WJEC生物 稳态 负反馈 血糖体温 肾单位ADH
稳态是 WJEC A level 生物学 Unit 3 里跨度最大的一块内容，从体温一直考到肾脏的渗透调节，中间还夹着血糖与激素。这一节的题目很少单独考定义，多半给一段实验数据或一张肾单位示意图，让考生把感受器、协调中心、效应器一串起来。这篇按 topic 3.7 的排法把主线走一遍，每个知识点后面都标出常见的失分写法。
Homeostasis stretches across the whole of Unit 3, from thermoregulation to the kidney, and exam questions rarely ask for a definition on its own. They usually supply data or a nephron diagram and expect a chain of receptor, coordination centre and effector. The notes below follow the WJEC sequence for topic 3.7 and flag the answers that quietly lose marks.
稳态的定义与影响酶活性的变量
稳态指的是把细胞外液的温度、水势、pH 与血糖浓度维持在一个窄范围内，而不是让这些数值固定不动。细胞泡在组织液里，组织液由血浆渗出形成，它的水势一变，细胞就会吸水膨胀或者失水皱缩。酶对温度与 pH 最敏感，偏离最适值几个单位，活性位点的形状就会改变，反应速率跟着掉下来。很多学生以为稳态就是不变，其实它是动态平衡，数值在设定点附近来回摆动。维持稳态还有一个容易被忽略的好处：生物体不再完全受外界环境摆布，可以迁移到更宽的地带活动。WJEC 的 specification 在 3.7 这一节里把核心体温、血糖浓度与血液水势并列为考查对象，这三条主线会轮流出现在同一张卷子上。
Homeostasis holds the internal environment inside narrow limits rather than frozen at a single value. Cells sit in tissue fluid, so a shift in its water potential makes them swell or crenate, and enzymes lose their shape once temperature or pH drifts from the optimum. WJEC names core temperature, blood glucose and blood water potential as the conditions to be examined in topic 3.7, and a strong answer always states which variable is being controlled before describing the control system, because the examiner is checking a sequence rather than a definition.
负反馈回路的五个环节与正反馈的例外
把回路拆开看，任何一条负反馈都包含设定点、感受器、协调中心、效应器与纠正反应五个环节，答题时漏掉任何一个都会被扣分。感受器负责检测偏离，协调中心多半是下丘脑或者胰岛，效应器则是腺体或肌肉。负反馈的意思是纠正方向与偏离方向相反，温度升上去就想办法散热。正反馈只在少数场合出现，例如分娩时子宫收缩引起催产素释放，收缩更强，偏离被放大而不是被拉回。阅卷时最常见的是把正反馈当成负反馈写反，句子读起来通顺，逻辑却整段作废。
A negative feedback loop is marked on five parts: set point, receptor, coordination centre, effector and corrective response. Positive feedback does the opposite and amplifies the deviation, which is why it appears only in special cases such as oxytocin driving stronger uterine contractions during labour. Marks disappear when a candidate describes the response but never states the direction of correction.
体温调节中的皮肤血流与骨骼肌战栗
体温调节的感受器分两处，皮肤里的外周温度感受器负责报告环境冷暖，下丘脑里的中枢感受器监控血液温度。体温高于设定点时，供应皮肤毛细血管的小动脉舒张，更多血液流近体表，辐射与对流带走热量，汗腺分泌增加，汗液蒸发再带走一部分潜热。体温低于设定点时，这些小动脉收缩，血液改走深层血管，散热减少，骨骼肌出现不自主的战栗，收缩产热增加。立毛肌收缩在人类身上几乎起不到保温作用，写进答案只能算补充，不能当成主要机制。常见的错误是写成血管收缩使血流加快，方向完全相反。
Skin thermoreceptors report the outside world while receptors in the hypothalamus monitor blood temperature. Heat loss rises when the arterioles feeding skin capillaries dilate and sweating increases, and heat is conserved by vasoconstriction while shivering generates extra heat in muscle. Human hair erection is often listed as insulation, yet it contributes very little and should not be presented as a main mechanism.
胰岛 β 与 α 细胞的激素分工
血糖的协调中心是胰腺里的朗格汉斯岛，β 细胞分泌胰岛素，α 细胞分泌胰高血糖素，两种细胞对同一个信号给出相反的反应。血糖升高时，进入 β 细胞的葡萄糖增多，呼吸作用加快，ATP 增加，钾通道关闭，细胞膜去极化，钙离子内流，含胰岛素的囊泡通过胞吐释放。血糖降低时 α 细胞释放胰高血糖素，靶器官主要是肝脏。两种激素都不通过神经冲动传递，属于体液调节，起效比神经慢，作用时间更长。很多学生以为胰岛素直接分解葡萄糖，正确的说法是它改变细胞膜上转运蛋白的数量与活性，让葡萄糖进入细胞。
The islets of Langerhans hold two cell types working in opposite directions: beta cells release insulin when glucose rises, alpha cells release glucagon when it falls. Insulin secretion runs through glucose entry, faster respiration, ATP-sensitive potassium channels closing, depolarisation and calcium-driven exocytosis of vesicles. Neither hormone travels along a neurone, so their effects start more slowly and last longer than a nervous response.
胰岛素与胰高血糖素在肝细胞里的两条通路
胰岛素与肝细胞、肌细胞膜上的受体结合，触发细胞内信号，让含葡萄糖转运蛋白的囊泡与细胞膜融合，葡萄糖顺浓度梯度进入细胞。进入肝细胞的葡萄糖被磷酸化后难以再流出，接着在酶催化下连成糖原，这个过程叫糖原生成。胰岛素还促进葡萄糖在呼吸作用中被氧化，并抑制糖原分解与糖异生。胰高血糖素走的是反向路径，它促进糖原分解为葡萄糖，也促进用氨基酸与甘油合成葡萄糖，后者叫糖异生。肾上腺素在紧张或运动时也升高血糖，机制与胰高血糖素相似，但它来自肾上腺髓质而不是胰岛。
Insulin binds receptors on liver and muscle cells and brings glucose transporters to the membrane, so glucose enters and is at once phosphorylated and trapped inside. In the liver it is polymerised into glycogen and oxidised in respiration, while glycogen breakdown and gluconeogenesis are suppressed. Glucagon reverses those steps, and adrenaline from the adrenal medulla behaves in a similar way during exercise or stress.
一型与二型糖尿病的区别与判断依据
一型糖尿病的起因是 β 细胞被自身免疫反应破坏，胰岛素分泌不足，患者通常较瘦且发病较急，治疗依靠注射胰岛素。二型糖尿病的细胞对胰岛素反应减弱，受体数量或信号通路出了问题，β 细胞后期也会逐渐衰竭，治疗先改饮食与运动，再用药物。两型的共同结果是血糖长期偏高，超过肾脏的重吸收能力后葡萄糖留在尿液中，尿液的溶质浓度升高，渗透压把更多水留在管内，患者出现多尿与口渴。这个现象把血糖调节与肾脏的渗透调节连在一起，是 WJEC 长答题喜欢用的跨节考点。判断两型不能只看年龄，β 细胞功能与自身抗体提供的证据更直接。
Type 1 diabetes follows autoimmune destruction of beta cells, so insulin has to be injected, while type 2 starts with cells responding poorly to insulin and later involves failing beta cells. Both raise blood glucose until the reabsorption capacity of the kidney is exceeded, and the resulting solute-rich urine pulls water out with it, producing polyuria and thirst. Examiners reward candidates who link this to osmoregulation instead of describing the two topics separately.
肾单位的结构与皮质髓质的分工
肾脏外面是皮质，里面是髓质，最内侧的肾盂连着输尿管，每个肾大约有一百万个肾单位。肾单位从肾小体开始，肾小体由肾小球与包曼氏囊组成，位置在皮质；接着是近曲小管，也在皮质；然后髓袢伸进髓质，再回到皮质的远曲小管，最后汇入集合管。血液由肾动脉进入，经入球小动脉进肾小球，再由出球小动脉离开，滤过的血液最后从肾静脉返回循环。环绕髓袢的毛细血管叫直小血管，它们与髓袢平行走向，这个排列是后面理解逆流机制的前提。画图题里最容易丢分的是把髓袢画成横穿皮质，方向一反，整题的逻辑都站不住。
Each kidney holds roughly a million nephrons, and their position explains their job: the renal corpuscle and proximal tubule lie in the cortex, the loop of Henle dips into the medulla and the collecting duct returns towards the pelvis. Blood arrives through the renal artery, passes the afferent arteriole into the glomerulus and leaves by the efferent arteriole before draining into the renal vein. Drawing the loop across the cortex rather than into the medulla is a common error that undermines the rest of the answer.
超滤的三层滤过屏障
超滤的动力来自压力差。入球小动脉的管径比出球小动脉粗，血液进入肾小球后出口更窄，肾小球内的静水压因此升高，把水和小分子压出毛细血管。滤过屏障有三层：毛细血管内皮上的窗孔、基膜、以及足细胞足突之间的裂孔。血细胞与血浆蛋白体积大，被留在血液里，葡萄糖、氨基酸、无机盐、尿素与水则进入包曼氏囊形成滤液。基膜带负电，带负电的蛋白更难通过，这一点常被忽略。很多学生以为超滤是主动运输，其实整个过程不消耗 ATP，靠的是血压。
Ultrafiltration is driven by pressure rather than by ATP. Because the afferent arteriole is wider than the efferent one, hydrostatic pressure builds up in the glomerulus and forces water and small solutes through three layers: the fenestrated endothelium, the basement membrane and the slits between podocyte foot processes. Cells and plasma proteins stay behind, which is why filtrate contains glucose and urea but not albumin, and why protein in the urine is treated as a sign of glomerular damage.
近曲小管的选择性重吸收与协同转运
近曲小管的上皮细胞朝向管腔的一侧有大量微绒毛，把表面积扩大很多倍，细胞内线粒体密集，供能充足。钠离子被主动泵出细胞进入血液，管内钠浓度因此降低，形成浓度梯度。葡萄糖与氨基酸利用这个梯度，通过协同转运蛋白从管腔进入细胞，再靠另一侧的转运蛋白进入血液。水顺着钠与葡萄糖造成的渗透梯度被重吸收，滤液中绝大部分水在这一段回到血液。葡萄糖在正常情况下被全部重吸收，所以健康人的尿液里检测不到葡萄糖，这一条是判断糖尿的关键依据。选择性三个字是答题关键词，它说明不同物质的重吸收比例并不相同。
The proximal tubule does most of the reclaiming, and its structure gives that away: dense microvilli on the lumen side and packed mitochondria behind them. Sodium is pumped out into the blood, and the resulting gradient drags glucose and amino acids in through cotransport proteins, with water following osmotically. Because glucose is normally recovered in full, its presence in urine is a diagnostic signal rather than a normal finding.
髓袢的逆流倍增如何造出髓质高渗
髓袢降支对水通透，对溶质通透性低；升支对水几乎不通透，却主动把钠与氯泵进髓质组织液。滤液在降支里因为水被抽出而浓缩，在升支里因为溶质被抽走而稀释，两个方向相反的流动叠加起来，让髓质从外到内形成越来越高的溶质浓度梯度。这个机制叫逆流倍增，它把髓质变成一个可以吸水的地方。直小血管与髓袢平行，血液下行时失去水得到溶质，上行时失去溶质得到水，浓度梯度因此不会被血流冲走。常见错误是漏掉直小血管，只写髓袢造梯度，答案拿不到完整分数。
The descending limb lets water leave but holds solutes back, while the ascending limb pumps sodium and chloride into the medulla and lets almost no water through. Those opposing flows build a steep solute gradient from cortex to pelvis, which is what makes water reabsorption in the collecting duct possible. Vasa recta run alongside the loop and exchange solutes and water in opposite directions, preserving the gradient that blood flow would otherwise wash away, so the loop and the vasa recta belong in one answer rather than two paragraphs.
ADH 与集合管的含水通道蛋白
血液水势下降时，下丘脑里的渗透压感受器检测到变化，神经垂体释放更多的抗利尿激素。ADH 随血液到达集合管与远曲小管，与膜上受体结合，促使含水通道蛋白插入细胞膜，水的通透性升高，更多水被重吸收回血液，尿量减少，尿液变浓。水势回升后感受器放电减少，ADH 分泌下降，形成一个闭合的负反馈。ADH 分泌不足会得尿崩症，患者排出大量稀尿，与糖尿病多尿的原因完全不同，前者缺激素，后者是溶质负荷过高。答题时把 ADH 写成肾上腺或者垂体前叶分泌是高频错误，它由下丘脑合成、神经垂体释放。
When blood water potential falls, osmoreceptors in the hypothalamus trigger more ADH release from the posterior pituitary. ADH binds receptors on the collecting duct and inserts aquaporins into the membrane, so more water is reclaimed and a smaller volume of concentrated urine is produced. Diabetes insipidus loses that hormone signal, which is a different cause of polyuria from the solute overload seen in diabetes mellitus.
脱氨基、尿素与不同动物的含氮废物
氨基酸不能像脂肪那样大量储存，多余的氨基酸在肝脏里脱去氨基，含氮部分转成氨，再经一系列反应变成尿素。尿素毒性比氨低得多，可以随血液运到肾脏排出，这就是肝脏与肾脏在排泄上的分工。不同动物排泄的含氮废物不同，鱼类多数直接排氨，因为水多，氨可以立刻稀释；鸟类与昆虫排尿酸，尿酸难溶，排出时几乎不带水，适合保水；哺乳动物排尿素，是在毒性与耗水之间取折中。这道题的得分点在于把废物的化学性质与生活环境联系起来，而不是背三种名字。
Surplus amino acids cannot be stored the way fat can, so the liver removes the amino group and converts the nitrogen into urea through an ammonia intermediate. Fish usually excrete ammonia directly, birds and insects excrete uric acid to save water, and mammals settle on urea as a compromise between toxicity and water loss. Marks come from linking each waste product to the habitat rather than from listing three names, and the water cost of excreting each compound is the comparison most questions are really asking for.
肾衰竭的透析与移植原理
肾功能衰竭时，尿素等废物在血液中积累，水与电解质平衡也失控。透析把血液引出体外，经过只允许小分子通过的半透膜，膜另一侧是配好的透析液，其中葡萄糖与无机盐浓度与正常血浆接近，尿素浓度为零。尿素顺着浓度差扩散进透析液，多余的水靠压力差被滤走，处理过的血液回到体内。透析不能替代肾脏的激素功能，患者常需配合药物与限制饮食。肾移植能恢复全部功能，代价是长期服用免疫抑制剂并承担排异风险。比较题要求写清两种方案的机制差别与生活影响，只写优缺点会丢分。
Dialysis pushes blood past a semipermeable membrane against fluid that matches plasma for glucose and salts yet contains no urea, so waste diffuses out and excess water is filtered off under pressure. It restores filtration but not the endocrine role of the kidney, which is why patients still need medication and dietary limits. A transplant restores both functions, at the cost of immunosuppression and the risk of rejection, so comparison questions expect the mechanism of each treatment as well as its long-term burden.
阅卷时最常见的三类失分点
WJEC 的 Unit 3 是两小时、九十分的笔试，占整个 A level 的四分之一，题目以结构化短答与延展写作为主，阅卷按点给分，所以答案要短、要有术语、要贴着设问。常见的失分点有三类：把被动过程写成主动运输，例如超滤；把激素的来源写错，例如说 ADH 由垂体前叶分泌；把水势与水含量混用，水势是决定水移动方向的量，不是水的多少。写数值时带上单位，写过程时把方向说清，写比较时两边都写，这三点能挽回不少分。备考时把 3.7 的每个过程画成流程图，标出感受器、协调中心与效应器，比反复读笔记有效。
Unit 3 is a two-hour paper worth a quarter of the A level, and marks are awarded point by point, so short answers built around the right terms score better than long descriptions. Three errors recur: calling ultrafiltration active transport, misplacing the source of ADH, and confusing water potential with water content. Redrawing each process in topic 3.7 as a flow diagram with receptor, coordination centre and effector labelled pays off more than rereading notes, and the same diagram usually answers a data question about one named variable.
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