📚 5.2 Excretion: Experimental Design | 5.2 排泄：实验设计
Excretion is a vital process in living organisms, responsible for the removal of toxic metabolic wastes such as carbon dioxide, urea, and excess salts. Understanding how excretory organs function and how their activity is regulated requires well-designed experiments. This article explores a variety of practical investigations and experimental approaches related to excretion, emphasising the principles of experimental design, variable control, and data interpretation.
排泄是生物体清除有毒代谢废物（如二氧化碳、尿素和多余盐分）的重要过程。要理解排泄器官如何工作及其活动如何受到调控，就需要精心设计的实验。本文探讨了一系列与排泄相关的实践探究和实验方法，着重介绍实验设计原则、变量控制和数据解读。
1. Introduction to Excretion and Experimental Design | 排泄与实验设计简介
Excretion should not be confused with egestion; it specifically involves metabolic wastes produced from cellular activities. In mammals, the kidneys, lungs, skin, and liver all contribute to excretion. Experimental design in this topic often involves measuring changes in urine composition, sweat rate, or exhaled CO₂ concentration under controlled conditions. A robust experiment identifies an independent variable (what you change), a dependent variable (what you measure), and several control variables (factors kept constant).
排泄不应与排遗混淆；它特指细胞活动产生的代谢废物。在哺乳动物中，肾脏、肺、皮肤和肝脏都参与排泄过程。本主题的实验设计通常涉及在受控条件下测量尿液成分、出汗速率或呼出气体中 CO₂ 浓度的变化。一个严谨的实验会明确自变量（你改变的因素）、因变量（你测量的指标）以及若干控制变量（需保持不变的因素）。
2. Measuring the Effect of Exercise on Urine Output | 测量运动对尿液输出量的影响
This investigation explores how physical activity influences kidney function and urine production. The independent variable could be the duration or intensity of exercise, while the dependent variable is the volume of urine produced over a fixed post-exercise period. Participants should empty their bladder before the experiment, consume a standardised volume of water (e.g., 500 mL), and then either rest (control group) or perform a set exercise routine (experimental group). Urine is collected at timed intervals, and its volume measured.
这项探究旨在考察体育活动如何影响肾功能和尿液生成。自变量可以是运动的时长或强度，因变量则是运动后固定时间段内产生的尿液量。受试者应在实验前排空膀胱，饮用规定量的水（如 500 mL），然后或休息（对照组）或完成设定的运动计划（实验组）。定时收集尿液并测量其体积。
Key control variables include ambient temperature, water intake, and the time since the last meal. To improve reliability, the experiment should be repeated on multiple participants, and the mean urine volume calculated. Ethical approval must be obtained for any experiment involving human subjects.
关键的控制变量包括环境温度、饮水量以及距上次进餐的时间。为提高可靠性，实验应对多名受试者重复进行，并计算平均尿量。凡是涉及人体受试者的实验，都必须获得伦理许可。
3. Investigating the Role of ADH Using a Model Kidney | 利用肾脏模型研究抗利尿激素的作用
It is not feasible to directly manipulate ADH levels in live humans in a school laboratory, so a model kidney using dialysis tubing can be employed. Fill a length of Visking tubing with a solution representing glomerular filtrate (containing glucose, urea, and ions) and tie both ends. Place the tubing in a beaker of water or a solution mimicking interstitial fluid. Adding a few drops of ADH (or a synthetic analogue such as vasopressin) to the external solution can be simulated by altering the permeability of the tubing. In practice, you can compare water movement by measuring the mass of the tubing before and after incubation.
在学校实验室中直接操控人体内的抗利尿激素（ADH）水平并不可行，因此可以采用透析管构建肾脏模型。取一段 Visking 透析管，注入代表肾小球滤液的溶液（含葡萄糖、尿素和离子），并扎紧两端。将透析管放入盛有水或模拟组织液的烧杯中。可以通过向外部溶液加入几滴 ADH（或其合成类似物如血管升压素）来模拟，实际上则可以对比透析管通透性的改变。在操作中，可通过称量透析管在孵育前后的质量变化来比较水分的移动。
If ADH increases the water permeability of the collecting duct, the water should enter the filtrate or leave the tubing depending on the osmotic gradient. Students can design experiments using solutions of varying osmolarity and record mass changes. This model helps illustrate that ADH promotes water reabsorption, leading to more concentrated urine.
如果 ADH 增加了集合管对水的通透性，根据渗透梯度，水应进入滤液或离开透析管。学生可以设计实验，使用不同渗透压的溶液并记录质量变化。这一模型有助于说明 ADH 促进水分重吸收，导致尿液浓缩。
4. Urinalysis: Testing for Glucose, Protein, and Ketones | 尿液分析：检测葡萄糖、蛋白质和酮体
urinalysis experiments use either test strips or chemical reagents. For glucose, Benedict's test is commonly performed: heat urine with Benedict' → green → yellow → brick-red precipitate (Cu₂O) depending on reducing sugar concentration.
尿液中出现异常成分可能提示糖尿病或肾损伤等疾病。常规尿液分析实验使用试纸条或化学试剂。检测葡萄糖常用本尼迪克特试验：将尿液与本尼迪克特试剂一同加热，根据还原糖浓度可观察到颜色由蓝 → 绿 → 黄 → 砖红色沉淀（Cu₂O）。
| Substance | Test | Positive result |
| Glucose | Benedict's test | Brick-red precipitate |
| Protein | Biuret test | Purple/violet colour |
| Ketones | Sodium nitroprusside (strip) | Pink to purple |
The independent variable could be the urine sample from different simulated conditions, and the dependent variable the colour intensity. Control variables include volume of reagent, temperature of water bath, and reaction time. Good experimental design mandates comparison with a negative control (distilled water) and a positive control (known concentration of glucose).
自变量可以是不间模拟条件下的尿液样本，因变量则是颜色深浅。控制变量包括试剂体积、水浴温度和反应时间。良好的实验设计要求与阴性对照（蒸馏水）和阳性对照（已知葡萄糖浓度的溶液）进行比对。
5. Effects of Different Beverages on Urine Volume and Concentration | 不同饮料对尿量和尿浓度的影响
This investigation examines how the intake of water, caffeine-containing drinks, or saline solutions alters urine output. Each participant consumes a fixed volume (e.g., 300 mL) of a specific beverage after emptying the bladder. Urine is collected every 30 minutes for 2 hours. Volume is measured, and urine specific gravity is determined using a urinometer or refractometer. Caffeine is a diuretic, so it is expected to increase urine output, while a hypertonic saline drink may temporarily reduce output as the body retains water to dilute the excess salt.
这项研究考察饮用水、含咖啡因饮料或盐水溶液如何改变尿液排出量。每名受试者在排空膀胱后饮用固定体积（如 300 mL）的特定饮料。每 30 分钟收集一次尿液，持续 2 小时。测量体积，并使用尿比重计或折射计测定尿比重。咖啡因是一种利尿剂，因此预期会增加尿液排出量；而高渗盐水饮料则可能暂时减少排出量，因为身体会保留水分以稀释多余的盐分。
A common pitfall is failing to control prior hydration status. All participants should follow the same fasting and fluid restriction guidelines for at least 2 hours before the experiment. The data can be plotted as a line graph (time vs. cumulative urine volume) to visualise diuretic effects.
一个常见的误区是未能控制实验前的水合状态。所有受试者应在实验前至少 2 小时遵循相同的禁食和限制饮水指南。可将数据绘制成折线图（时间 vs. 累积尿量），以直观展示利尿效果。
6. Osmotic Regulation: Using Dialysis Tubing to Simulate Tubular Reabsorption | 渗透调节：用透析管模拟肾小管重吸收
A further model-based experiment uses dialysis tubing to represent a kidney tubule and investigates how the composition of the surrounding fluid influences what is reabsorbed. Fill the tubing with a simulated filtrate (water, glucose, starch, sodium chloride) and immerse it in distilled water or a salt solution. After 20 minutes, test the external solution for the presence of glucose (using Benedict's) and starch (iodine test).
另一个基于模型的实验用透析管代表肾小管，探究周围液体的组成如何影响重吸收的物质。将模拟滤液（含水、葡萄糖、淀粉、氯化钠）注入透析管，并浸入蒸馏水或盐溶液中。20 分钟后，检测外部溶液中是否出现葡萄糖（用本尼迪克特法）和淀粉（碘液检测）。
The expectation is that glucose and ions will diffuse across the semi-permeable membrane, but large molecules like starch will be retained, mirroring selective reabsorption in the proximal convoluted tubule. This setup reinforces the concept that the kidney reabsorbs useful substances while waste products remain in the filtrate to be excreted.
预期结果是葡萄糖和离子会透过半透膜扩散，而像淀粉这样的大分子则被截留，这模拟了近曲小管的选择性重吸收。此装置强化了以下概念：肾脏重吸收有用物质，而废物则留在滤液中被排出。
7. Sweat Rate Measurement under Varying Environmental Conditions | 不同环境条件下的出汗率测量
filter paper patches (e.g., 5 cm × 5 cm) can be attached to the forehead or forearm of a subject. After a fixed period of physical activity or exposure to a warm environment, the papers are re-weighed. The difference in mass gives the mass of sweat produced.
皮肤通过分泌汗液参与排泄，汗液中含有水分、盐分及少量尿素。要测量出汗率，可将预先称重的滤纸块（如 5 cm × 5 cm）贴附于受试者的前额或前臂。经过固定时间的体育活动或热环境中暴露后，再次称重。两次质量之差即为产生的汗液质量。
The independent variable can be the ambient temperature or humidity level, while the dependent variable is sweat mass per unit time. It is crucial to control airflow, as evaporation is affected by wind speed. Anhydrous cobalt chloride paper, which turns from blue to pink in the presence of water, can also be used qualitatively to demonstrate active sweat glands.
自变量可以是环境温度或湿度水平，因变量则是单位时间的出汗质量。控制气流至关重要，因为风速会影响蒸发速率。无水氯化钴试纸遇水由蓝变粉，也可用于定性显示活跃的汗腺。
8. Kidney Dissection and Observation of Nephron Structure | 肾脏解剖与肾单位结构观察
While a dissection is not an experiment in the hypothesis-testing sense, it serves as a critical observational design activity. A mammalian kidney (e.g., pig or sheep) is dissected longitudinally to expose the cortex, medulla, renal pelvis, and ureter. Students can measure the thickness of different regions and link structure to function. Using a hand lens or microscope, the arrangement of renal corpuscles and tubules can be sketched.
尽管解剖并非通常意义上的假设检验实验，但它是一项关键的观察性设计活动。纵向剖开哺乳动物肾脏（如猪肾或羊肾），可暴露皮质、髓质、肾盂和输尿管。学生可测量不同区域的厚度，并将结构与其功能关联起来。使用放大镜或显微镜，可绘制出肾小体和肾小管的排列草图。
Good experimental design involves systematic recording of observations, calibrated measurements using a ruler or calipers, and careful labelling of diagrams. Ethical considerations require sourcing specimens from animals already slaughtered for food, wearing gloves, and disinfecting work surfaces afterwards.
良好的实验设计包含系统性地记录观察结果、借助直尺或卡尺进行校准测量，以及仔细标注图示。伦理方面的考量包括：要使用已在食品加工中屠宰的动物器官、佩戴手套并在操作后对台面进行消毒。
9. Monitoring Carbon Dioxide Excretion during Exercise | 监测运动过程中的二氧化碳排出
The lungs excrete carbon dioxide, a waste product of aerobic respiration. A simple investigation can measure the rate of CO₂ release before and after exercise using hydrogencarbonate indicator or a CO₂ sensor. The indicator changes from red to yellow as CO₂ concentration increases. The independent variable is the level of physical activity, and the dependent variable is the time taken for the indicator to change colour or the CO₂ concentration in parts per million (ppm).
肺排出二氧化碳，后者是有氧呼吸的废物。可使用碳酸氢盐指示剂或 CO₂ 传感器，测量运动前后 CO₂ 的释放速率。随着 CO₂ 浓度升高，指示剂由红变黄。自变量是体力活动水平，因变量是指示剂变色所需的时间或 CO₂ 浓度（ppm）。
A respirometer setup with an absorption tube for CO₂ (sodium hydroxide) can specifically measure O₂ consumption, but integrating a CO₂ probe allows direct monitoring of excretion. Control variables include the volume of the chamber, temperature, and the person's breathing pattern.
使用带 CO₂ 吸收管（氢氧化钠）的呼吸计装置可专门测量耗氧量，但如果集成 CO₂ 探头则可直接监测排泄过程。控制变量包括呼吸室容积、温度以及受试者的呼吸模式。
10. Ethical Considerations and Safety in Excretion Experiments | 排泄实验中的伦理与安全考虑
Working with bodily fluids such as urine and sweat requires strict hygiene protocols. Urine samples should be treated as potentially hazardous biological material; wear disposable gloves, use sealed containers, and disinfect spills immediately. When using dialysis tubing and chemical reagents, safety goggles must be worn. Benedict's reagent is corrosive and requires heating with caution.
处理尿液和汗液等体液时，必须遵守严格的卫生规程。应将尿液样本视为有潜在危险的生物材料；佩戴一次性手套，使用密封容器，并立即消毒溢出物。在使用透析管和化学试剂时，必须佩戴护目镜。本尼迪克特试剂具有腐蚀性，加热时需谨慎。
Human subject experiments mandate informed consent, the right to withdraw, and anonymisation of data. All procedures should be approved by a teacher or ethics committee. No invasive techniques should be employed in a school setting.
涉及人体受试者的实验要求参与者知情同意、有权退出，并对数据进行匿名化处理。所有操作均应获得教师或伦理委员会的批准。在学校环境中不得采用任何侵入性技术。
11. Designing Your Own Excretion Experiment: A Step-by-Step Guide | 设计你自己的排泄实验：分步指南
Step 1: Form a hypothesis. For example, 'Drinking a high-salt solution will reduce urine output over the following 60 minutes compared to an equal volume of plain water.'
步骤一：提出假设。 例如，“与等量白水相比，饮用高盐溶液会减少随后 60 分钟内的尿液排出量。”
Step 2: Identify variables. Independent variable: type of drink (salt solution vs. water). Dependent variable: urine volume. Control variables: drink volume, temperature, time, participant's initial hydration status.
步骤二：识别变量。 自变量：饮料种类（盐溶液 vs. 白水）。因变量：尿液体积。控制变量：饮用量、温度、时间、受试者初始水合状态。
Step 3: Write a detailed procedure. Include instructions for fluid intake, timing of urine collections, and measuring instruments. Ensure reproducibility by specifying quantities and equipment precisely.
步骤三：撰写详细步骤。 包含饮水操作、尿液收集时间安排和测量工具的说明。通过精确描述数量和设备来确保可重复性。
Step 4: Present and analyse data. Use a table to record raw data and a graph to show trends. Calculate the mean and, if appropriate, perform a simple statistical test such as the t-test.
步骤四：展示和分析数据。 用表格记录原始数据，用图表展示趋势。计算平均值，并酌情进行如 t 检验之类的简单统计检验。
Step 5: Draw conclusions. Relate findings to the physiological role of osmoregulation and ADH. Discuss limitations, such as small sample size or individual variations in kidney sensitivity.
步骤五：得出结论。 将发现与渗透调节和 ADH 的生理作用联系起来。讨论实验的局限性，如样本量小或个体肾脏敏感性的差异。
12. Common Pitfalls and How to Avoid Them | 常见误区及避免方法
Pitfall 1: Inadequate control of water intake. Even a small sip of water during the experiment can unpredictably alter urine output. Ensure strict supervision and provide a fixed volume.
误区一：饮水控制不严。 实验期间哪怕小啜一口水也可能无法预测地改变尿量。应严格监督并提供固定饮水量。
Pitfall 2: Not accounting for bladder residual volume. The bladder may not be completely empty, introducing variability. Instruct participants to void completely at the start, and consider a short preliminary collection period.
误区二：未考虑膀胱残余量。 膀胱可能未完全排空，从而引入变异性。指导受试者在开始时彻底排空，并可考虑设置一个短暂的预收集期。
Pitfall 3: Confusing correlation with causation. If urine output increases after drinking coffee, one infers caffeine is a diuretic, but other factors like the volume of fluid consumed also matter. Use a control group drinking the same volume of water.
误区三：混淆相关与因果。 如果饮用咖啡后尿量增加，可推断咖啡因是利尿剂，但所饮用的液体体积等其他因素也很关键。应设置饮用等量白水的对照组。
Pitfall 4: Ignoring temperature effects. In hot conditions, more water is lost through sweat, reducing urine volume irrespective of treatment. Standardise environmental temperature or measure both sweat and urine output.
误区四：忽视温度效应。 在炎热条件下，更多水分通过汗液流失，因而无论施加何种处理尿量都会减少。应统一环境温度或同时测量出汗量和尿量。
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