in the specification is ADH, while insulin and glucagon sit in the wider physiology work; AQA 7402 section 3.6.4 and Edexcel Biology A 9BN0 treat blood glucose as a topic in its own right, so it repays revision across boards; beta cells in the islets of Langerhans release insulin when blood glucose rises, prompting liver cells to store glucose as glycogen and increasing uptake and respiration in body cells. Alpha cells release glucagon when glucose falls, breaking glycogen down again and building new glucose from amino acids and glycerol; the two hormones oppose each other and both loops close by negative feedback. Type 1 diabetes follows autoimmune destruction of beta cells and needs injected insulin, whereas type 2 usually involves reduced responsiveness in target cells and is closely tied to body mass and activity levels.
肾衰竭 透析与移植的取舍
肾功能下降到无法维持内环境时，尿素和肌酐在血液里累积，钾、钠、磷酸盐的浓度失衡，水排不出去引起水肿和血压升高，红细胞生成素与活性维生素 D 的合成也一并受影响。血液透析把血液引到体外，经半透膜与透析液交换，尿素等小分子顺浓度梯度被带走，一周要去医院三次、每次三到四小时，饮食上还得限制蛋白质、钾和液体摄入。腹膜透析利用患者自己的腹膜当半透膜，透析液灌进腹腔再放出，可以在家操作，代价是腹腔感染的风险和每天多次换液的流程。肾移植的长期生活质量最好，但要等合适的供体、配型要过关，术后长期服用免疫抑制剂来压低排异反应，感染风险随之上升。考纲特意点到器官来源与费用带来的伦理问题，论述题里把治疗方式、代价、风险逐一对照再给出判断，比单写「移植最好」得分高得多。
When kidney function falls far enough, urea and creatinine accumulate, potassium, sodium and phosphate drift out of range, water is retained and blood pressure climbs, while erythropoietin and active vitamin D production also suffer; haemodialysis passes blood through a machine where it meets dialysis fluid across a partially permeable membrane, letting small molecules such as urea leave down their concentration gradient; sessions run three times a week for three to four hours and come with restrictions on protein, potassium and fluid intake. Peritoneal dialysis uses the patient's own peritoneum as the membrane and can be managed at home, at the cost of infection risk and repeated fluid exchanges; A transplant offers the best long-term quality of life but depends on a matched donor and lifelong immunosuppressants, which raise the risk of infection. WJEC highlights the ethical questions raised by organ supply and cost, and essays that weigh each treatment against its drawbacks score better than a blanket preference for transplantation.
沙漠与淡水动物的排泄适应
不同栖息地排出的含氮废物类型不同，这是水分收支的结果而不是随意选择。硬骨鱼把氨直接排进水中，氨毒性大但极易溶于水，周围水体随时可以稀释；软骨鱼把氨转成尿素留在血液里，用略高于海水的体液渗透浓度减少失水。爬行类、鸟类和陆生昆虫把氨转成尿酸，尿酸溶解度低，可以以半固体形式排出，损失的水分很少。哺乳动物选择尿素，因为尿素毒性低于氨、又能用较少的水带走。髓袢长度与环境的水分供应也对应：沙漠啮齿类的髓袢极长，髓质梯度陡，尿液浓度可以显著高于血浆；淡水鱼髓袢短而肾小球大，主要任务是排出渗入体内的大量水分。这一节的问法通常是给出某种动物的栖息地与尿浓度数据，让你解释结构与功能的对应关系，回答时把髓袢长度、逆流倍增和可用水量三点连起来，不要只描述数据。
Which nitrogenous waste an animal makes follows from its water budget rather than from chance; bony fish release ammonia straight into the surrounding water, where it is highly soluble and quickly diluted; cartilaginous fish convert it to urea and keep it in the blood, using a body fluid slightly more concentrated than seawater to reduce water loss. Reptiles, birds and terrestrial insects convert ammonia to uric acid, which is barely soluble and can be voided as a semi-solid paste, conserving water; mammals settle on urea because it is less toxic than ammonia yet still cheap to excrete in water. Loop length tracks the water available in the habitat, so desert rodents carry extremely long loops, a steep medullary gradient and urine far more concentrated than plasma, while freshwater fish have short loops and large glomeruli to dump the water that enters across their gills and skin.
牛津AQA国际GCSE生物?OxfordAQA International GCSE Biology (9201): Onl?进口原版¥399.0📱 去京东购买京东精选
需要一对一辅导？Need One-on-One Tutoring?
A-Level / GCSE / IB / KS2-KS3 全科辅导
16621398022 同微信
Visit aleveler.com
或截图后用「扫一扫 → 相册」选择本图