IGCSE Biology Excretion: Kidney, Nephron and Osmoregulation — IGCSE 生物排泄：肾脏、肾单位与渗透调节

一、什么是排泄？区分排泄与排遗 | 1. What Is Excretion? Distinguishing Excretion from Egestion

排泄（excretion）是指生物体将细胞代谢过程中产生的废物从体内排出的过程。这些废物包括二氧化碳、尿素、多余的水分和多余的盐分。排泄的本质是清除「细胞自己制造出来的」代谢废物，而不是清除消化道里未被消化的食物残渣。理解这一点，是学好 IGCSE 生物「排泄」这一章的第一步，也是考试中最容易混淆的概念之一。

Excretion is the removal of waste products produced by the body’s cells during metabolism. These wastes include carbon dioxide, urea, excess water and excess salts. The key point is that excretion removes metabolic wastes that the body’s own cells have produced, rather than undigested food remains in the digestive tract. Understanding this distinction is the first step to mastering the “Excretion” chapter in IGCSE Biology, and it is one of the most commonly confused ideas in exams.

与排泄容易混淆的概念是「排遗」（egestion）。排遗指的是将未被消化、未被吸收的食物残渣以粪便的形式排出体外。这些残渣从来就没有真正进入过细胞，它们只是「路过」了消化道而已。因此，排便属于排遗，而不是排泄。

The concept easily confused with excretion is egestion. Egestion refers to the removal of undigested, unabsorbed food remains from the body in the form of faeces. These remains never actually entered the body’s cells; they simply passed through the digestive tract. Therefore, defecation is an example of egestion, not excretion.

考试中经常会出现这样的判断题：「排便是一种排泄。」答案是「错误」，因为粪便不是代谢废物。同样，「呼气排出二氧化碳」是排泄，因为二氧化碳是细胞呼吸作用产生的代谢废物。牢牢记住「代谢废物」这四个字，就能在选择题和简答题中准确判断。

Exam questions often ask: “Defecation is a form of excretion.” The answer is “False”, because faeces are not metabolic wastes. By contrast, “breathing out carbon dioxide” is excretion, because carbon dioxide is a metabolic waste produced by cellular respiration. As long as you remember the phrase “metabolic waste”, you will be able to judge correctly in multiple-choice and short-answer questions.

二、人体的三大排泄器官及其废物 | 2. The Body’s Three Main Excretory Organs and Their Waste Products

人体主要通过三个器官完成排泄任务：肺（lungs）、皮肤（skin）和肾脏（kidneys）。每一个器官负责清除特定类型的代谢废物，它们分工明确，共同维持着人体内环境的稳定。IGCSE 考试要求你能够清楚地列出每个器官所排泄的废物。

The human body carries out excretion through three main organs: the lungs, the skin and the kidneys. Each organ is responsible for removing a specific type of metabolic waste. They have clearly divided roles and work together to maintain a stable internal environment. IGCSE exams require you to clearly list the wastes removed by each organ.

肺通过呼气排出二氧化碳。细胞呼吸作用会产生二氧化碳，二氧化碳溶解在血液中运输到肺，在肺泡处通过气体交换扩散到空气中，最终被呼出体外。肺同时也会排出少量的水蒸气，这一点在寒冷的天气里呼出「白气」时就能直观地看到。

The lungs remove carbon dioxide through exhalation. Cellular respiration produces carbon dioxide, which is transported dissolved in the blood to the lungs. At the alveoli, carbon dioxide diffuses into the air during gas exchange and is finally breathed out. The lungs also remove a small amount of water vapour, which you can see directly when you breathe out “white breath” on a cold day.

皮肤通过汗液排出多余的水分和盐分。汗腺将血液中的水、盐和少量尿素带到皮肤表面，汗液蒸发时还能帮助身体散热，因此皮肤同时承担着排泄和体温调节的双重功能。需要注意的是，出汗的主要作用是降温，而排出尿素只是附带的效果，真正大量清除尿素的任务由肾脏完成。

The skin removes excess water and salts through sweat. Sweat glands bring water, salts and a small amount of urea from the blood to the skin surface. As sweat evaporates it also helps cool the body, so the skin has the dual function of excretion and temperature regulation. Note that the main purpose of sweating is cooling, while removing urea is only a side effect; the job of removing large amounts of urea belongs to the kidneys.

肾脏是人体最重要的排泄器官，它通过产生尿液来清除尿素、多余的水分和多余的盐分。尿素是肝脏将多余的氨基酸脱氨后产生的含氮废物，它对细胞有毒，必须及时排出。肾脏每天过滤约 180 升的血液滤液，最终只产生约 1.5 升的尿液，可见其回收效率之高。

The kidneys are the most important excretory organs. They remove urea, excess water and excess salts by producing urine. Urea is a nitrogenous waste produced when the liver deaminates excess amino acids; it is toxic to cells and must be removed promptly. The kidneys filter about 180 litres of blood filtrate every day, yet only produce about 1.5 litres of urine, which shows how efficient their reabsorption is.

三、泌尿系统：肾脏、输尿管、膀胱与尿道 | 3. The Urinary System: Kidneys, Ureters, Bladder and Urethra

肾脏并不是孤立工作的，它与输尿管（ureter）、膀胱（bladder）和尿道（urethra）共同构成了泌尿系统。理解这条「尿液生产线」的走向，能帮助你理清尿液从产生到排出的完整路径，这也是 IGCSE 生物识图题的高频考点。

The kidneys do not work in isolation. Together with the ureters, the bladder and the urethra, they form the urinary system. Understanding the direction of this “urine production line” helps you work out the complete path of urine from production to excretion, which is a high-frequency topic in IGCSE Biology diagram questions.

人体有一对肾脏，位于腰部脊柱两侧。血液经由肾动脉（renal artery）流入肾脏，经过过滤和重吸收后，净化后的血液经肾静脉（renal vein）流出。肾脏内部产生的尿液一滴一滴地汇入输尿管，输尿管是一根细长的管道，负责把尿液从肾脏输送到膀胱。

Humans have a pair of kidneys, located on either side of the spine in the lower back. Blood enters the kidney through the renal artery, and after filtration and reabsorption, the purified blood leaves through the renal vein. The urine produced inside the kidney drips into the ureter, a thin tube that carries urine from the kidney to the bladder.

膀胱是一个肌肉发达的储存器官，用来暂时储存尿液。当膀胱充盈到一定程度时，大脑会接收到信号，产生排尿的冲动。尿道是连接膀胱与体外的管道，尿液最终通过尿道排出体外。请注意区分「输尿管」（ureter）和「尿道」（urethra）这两个单词，它们的拼写非常接近，考试中常用来设置陷阱。

The bladder is a muscular storage organ that temporarily stores urine. When the bladder fills to a certain level, the brain receives a signal and produces the urge to urinate. The urethra is the tube connecting the bladder to the outside of the body, and urine finally leaves the body through it. Be careful to distinguish the words “ureter” and “urethra”; their spellings are very similar and they are often used to set traps in exams.

四、肾脏的内部结构：皮质、髓质与肾盂 | 4. Inside the Kidney: Cortex, Medulla and Pelvis

把肾脏纵向切开，可以看到三个明显的区域：最外层的皮质（cortex）、内层的髓质（medulla）以及中央的肾盂（pelvis）。皮质呈深红色，是超滤作用发生的场所；髓质颜色较浅，含有肾单位的亨利袢和集合管；肾盂是一个中空的腔，负责收集尿液并将其导入输尿管。

Cutting a kidney lengthwise reveals three distinct regions: the outer cortex, the inner medulla and the central pelvis. The cortex is dark red and is where ultrafiltration takes place. The medulla is lighter in colour and contains the loop of Henle and collecting ducts of the nephrons. The pelvis is a hollow cavity that collects urine and directs it into the ureter.

皮质之所以颜色更深，是因为它布满了肾小球（glomeruli），这些球状的毛细血管网让皮质富含血液。髓质则呈现条纹状的外观，这些条纹实际上是许多平行的管道。IGCSE 的识图题常常要求你在肾脏剖面图上标注 cortex、medulla 和 pelvis 的位置，务必熟练。

The cortex appears darker because it is packed with glomeruli, the ball-shaped networks of capillaries that make the cortex rich in blood. The medulla has a striped appearance, and these stripes are actually many parallel tubules. IGCSE diagram questions often ask you to label the positions of the cortex, medulla and pelvis on a cross-section of the kidney, so practise these labels thoroughly.

此外，肾脏还有两个重要的血管：肾动脉把含尿素的血液送进肾脏，肾静脉把净化后的血液带走。肾动脉的血比肾静脉的血含有更多的尿素，但两者都含有相似浓度的葡萄糖，因为葡萄糖会被肾脏重新吸收回血液。理解这两条血管的成分差异，是回答相关数据题的关键。

The kidney also has two important blood vessels: the renal artery carries urea-containing blood into the kidney, and the renal vein carries purified blood away. Blood in the renal artery contains more urea than blood in the renal vein, but both contain similar concentrations of glucose, because glucose is reabsorbed back into the blood by the kidney. Understanding the composition differences between these two vessels is key to answering related data questions.

五、肾单位：肾脏的功能单位 | 5. The Nephron: The Functional Unit of the Kidney

每个肾脏内部含有大约一百万个微小的过滤单元，这些单元叫做肾单位（nephron）。肾单位是真正执行过滤和重吸收功能的结构，可以说，理解肾单位就等于理解了肾脏的工作原理。每个肾单位都由肾小体和肾小管两部分组成。

Each kidney contains about one million tiny filtering units called nephrons. The nephron is the structure that actually carries out filtration and reabsorption. In other words, understanding the nephron is understanding how the kidney works. Each nephron consists of two parts: the renal corpuscle and the renal tubule.

肾小体位于皮质，由肾小球（glomerulus）和包绕着它的肾小囊（Bowman’s capsule）组成。肾小球是一团毛细血管，血液在这里被高压过滤。肾小囊像一个杯状的「接水器」，收集从肾小球滤出的液体，这些液体就是原尿（glomerular filtrate），也叫滤液。

The renal corpuscle is located in the cortex and consists of the glomerulus and the Bowman’s capsule that surrounds it. The glomerulus is a knot of capillaries where blood is filtered under high pressure. The Bowman’s capsule acts like a cup-shaped “receiver”, collecting the liquid filtered out of the glomerulus. This liquid is called the glomerular filtrate.

肾小管从肾小囊延伸出来，依次经过近曲小管（proximal convoluted tubule）、亨利袢（loop of Henle）、远曲小管（distal convoluted tubule），最后汇入集合管（collecting duct）。滤液沿着这条管道流动的过程中，有用的物质被重新吸收回血液，最终剩下的液体就变成了尿液。

The renal tubule extends from the Bowman’s capsule and passes through the proximal convoluted tubule, the loop of Henle and the distal convoluted tubule, finally joining the collecting duct. As the filtrate flows along this tubule, useful substances are reabsorbed back into the blood, and the remaining liquid eventually becomes urine.

六、超滤作用：血液如何在肾小球中被过滤 | 6. Ultrafiltration: How Blood Is Filtered in the Glomerulus

超滤作用（ultrafiltration）发生在肾小球。血液从较宽的入球小动脉（afferent arteriole）流入肾小球，再从较窄的出球小动脉（efferent arteriole）流出。由于「入口宽、出口窄」，肾小球内部形成了很高的血压，这个高压把血液中的小分子物质强行「挤」过滤过膜，进入肾小囊。

Ultrafiltration takes place in the glomerulus. Blood flows into the glomerulus through the wide afferent arteriole and leaves through the narrower efferent arteriole. Because the entrance is wide and the exit is narrow, a high blood pressure builds up inside the glomerulus. This high pressure forces small molecules in the blood through the filtration membrane into the Bowman’s capsule.

过滤膜像一个精细的筛子，它允许小分子通过，却挡住大分子和血细胞。能够通过的物质包括水、葡萄糖、氨基酸、尿素和盐离子；被挡住的物质包括红细胞、白细胞、血小板，以及血浆蛋白这样的大分子蛋白质。因此，正常情况下健康人的尿液中既没有血细胞，也没有蛋白质。

The filtration membrane acts like a fine sieve, allowing small molecules to pass while blocking large molecules and blood cells. Substances that can pass through include water, glucose, amino acids, urea and salt ions. Substances that are blocked include red blood cells, white blood cells, platelets and large proteins such as plasma proteins. This is why, under normal conditions, a healthy person’s urine contains neither blood cells nor protein.

这里有一个考试重点：肾小球滤液中葡萄糖和尿素的浓度，与血浆中的浓度基本相同，因为这两者都是能够自由通过滤膜的小分子。但滤液中不应该出现蛋白质和血细胞。如果验尿时发现尿液中含有蛋白质或红细胞，往往说明肾小球的滤膜受损了。

Here is a key exam point: the concentration of glucose and urea in the glomerular filtrate is roughly the same as in blood plasma, because both are small molecules that pass freely through the filter. However, the filtrate should not contain protein or blood cells. If a urine test reveals protein or red blood cells in the urine, it usually indicates damage to the glomerular filtration membrane.

七、选择性重吸收：有用的物质如何回到血液 | 7. Selective Reabsorption: How Useful Substances Return to the Blood

超滤作用每天会产生约 180 升的滤液，其中含有大量对人体有用的葡萄糖、氨基酸、水分和盐分。如果这些物质都随尿液排出，人体很快就会被「掏空」。因此，肾小管会对滤液进行「选择性重吸收」（selective reabsorption），把有用的物质重新送回血液。

Ultrafiltration produces about 180 litres of filtrate every day, containing large amounts of useful glucose, amino acids, water and salts. If all these substances were lost in urine, the body would quickly be depleted. Therefore, the renal tubule carries out selective reabsorption, returning useful substances to the blood.

大部分重吸收发生在近曲小管。在这里，所有的葡萄糖和大部分氨基酸、水分、盐分通过主动运输和扩散等方式被重新吸收，进入包绕在肾小管周围的毛细血管。葡萄糖的重吸收需要消耗能量（主动运输），这也是「选择性」一词的含义：有用的物质被专门回收，废物则被留下。

Most reabsorption occurs in the proximal convoluted tubule. Here, all of the glucose and most of the amino acids, water and salts are reabsorbed by active transport and diffusion into the capillaries surrounding the tubule. The reabsorption of glucose requires energy (active transport), and this is the meaning of the word “selective”: useful substances are specifically recovered while wastes are left behind.

亨利袢和集合管负责调节水分的重吸收。亨利袢通过「逆流倍增」机制在髓质中建立起高浓度的盐环境，使得水分能够顺浓度梯度从集合管中被吸收。最终，经过这一系列重吸收后，原本 180 升的滤液被浓缩成约 1.5 升的尿液，其中富含尿素等废物。

The loop of Henle and the collecting duct regulate the reabsorption of water. The loop of Henle uses a “countercurrent multiplier” mechanism to build up a high salt concentration in the medulla, allowing water to be reabsorbed from the collecting duct along its concentration gradient. In the end, after this series of reabsorption processes, the original 180 litres of filtrate is concentrated into about 1.5 litres of urine, rich in urea and other wastes.

考试中一个经典结论是：正常尿液中不含葡萄糖，因为葡萄糖在近曲小管中已被全部重吸收。如果某人的尿液中出现葡萄糖，可能意味着其血糖浓度过高（超过了肾脏的重吸收能力），这正是糖尿病「糖尿」这一名称的由来。

A classic exam conclusion is that normal urine contains no glucose, because all of it has been reabsorbed in the proximal convoluted tubule. If glucose appears in a person’s urine, it may mean their blood glucose level is too high, exceeding the kidney’s reabsorption capacity. This is exactly the origin of the “sugar in urine” symptom that gives diabetes part of its name.

八、渗透调节与抗利尿激素（ADH）| 8. Osmoregulation and Antidiuretic Hormone (ADH)

人体需要把血液中的水分含量维持在一个稳定的范围内，这个过程叫做渗透调节（osmoregulation）。当人体缺水时（例如剧烈运动大量出汗后），血液中的水分减少、渗透压升高，此时肾脏必须减少排水、浓缩尿液；反之，当饮水过多时，肾脏则增加排水、稀释尿液。

The body needs to keep the water content of the blood within a stable range, a process called osmoregulation. When the body is short of water (for example, after heavy exercise with heavy sweating), the water in the blood decreases and the blood’s solute concentration rises. At this time the kidneys must reduce water loss and produce concentrated urine. Conversely, when too much water has been drunk, the kidneys increase water loss and produce dilute urine.

这个过程由抗利尿激素（ADH）精确调控。ADH 由脑部的下丘脑感知信号后，通过垂体释放到血液中。当血液缺水变浓时，垂体释放更多的 ADH；ADH 作用于集合管，使其对水的通透性增加，于是更多的水被重吸收回血液，尿液变得更浓、更少。

This process is precisely controlled by antidiuretic hormone (ADH). ADH is released into the blood by the pituitary gland after the hypothalamus in the brain detects the signal. When the blood becomes more concentrated due to water shortage, the pituitary releases more ADH. ADH acts on the collecting duct, increasing its permeability to water, so more water is reabsorbed into the blood and the urine becomes more concentrated and smaller in volume.

相反，当人大量饮水后，血液被稀释，垂体减少释放 ADH，集合管对水的通透性下降，更多的水随尿液排出，尿液变稀、变多。这个过程是一个典型的「负反馈」调节机制：身体检测到变化，然后做出相反方向的调节，使内环境恢复稳定。

Conversely, after drinking a lot of water, the blood becomes diluted and the pituitary releases less ADH. The collecting duct’s permeability to water decreases, so more water is lost in the urine and the urine becomes more dilute and larger in volume. This is a typical negative feedback mechanism: the body detects a change and then adjusts in the opposite direction to restore a stable internal environment.

IGCSE 考试常要求你用「喝水过多」或「出汗过多」的情景，描述 ADH 的分泌变化及其对尿液的影响。记住这个口诀：血浓 → ADH 多 → 尿少而浓；血稀 → ADH 少 → 尿多而稀。

IGCSE exams often ask you to describe changes in ADH secretion and their effect on urine using scenarios such as “drinking too much water” or “sweating too much”. Remember this rule: concentrated blood leads to more ADH, which leads to less, more concentrated urine; dilute blood leads to less ADH, which leads to more, more dilute urine.

九、尿液与血液的成分对比：一张表格看清差异 | 9. Comparing Urine and Blood: A Table of Key Differences

理解尿液与血液在成分上的差异，是掌握排泄这一章的重要一环。下面的表格总结了血浆、肾小球滤液和尿液三种液体在关键成分上的区别，帮助你快速复习和记忆。

Understanding the composition differences between urine and blood is an important part of mastering the excretion chapter. The table below summarises the differences among blood plasma, glomerular filtrate and urine in key components, helping you review and memorise quickly.

成分 Component	血浆 Plasma	肾小球滤液 Filtrate	尿液 Urine
水 Water	有 Yes	有 Yes	有（减少）Yes (reduced)
葡萄糖 Glucose	有 Yes	有 Yes	无 No
尿素 Urea	有（少量）Yes (little)	有 Yes	有（高浓度）Yes (high)
蛋白质 Protein	有 Yes	无 No	无 No
血细胞 Blood cells	有 Yes	无 No	无 No

从表格中可以看出，血浆和滤液最大的区别在于蛋白质：蛋白质因为分子太大，无法通过肾小球的滤膜，所以滤液中没有蛋白质。滤液和尿液最大的区别在于葡萄糖和尿素浓度：葡萄糖被全部重吸收而消失，尿素则因为水分被大量重吸收而被浓缩，浓度大幅升高。

From the table, the biggest difference between plasma and filtrate is protein: protein molecules are too large to pass through the glomerular filter, so the filtrate contains no protein. The biggest difference between filtrate and urine lies in glucose and urea concentration: glucose disappears because it is completely reabsorbed, while urea becomes more concentrated because large amounts of water are reabsorbed.

这类对比表是 IGCSE 数据题和选择题的常见素材。考试可能给你一张尿液成分化验单，让你判断哪一份样本来自健康人、哪一份来自糖尿病患者，或者哪一份显示肾脏受损。掌握「尿中无糖、无蛋白、无血细胞」这条原则，就能轻松应对。

Comparison tables like this are common material for IGCSE data questions and multiple-choice questions. The exam may give you a urine test report and ask you to judge which sample comes from a healthy person, which from a diabetic, or which shows kidney damage. Mastering the principle “no glucose, no protein and no blood cells in urine” will let you handle these questions with ease.

十、肾衰竭的应对：透析与肾移植 | 10. Treating Kidney Failure: Dialysis and Kidney Transplant

当肾脏因为疾病或损伤而丧失过滤功能时，尿素等废物会在血液中积累，危及生命，这种情况叫做肾衰竭（kidney failure）。现代医学有两种主要的应对方法：透析（dialysis）和肾移植（kidney transplant）。IGCSE 考试要求你能够比较这两种方法的优缺点。

When the kidneys lose their filtering function because of disease or injury, wastes such as urea build up in the blood and threaten life. This condition is called kidney failure. Modern medicine offers two main treatments: dialysis and kidney transplant. IGCSE exams require you to compare the advantages and disadvantages of these two methods.

透析利用「透析机」（dialysis machine）模拟肾脏的过滤功能。患者的血液被抽出体外，流过一层半透膜，膜的另一侧是特制的透析液（dialysis fluid）。透析液中含有与健康血液浓度相近的葡萄糖和盐，但不含尿素。由于浓度梯度，血液中的尿素会扩散到透析液中，而血液中多余的盐和水也会被清除，葡萄糖则保持在血液中。血液经过净化后再流回患者体内。

Dialysis uses a dialysis machine to mimic the kidney’s filtering function. The patient’s blood is drawn out of the body and passed over a partially permeable membrane, on the other side of which is a special dialysis fluid. The dialysis fluid contains glucose and salts at concentrations similar to healthy blood, but no urea. Because of the concentration gradient, urea in the blood diffuses into the dialysis fluid, while excess salts and water are also removed from the blood; glucose stays in the blood. The purified blood then flows back into the patient’s body.

透析的优点是不需要大手术，也不需要等待器官捐献；缺点是患者必须定期（通常每周数次）到医院接受数小时的治疗，生活受到很大限制，而且需要严格控制饮食。肾移植则是把健康的肾脏移植到患者体内，优点是患者可以恢复正常生活，无需频繁透析；缺点是需要找到匹配的供体，术后需终身服用免疫抑制药物以防排斥。

The advantage of dialysis is that it requires no major surgery and no waiting for organ donation. The disadvantage is that patients must regularly visit the hospital (usually several times a week) for hours of treatment, which greatly restricts their lives, and they must strictly control their diet. A kidney transplant involves transplanting a healthy kidney into the patient. The advantage is that the patient can return to a normal life without frequent dialysis; the disadvantage is the need to find a matched donor, and the patient must take immunosuppressant drugs for life to prevent rejection.

透析液与血液之间的物质交换原理，是 IGCSE 生物中非常经典的分析题。关键在于理解「透析液不含尿素，且盐和葡萄糖浓度与血液相近」，这样才能用扩散的知识解释为什么尿素被清除、而葡萄糖和盐不被流失。答题时紧扣「浓度梯度」和「扩散」这两个关键词。

The principle of substance exchange between dialysis fluid and blood is a very classic analysis question in IGCSE Biology. The key is to understand that “the dialysis fluid contains no urea, and its salt and glucose concentrations are similar to those of blood”, so that you can use the idea of diffusion to explain why urea is removed while glucose and salts are not lost. When answering, stick closely to the two keywords “concentration gradient” and “diffusion”.

Summary | 总结

排泄是清除细胞代谢废物的过程，与清除食物残渣的排遗是两回事。人体通过肺排出二氧化碳、通过皮肤排出汗液、通过肾脏排出尿素，其中肾脏是最重要的排泄器官。泌尿系统由肾脏、输尿管、膀胱和尿道组成，尿液沿着这条路径从产生到排出。

Excretion is the removal of cellular metabolic wastes, which is different from egestion, the removal of food remains. The body removes carbon dioxide through the lungs, sweat through the skin and urea through the kidneys, with the kidneys being the most important excretory organs. The urinary system consists of the kidneys, ureters, bladder and urethra, and urine travels along this path from production to excretion.

肾脏的功能单位是肾单位。血液在肾小球中经历超滤作用，小分子物质进入肾小囊形成滤液；随后在肾小管中经历选择性重吸收，葡萄糖被全部回收，大部分水和盐也被回收。最终产生的尿液含有高浓度的尿素，但不含葡萄糖、蛋白质和血细胞。抗利尿激素（ADH）通过负反馈机制调节水分的重吸收，维持血液渗透压的稳定。

The functional unit of the kidney is the nephron. Blood undergoes ultrafiltration in the glomerulus, where small molecules enter the Bowman’s capsule to form the filtrate; this is followed by selective reabsorption in the renal tubule, where all glucose and most water and salts are recovered. The final urine contains a high concentration of urea but no glucose, protein or blood cells. Antidiuretic hormone (ADH) regulates water reabsorption through a negative feedback mechanism, keeping the blood’s solute concentration stable.

当肾脏衰竭时，可以用透析或肾移植来替代其功能。透析依靠浓度梯度在半透膜两侧进行物质交换，而肾移植则能让患者恢复正常生活。掌握排泄、超滤、重吸收和渗透调节这四个核心概念，你就能从容应对 IGCSE 生物中关于「排泄」的所有题型。

When the kidneys fail, dialysis or a kidney transplant can replace their function. Dialysis relies on concentration gradients to exchange substances across a partially permeable membrane, while a kidney transplant allows the patient to return to a normal life. Once you master the four core concepts of excretion, ultrafiltration, reabsorption and osmoregulation, you will be ready for every “excretion” question in IGCSE Biology.

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