📚 5.2 Excretion: Exam-focused Practice | 排泄：真题精练
Welcome to your focused revision on excretion. This article breaks down the most common exam-style questions and the key knowledge you must demonstrate. We will cover definitions, liver functions, kidney anatomy, the nephron’s ultrafiltration and selective reabsorption, osmoregulation, and medical applications such as dialysis. By working through these topics with bilingual explanations, you will be ready to tackle any A‑level excretion paper with confidence.
欢迎来到排泄章节的真题精练。本文将拆解最常见的考试题型以及你必须掌握的核心知识。我们将涵盖排泄的定义、肝脏功能、肾脏解剖、肾单位的超滤和选择性重吸收、渗透调节以及透析等医学应用。通过双语讲解逐一突破这些主题，你将能够自信地应对任何A‑level排泄考卷。
1. What Is Excretion? Key Definitions | 什么是排泄？关键定义
Excretion is the removal of metabolic waste products from the body. Many students confuse excretion with egestion, so exam questions frequently ask you to distinguish between the two. Metabolic wastes include carbon dioxide from respiration, urea from deamination of amino acids, uric acid from nucleic acid breakdown, and bile pigments from haemoglobin breakdown. Be prepared to state that egestion is the elimination of undigested food from the gut, which is not a metabolic process.
排泄是将代谢废物从体内排出的过程。很多学生容易混淆排泄与排遗，因此考题经常会要求你区分两者。代谢废物包括呼吸作用产生的二氧化碳、氨基酸脱氨产生的尿素、核酸分解产生的尿酸以及血红蛋白分解产生的胆色素。要准备好说明排遗是未消化的食物从肠道排出，并不属于代谢过程。
A typical 2‑mark question might ask: “Define excretion and give one example of a metabolic waste product.” Write: “Excretion is the removal of metabolic waste products from the body, e.g. urea.” Keep it concise but precise.
常见的2分题会问：“定义排泄并给出一个代谢废物例子。”答案写成：“排泄是将代谢废物从体内排出的过程，例如尿素。”保持简洁但准确。
2. The Liver’s Role in Excretion | 肝脏在排泄中的作用
The liver is central to metabolism and produces urea via the ornithine cycle. Exam questions expect you to describe deamination: the removal of the amine group from an amino acid, forming ammonia and a keto acid. Ammonia is highly toxic, so the liver converts it to urea, which is less toxic and water‑soluble. Urea is then transported in the blood plasma to the kidneys for excretion.
肝脏是代谢的核心器官，通过鸟氨酸循环生成尿素。考试要求你描述脱氨作用：从氨基酸上移除氨基，生成氨和酮酸。氨的毒性非常强，因此肝脏将其转化为毒性较低且水溶性的尿素。随后尿素随血浆运输到肾脏排出体外。
You should also recall that the liver breaks down haemoglobin to bilirubin, which is excreted in bile. An exam question may ask: “Explain how the liver contributes to excretion.” The answer must mention deamination, urea formation, and bile pigment excretion.
你还需要知道肝脏将血红蛋白分解为胆红素，后者随胆汁排出。考题可能会问：“解释肝脏如何参与排泄。”答案必须提及脱氨作用、尿素生成和胆色素排泄。
3. Kidney Macroscopic Structure | 肾脏宏观结构
Knowing how to label a vertical section through a kidney is a fundamental exam skill. Identify the cortex (outer darker region), medulla (inner region containing pyramids), renal pelvis (collecting area that leads to the ureter), and the ureter itself. The renal arteries bring blood to the kidney, and the renal veins return blood to the vena cava.
学会标注肾脏的纵切面是一项基本的考试技能。你需要标出皮质（外层颜色较深的区域）、髓质（包含肾锥体的内部区域）、肾盂（通向输尿管的集合区域）以及输尿管本身。肾动脉将血液送入肾脏，肾静脉将血液送回腔静脉。
A popular exam task provides a diagram and asks you to name the parts labelled A, B, C and D. Remember: the fibrous capsule surrounds the kidney and protects it. If the question mentions urine collection, you are likely pointing to the renal pelvis.
常见的考题会给出一张示意图，要求你命名图中A、B、C、D所对应的结构。记住：纤维囊包绕并保护肾脏。如果题目提到尿液收集，你指的很可能就是肾盂。
4. The Nephron: Functional Unit | 肾单位：功能单位
The nephron is the microscopic tubule that forms urine. In your exam, you must be able to draw and label the glomerulus, Bowman’s capsule, proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT), and collecting duct. The glomerulus is a knot of capillaries inside the cup‑shaped Bowman’s capsule; the structure together is called the renal corpuscle.
肾单位是形成尿液的微观管道。在考试中，你必须能够画出并标注肾小球、鲍曼氏囊、近曲小管、亨勒袢、远曲小管和集合管。肾小球是位于杯状鲍曼氏囊内的一团毛细血管；这一结构整体称为肾小体。
Exam tips: use fine lines and clearly mark the direction of filtrate flow. A common mistake is drawing the loop of Henle too short; it should descend deep into the medulla. Be sure to indicate that the proximal convoluted tubule is in the cortex while the loop dips into the medulla.
考试技巧：用细线条作画，并清楚地标出滤液流动方向。常见的错误是把亨勒袢画得太短；它应该深深下降到髓质。务必指出近曲小管位于皮质，而亨勒袢深入髓质。
5. Ultrafiltration in the Renal Corpuscle | 肾小体中的超滤
Ultrafiltration occurs due to the high hydrostatic pressure in the glomerular capillaries. The afferent arteriole is wider than the efferent arteriole, building pressure that forces water, ions, glucose, amino acids, and urea out of the blood and into the Bowman’s capsule. The basement membrane acts as a filter, preventing large molecules like plasma proteins and blood cells from passing through.
超滤的发生是由于肾小球毛细血管中的高静水压。入球小动脉比出球小动脉宽，由此产生的高压将水、离子、葡萄糖、氨基酸和尿素从血液压入鲍曼氏囊。基底膜充当滤器，阻止血浆蛋白和血细胞等大分子通过。
Many exam papers ask: “Explain how the structure of the glomerulus and Bowman’s capsule facilitates ultrafiltration.” You need to mention the fenestrated capillary endothelium, the porous basement membrane, and the podocytes with filtration slits. These features create a selective barrier based on size.
很多试卷会问：“解释肾小球和鲍曼氏囊的结构如何促进超滤。”你需要提到有孔毛细血管内皮、多孔的基底膜以及带有滤过裂隙的足细胞。这些结构构成了基于分子大小进行选择性的屏障。
6. Selective Reabsorption in the PCT | 近曲小管中的选择性重吸收
The glomerular filtrate contains useful substances such as glucose, amino acids, vitamins, and a large amount of water. The proximal convoluted tubule is responsible for reabsorbing all glucose, all amino acids, and about 85% of water and sodium ions. This occurs via co‑transport proteins and facilitated diffusion. The cells of the PCT have many mitochondria to provide ATP for active transport, and microvilli to increase surface area.
肾小球滤液含有葡萄糖、氨基酸、维生素以及大量水分等有用物质。近曲小管负责重吸收全部葡萄糖、全部氨基酸以及大约85%的水和钠离子。这一过程通过协同转运蛋白和易化扩散完成。近曲小管细胞含有大量线粒体，为主动运输提供ATP，并具有微绒毛以增加表面积。
Expect to label or describe the co‑transport of sodium ions and glucose. Sodium ions are actively pumped out of the PCT cell into the blood, creating a concentration gradient that drives sodium–glucose symport activity. Glucose then moves into the blood by facilitated diffusion. Any glucose appearing in urine indicates the transport maximum has been exceeded, as in diabetes.
预计需要标注或描述钠离子与葡萄糖的协同转运。钠离子被主动泵出近曲小管细胞进入血液，产生的浓度梯度驱动钠‑葡萄糖同向转运。随后葡萄糖通过易化扩散进入血液。尿中出现葡萄糖表明转运极限已被超过，例如糖尿病的情况。
7. The Loop of Henle and Countercurrent Multiplication | 亨勒袢与逆流倍增
The loop of Henle creates a high solute concentration in the medulla tissue, enabling the production of concentrated urine. The descending limb is permeable to water but not to ions, so water leaves by osmosis. The ascending limb is impermeable to water but actively transports Na⁺ and Cl⁻ out into the medulla. This countercurrent flow maintains the gradient.
亨勒袢在髓质组织中营造了高溶质浓度，为产生浓缩尿液提供了条件。降支对水通透但对离子不通透，因此水通过渗透作用离开。升支对水不通透，但能主动将Na⁺和Cl⁻转运到髓质。这种逆流流动维持了浓度梯度。
When answering an exam question on osmoregulation, you must link the medullary concentration gradient to water reabsorption in the collecting duct. The longer the loop of Henle (as in desert mammals), the greater the ability to concentrate urine and conserve water. This is a classic adaptation question.
在回答渗透调节的考题时，必须将髓质浓度梯度与集合管中的水重吸收联系起来。亨勒袢越长（如沙漠哺乳动物），浓缩尿液和保存水分的能力就越强。这是一个经典的适应性问题。
8. Osmoregulation and ADH | 渗透调节与抗利尿激素
Osmoregulation is the control of water potential in the blood. The hypothalamus contains osmoreceptors that detect a decrease in blood water potential (dehydration). This stimulates the posterior pituitary gland to release antidiuretic hormone (ADH) into the blood. ADH increases the permeability of the collecting duct and DCT to water by inserting aquaporins into the membranes, allowing more water reabsorption and producing a small volume of concentrated urine.
渗透调节是对血液水势的控制。下丘脑含有渗透压感受器，能够探测到血液水势的下降（脱水）。这会刺激垂体后叶向血液中释放抗利尿激素（ADH）。ADH通过将水通道蛋白插入细胞膜，增加集合管和远曲小管对水的通透性，使更多的水被重吸收，从而产生量少浓缩的尿液。
A typical exam question features a graph of plasma ADH concentration against plasma water potential or urine volume. You must interpret the negative correlation: when water potential falls, ADH rises, urine volume drops. Remember that in the absence of ADH (e.g. diabetes insipidus), the collecting duct remains impermeable, leading to large volumes of dilute urine.
典型的考题会给出血浆ADH浓度与血浆水势或尿量的曲线图。你必须解读这种负相关：当水势下降时，ADH升高，尿量减少。记住，在缺乏ADH的情况下（例如尿崩症），集合管保持不通透，导致排出大量稀释的尿液。
9. Kidney Failure and its Treatment | 肾衰竭及其治疗
Kidney failure can result from diabetes, high blood pressure, or infection. When kidneys fail, waste products accumulate in the blood, fluid balance is disturbed, and the patient may require dialysis or a kidney transplant. Haemodialysis involves passing the patient’s blood through a machine with a partially permeable membrane, where wastes diffuse into dialysis fluid while blood cells and proteins are retained.
肾衰竭可由糖尿病、高血压或感染引起。当肾脏衰竭时，血液中的废物积聚，体液平衡被扰乱，患者可能需要透析或肾移植。血液透析是将患者的血液通过带有部分透过膜的机器，在透析液中废物扩散进入，而血细胞和蛋白质被保留。
The dialysis fluid must contain the same concentration of glucose and ions as healthy blood plasma to prevent loss of essential substances through diffusion. However, it contains no urea, ensuring a steep concentration gradient that maximises urea removal. Be ready to compare haemodialysis with peritoneal dialysis, which uses the body’s own peritoneum as the membrane.
透析液必须含有与健康血浆相同浓度的葡萄糖和离子，以防止必需物质通过扩散而流失。但透析液中不含尿素，这样可以形成陡峭的浓度梯度，以最大限度地清除尿素。要做好准备比较血液透析与腹膜透析，后者是利用人体自身的腹膜作为膜。
10. Urine Analysis and Composition | 尿液分析与成分
Exam questions frequently present urine test results and ask you to interpret findings. Normal urine contains water, urea, uric acid, creatinine, and various ions, but no glucose or proteins. The presence of protein (proteinuria) suggests damage to the glomerular basement membrane, making it more permeable. Glucose in urine (glycosuria) is a hallmark of diabetes mellitus when blood glucose exceeds the renal threshold.
考题常常给出尿液检测结果，要求你解读发现。正常尿液中含有水、尿素、尿酸、肌酐以及各种离子，但不含葡萄糖或蛋白质。存在蛋白质（蛋白尿）提示肾小球基底膜受损，通透性增大。尿液中出现葡萄糖（糖尿）是糖尿病的标志，此时血糖浓度超过了肾糖阈。
You may also be asked to calculate the concentration of a substance in urine given a table of volumes and masses. Remember the formula: concentration = mass / volume. Be careful with unit conversions; often volumes are given in mL but concentrations in g L⁻¹, so you must convert to litres.
你还可能需要根据提供的体积和质量表格，计算某物质在尿液中的浓度。记住公式：浓度 = 质量 / 体积。注意单位换算；通常体积以mL给出，但浓度的单位为g L⁻¹，因此必须换算成升。
11. Common Exam Mistakes and How to Avoid Them | 常见考试错误及如何避免
Many students lose marks by mixing up ultrafiltration and selective reabsorption. Ultrafiltration is based purely on size and pressure, occurring in the renal corpuscle, while selective reabsorption uses transporters and energy in the PCT. Also, never state that “the kidney filters the blood to remove waste” without specifying that large molecules remain in the blood. The filtrate is not urine; urine is the final product after reabsorption and secretion.
很多学生因为混淆超滤和选择性重吸收而失分。超滤仅基于分子大小和压力，发生在肾小体；而选择性重吸收则利用转运蛋白和能量，在近曲小管进行。同时，切勿只说“肾脏过滤血液以除去废物”，而应明确指出大分子留在血液中。滤液不是尿液；尿液是经过重吸收和分泌后的最终产物。
Another pitfall is forgetting to mention the role of the medullary gradient when explaining water reabsorption. Water follows Na⁺ and Cl⁻ only when the collecting duct is made permeable by ADH. Write clearly and avoid vague phrases like “water moves out”. Always state “by osmosis down the water potential gradient”.
另一个易错点是，在解释水重吸收时忘记提及髓质梯度的作用。水只有在ADH使集合管通透时才跟随Na⁺和Cl⁻。表述要清晰，避免使用“水分离开”这类模糊的说法。一定要写明“通过渗透作用顺着水势梯度移动”。
12. Practice Exam-Style Question with Model Answer | 模拟真题与标准答案
Question: Describe the roles of the loop of Henle and the collecting duct in the production of concentrated urine. (6 marks)
Model answer: The ascending limb of the loop of Henle actively transports Na⁺ and Cl⁻ out into the medulla tissue, creating a high solute concentration in the medullary interstitial fluid. The descending limb is permeable to water, so water leaves the filtrate by osmosis. This countercurrent multiplier system maintains a steep concentration gradient from the cortex to the medulla. When ADH is present, it increases the permeability of the collecting duct to water. As the collecting duct passes through the medulla, water moves out by osmosis into the hypertonic interstitial fluid, resulting in the production of a small volume of concentrated urine.
题目：描述亨勒袢和集合管在浓缩尿液生成中的作用。（6分）
标准答案：亨勒袢升支主动将Na⁺和Cl⁻转运到髓质组织中，使髓质组织液具有高溶质浓度。降支对水通透，因此水分通过渗透离开滤液。这种逆流倍增系统维持了从皮质到髓质的陡峭浓度梯度。当ADH存在时，它增加了集合管对水的通透性。当集合管经过髓质时，水分通过渗透作用进入高渗的组织液中，从而导致产生量少浓缩的尿液。
By studying this model answer, you can see how to link structure to function, use the correct terminology (‘countercurrent multiplier’, ‘hypertonic’, ‘osmosis’), and address both components of the question. Practice writing similar structured responses under timed conditions.
通过学习这份标准答案，你可以看到如何将结构与功能联系起来，使用正确的术语（｀逆流倍增器｀、｀高渗的｀、｀渗透作用｀），并涵盖问题的两个部分。在限时条件下练习书写类似的结构化答案。
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