📚 Excretory Products of the Lungs, Kidneys and Skin | 肺、肾和皮肤的排泄产物
Excretion is the removal of metabolic waste products from the body. In IGCSE Biology, you need to know exactly which waste substances are eliminated by the lungs, kidneys, and skin, and why this process is vital for maintaining a stable internal environment.
排泄是指将代谢废物从体内清除的过程。在IGCSE生物学中，你需要准确了解肺部、肾脏和皮肤分别排出哪些废物，以及为什么这一过程对维持稳定的内部环境至关重要。
1. What is Excretion? | 什么是排泄？
Excretion refers specifically to the removal of waste products formed by metabolic reactions inside cells. These wastes can be toxic if they accumulate. It is not the same as egestion, which is the elimination of undigested food material from the gut.
排泄特指清除细胞内代谢反应形成的废物。这些废物若累积会具有毒性。它不同于排遗，排遗是指从肠道排出未消化的食物残渣。
Carbon dioxide, urea, excess water, and mineral ions are the main excretory products in humans. The organs responsible are the lungs, kidneys, and skin.
二氧化碳、尿素、多余的水和矿物离子是人体主要的排泄产物。负责排泄的器官是肺、肾和皮肤。
2. Excretory Role of the Lungs | 肺的排泄作用
The lungs excrete carbon dioxide and water vapour. These are waste products of aerobic respiration, which occurs in almost every living cell.
肺部排出二氧化碳和水蒸气。它们是有氧呼吸产生的废物，有氧呼吸几乎发生在每一个活细胞内。
Carbon dioxide dissolves in blood plasma, forming a weak acid that can lower blood pH. It is transported to the lungs, where it diffuses from the blood into the alveoli and is breathed out. Water vapour is also lost as moist air is exhaled.
二氧化碳溶解在血浆中，形成弱酸会降低血液的pH值。它被运送到肺部，从血液扩散进入肺泡并被呼出。呼气时湿润空气的排出也会带走水蒸气。
The equation for aerobic respiration shows why these wastes appear:
Glucose + Oxygen → Carbon dioxide + Water (+ energy)
有氧呼吸的方程式表明为何会产生这些废物：
葡萄糖 + 氧气 → 二氧化碳 + 水 (+ 能量)
3. Excretory Role of the Kidneys | 肾脏的排泄作用
The kidneys remove urea, excess water, and excess mineral ions from the blood. Urea is formed in the liver from the breakdown of excess amino acids — a process called deamination.
肾脏从血液中清除尿素、多余的水和多余的矿物离子。尿素是肝脏在分解多余氨基酸时形成的——这一过程称为脱氨作用。
Urea is toxic in high concentrations. The kidneys filter it out of the blood and combine it with water to form urine, which is then stored in the bladder before being released.
高浓度的尿素具有毒性。肾脏将尿素从血液中滤出，并与水结合形成尿液，尿液储存在膀胱中随后排出。
By adjusting the volume of water reabsorbed, the kidneys also regulate the water balance of the blood. This is a key example of homeostasis.
通过调节重吸收的水量，肾脏还能调控血液中的水分平衡。这是稳态调节的一个重要例子。
4. Excretory Role of the Skin | 皮肤的排泄作用
The skin excretes water, mineral ions, and a small amount of urea through sweat. Sweat glands in the dermis release sweat onto the skin surface.
皮肤通过汗液排出水、矿物离子以及少量的尿素。真皮中的汗腺将汗液分泌到皮肤表面。
Although the primary function of sweating is to cool the body by evaporation, the loss of these dissolved substances makes it an excretory process. The amount of urea lost through skin is minimal compared to that lost via the kidneys.
尽管出汗的主要功能是通过蒸发冷却身体，但溶解物质的流失使其成为一个排泄过程。与肾脏相比，通过皮肤排出的尿素的量极少。
5. Urea Production and Removal | 尿素的产生与清除
Excess amino acids cannot be stored in the body. In the liver, the amino group (–NH₂) is removed and converted into ammonia, which is quickly converted to urea because ammonia is highly toxic.
多余的氨基酸无法在体内储存。在肝脏中，氨基（–NH₂）被移除并转化为氨，而氨又迅速转化为尿素，因为氨的毒性很强。
Urea then travels in the blood plasma to the kidneys, where it is filtered out. The concentration of urea in urine is far higher than in the blood, showing that the kidneys actively concentrate this waste for safe disposal.
尿素随后通过血浆运送到肾脏，并被过滤出来。尿液中的尿素浓度远高于血液，这表明肾脏能主动浓缩这种废物以安全清除。
6. Water and Ion Balance | 水分与离子平衡
Excretory organs also help maintain the water and ion balance of the body. The kidneys can reabsorb more water under the influence of ADH (antidiuretic hormone) when the body is dehydrated.
排泄器官还有助于维持人体的水分和离子平衡。身体脱水时，肾脏在抗利尿激素（ADH）的影响下可重吸收更多的水。
The skin loses water and salts during sweating, which must be replaced by drinking fluids and eating mineral-rich foods. The lungs lose water continuously through exhalation.
皮肤在出汗时失去水分和盐分，必须通过饮水和摄取富含矿物质的食物来补充。肺部则通过呼气持续失去水分。
7. Comparison of Excretory Products | 排泄产物对比
| Organ | 主要排泄产物 | 代谢来源 |
|---|---|---|
| Lungs | CO₂, H₂O (水蒸气) | 有氧呼吸 |
| Kidneys | 尿素, 多余水, 离子 | 脱氨作用, 饮食摄入 |
| Skin | 水, 盐, 少量尿素 | 代谢活动, 血液溶质 |
The table above summarises the key excretory products and their origins. Notice that all three organs share the elimination of water, but the kidneys dominate in urea removal.
上表总结了主要的排泄产物及其来源。值得注意的是，三个器官都参与水的排出，但肾脏在尿素清除中占主导地位。
8. Why Excretion is Essential | 为何排泄至关重要
Without excretion, waste products would accumulate to toxic levels. For example, high CO₂ levels cause blood to become too acidic, disrupting enzyme activity. Excess urea can cause neurological damage, and severe water–ion imbalance can lead to cell dysfunction.
没有排泄，废物就会累积到有毒水平。例如，高浓度二氧化碳会使血液酸性过强，破坏酶活性。过量尿素可引起神经损伤，严重的水-离子失衡则会导致细胞功能障碍。
Thus, excretion is not just about removing unwanted materials — it is a fundamental part of homeostasis, keeping internal conditions within narrow limits necessary for life.
因此，排泄不仅仅是清除无用物质——它是稳态调节的基本组成部分，能使内部环境维持在生命所需的小范围内。
9. Link to the Kidney Nephron | 与肾单位的关联
In the kidney, blood is filtered in tiny structures called nephrons. The glomerulus filters water, urea, ions, and glucose out of the blood, forming glomerular filtrate. As this fluid flows through the nephron tubule, useful substances like glucose and most water are reabsorbed back into the blood.
在肾脏中，血液在微小的结构——肾单位中被过滤。肾小球将水、尿素、离子和葡萄糖滤出血液，形成肾小球滤液。当这些液体流经肾小管时，葡萄糖等有用物质和大部分水被重吸收回血液。
What remains in the tubule is urine, containing urea, excess water, and excess ions. This shows that the kidney not only excretes but also conserves vital resources. For the IGCSE, knowing this basic function without all the detailed steps is sufficient, but understanding the balance is key.
肾小管中剩下的就是尿液，其中含有尿素、多余的水和离子。这表明肾脏不仅是排泄器官，还能保留重要资源。对IGCSE而言，了解这一基本功能（无需所有细节步骤）即可，但理解平衡是关键。
10. Common Misconceptions | 常见误区
Misconception: "Sweat is mainly urea."
Fact: Sweat is about 99% water, with small amounts of salts and only traces of urea. The kidney is the main urea remover.
误区："汗液主要含尿素。"
事实：汗液中约99%是水，含有少量盐分和极微量的尿素。肾脏才是主要的尿素清除器官。
Misconception: "The lungs excrete only carbon dioxide."
Fact: The lungs also excrete a significant amount of water vapour, which can be seen on a cold day as breath condensation.
误区："肺部只排出二氧化碳。"
事实：肺部还排出大量水蒸气，这一点在寒冷天气时呼出的水汽中可见。
Misconception: "Faeces is an excretory product."
Fact: Faeces contain mainly undigested fibre, bacteria, and bile pigments — not metabolic waste. This process is egestion, not excretion.
误区："粪便是排泄物。"
事实：粪便中主要包含未消化的纤维、细菌和胆色素——并非代谢废物。这一过程是排遗，不是排泄。
11. Excretion and Homeostasis Summary | 排泄与稳态总结
Each excretory organ contributes to homeostasis in a unique way. The lungs regulate blood pH by removing CO₂; the kidneys control water, ion, and urea balance; the skin aids thermoregulation while excreting small amounts of waste.
每个排泄器官都以其独特方式参与稳态。肺部通过清除CO₂来调节血液pH值；肾脏调控水分、离子和尿素的平衡；皮肤在辅助温度调节的同时排出少量废物。
If one organ fails, the others cannot fully compensate. Kidney failure, for instance, requires dialysis to perform excretory functions artificially, highlighting the essential nature of these processes.
如果某个器官衰竭，其他器官无法完全代偿。例如，肾衰竭就需要通过透析来人工实现排泄功能，这突显了这些过程的不可或缺性。
12. Exam Tips for Edexcel IGCSE Biology | Edexcel IGCSE 生物学应试提示
You may be asked to state the main excretory product of each organ. Use precise scientific terms: "carbon dioxide and water vapour" for lungs, "urea, excess water, excess mineral ions" for kidneys, "water, salts and a small amount of urea" for skin.
你可能会被要求列出每个器官的主要排泄产物。要使用准确的科学术语：肺部是"二氧化碳和水蒸气"，肾脏是"尿素、多余的水和多余矿物离子"，皮肤是"水、盐和少量尿素"。
Be ready to explain why carbon dioxide must be removed (it forms an acidic solution in the blood) and why urea is toxic. Diagrams of the excretory system may appear, and you should label kidneys, ureters, bladder, urethra, lungs, and skin as excretory organs.
应准备好解释为什么必须清除二氧化碳（它在血液中形成酸性溶液）以及为什么尿素具有毒性。试卷中可能出现排泄系统示意图，你应该标出肾、输尿管、膀胱、尿道、肺和皮肤作为排泄器官。
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