📚 3.2 Transport in Animals Exam Practice | 3.2 动物体内运输真题精练
This article provides focused exam practice on the key concepts of transport in animals, featuring typical A Level questions with model answers and examiner-style commentary. Work through each section to master the structure and function of the circulatory system, hemoglobin, the cardiac cycle, tissue fluid formation, and more.
本文提供动物体内运输关键概念的真题精练，涵盖典型的 A Level 考题、标准答案及考官点评风格解析。逐节练习，掌握循环系统的结构与功能、血红蛋白、心动周期、组织液形成等重要考点。
1. Haemoglobin and Oxygen Dissociation Curve | 血红蛋白与氧合解离曲线
Question: Describe the oxygen dissociation curve of adult haemoglobin and explain how its shape is related to the cooperative binding of oxygen. (5 marks)
题目：描述成人血红蛋白的氧合解离曲线，并解释其形状如何与氧的协同结合相关。（5分）
Answer: The curve is S-shaped (sigmoid). At low pO₂, the gradient is shallow because haemoglobin has low affinity for the first oxygen molecule. After the first oxygen binds, the haemoglobin undergoes conformational changes, making it easier for subsequent oxygen molecules to bind – this is cooperative binding. This causes the curve to steepen. At high pO₂, the curve plateaus as the haemoglobin becomes nearly saturated. The sigmoid shape ensures effective loading in the lungs and unloading in respiring tissues.
答案：曲线呈 S 形（sigmoid）。在低氧分压下，曲线斜率小，因为血红蛋白对第一个氧分子的亲和力低。第一个氧结合后，血红蛋白发生构象变化，使后续氧分子更容易结合——这就是协同结合。曲线因此变陡。在高氧分压下，血红蛋白接近饱和，曲线趋于平台。S 形曲线保证了在肺部有效装载氧气、在呼吸组织有效卸载氧气。
Key examiner points:
考官评分要点：
- Correctly identifies sigmoid shape; 正确指出 S 形；
- Links initial low affinity to low saturation; 将初始低亲和力与低饱和度关联；
- Explains conformational change and cooperativity; 解释构象变化与协同性；
- Relates shape to physiological loading/unloading. 将形状与生理性装载/卸载关联。
2. Bohr Effect and CO₂ Transport | 波尔效应与二氧化碳运输
Question: Explain the Bohr effect and describe how it enhances oxygen delivery to actively respiring tissues. (4 marks)
题目：解释波尔效应，并说明它如何促进氧气向活跃呼吸组织的输送。（4分）
Answer: The Bohr effect refers to the decrease in haemoglobin’s affinity for oxygen when carbon dioxide concentration increases or pH decreases. In actively respiring tissues, CO₂ production is high, which leads to the formation of carbonic acid, lowering pH. This causes more oxygen to be released from oxyhaemoglobin. The dissociation curve shifts to the right, meaning at a given pO₂, haemoglobin saturation is lower, unloading more oxygen.
答案：波尔效应是指当二氧化碳浓度升高或 pH 降低时，血红蛋白对氧的亲和力下降。在活跃呼吸的组织中，CO₂ 生成量高，形成碳酸使 pH 下降，导致氧合血红蛋白释放更多氧气。解离曲线右移，意味着在给定氧分压下，血红蛋白饱和度更低，从而卸载更多氧气。
Additional note – chloride shift: To maintain electrochemical neutrality, chloride ions enter the red blood cell in exchange for bicarbonate ions moving out; this is the chloride shift, which supports CO₂ transport.
附加说明——氯转移：为维持电中性，氯离子进入红细胞以交换移出碳酸氢根离子，这就是氯转移，它支持了 CO₂ 的运输。
3. Cardiac Cycle and Pressure Changes | 心动周期与压力变化
Question: The table below shows pressures in different heart chambers and vessels. Analyse the changes during one cardiac cycle and explain how pressure differences drive blood flow. (6 marks)
题目：下表显示不同心腔和血管中的压力。分析一个心动周期中的变化，并解释压力差如何驱动血流。（6分）
| Phase | Left atrium (mmHg) | Left ventricle (mmHg) | Aorta (mmHg) |
|---|---|---|---|
| Atrial systole | ↑ 8 → 10 | ↑ 5 → 12 | 80 |
| Ventricular systole (isovolumetric) | ↓ | ↑ rapidly 12 → 120 | 80 |
| Ventricular ejection | low | ↑ 120 → 130 | ↑ 80 → 120 |
| Ventricular diastole | ↑ (filling) | ↓ to near 0 | ↓ 120 → 80 |
Answer: During atrial systole, atrial pressure rises slightly and forces blood into the ventricle. As ventricular systole begins, ventricular pressure rises sharply while all valves are closed (isovolumetric contraction). Once ventricular pressure exceeds aortic pressure, the semilunar valves open and blood is ejected into the aorta. During diastole, ventricular pressure falls below atrial pressure, allowing the AV valves to open and passive ventricular filling to occur. Pressure differences thus ensure unidirectional blood flow.
答案：在心房收缩期，心房压轻度升高，将血液挤入心室。心室开始收缩时，所有瓣膜关闭，心室压迅速升高（等容收缩期）。当心室压超过主动脉压，半月瓣打开，血液射入主动脉。舒张期，心室压低于心房压，房室瓣打开，实现心室被动充盈。压力差因此保证了血液的单向流动。
4. Structure of Blood Vessels | 血管结构与适应性
Question: Compare the structure of arteries, veins and capillaries and explain how each is adapted to its function. (6 marks)
题目：比较动脉、静脉和毛细血管的结构，并解释它们各自如何适应其功能。（6分）
| Feature | Artery | Capillary | Vein |
|---|---|---|---|
| Wall thickness | Thick, with elastic fibres and smooth muscle | One cell thick (endothelium) | Thinner, less muscle and elastic tissue |
| Lumen | Relatively narrow | Very narrow, just enough for RBCs in single file | Wide, with valves |
| Valves | No | No | Yes, prevent backflow |
| Functional adaptation | Withstand high pressure, maintain blood flow via elastic recoil | Short diffusion distance for exchange of gases and nutrients | Low pressure, return blood to heart with the aid of valves and skeletal muscle pump |
Mark scheme focus:
评分要点：
- Arteries have thick walls to handle high pressure; 动脉壁厚以应对高压；
- Elastic fibres allow stretch and recoil, smoothing blood flow; 弹性纤维可伸展回缩，平缓血流；
- Capillaries are extremely thin to minimise diffusion distance; 毛细血管极薄，使扩散距离最小化；
- Veins contain valves and rely on skeletal muscle contraction to return blood. 静脉含有瓣膜，依赖骨骼肌收缩回送血液。
5. Tissue Fluid Formation and Return | 组织液的形成与回流
Question: Explain how tissue fluid is formed at the arterial end of a capillary and how it is returned to the circulatory system at the venous end. (6 marks)
题目：解释组织液如何在毛细血管动脉端形成，以及如何在静脉端返回循环系统。（6分）
Answer: At the arterial end of a capillary, the hydrostatic pressure of the blood (Pc) is higher than the osmotic pressure of plasma proteins (πc). This pressure difference forces water and small solutes out through the capillary walls, forming tissue fluid. At the venous end, Pc has dropped, while πc remains relatively constant. Now the osmotic pull exceeds the hydrostatic pressure, drawing water back into the capillary. Any excess tissue fluid is drained by the lymphatic system and eventually returned to the blood via the subclavian veins.
答案：在毛细血管动脉端，血液静水压（Pc）高于血浆蛋白渗透压（πc），压力差使水和小分子溶质透出毛细血管壁，形成组织液。在静脉端，Pc 降低，而 πc 相对恒定，渗透压拉力超过静水压，将水拉回毛细血管。多余的组织液由淋巴系统引流，最终经锁骨下静脉返回血液。
The relationship can be expressed as:
这一关系可表示为：
Net filtration pressure = (Pc + πi) − (πc + Pi)
where Pc = capillary hydrostatic pressure, πi = interstitial fluid osmotic pressure, πc = plasma colloid osmotic pressure, Pi = interstitial fluid hydrostatic pressure.
其中 Pc = 毛细血管静水压，πi = 组织液胶体渗透压，πc = 血浆胶体渗透压，Pi = 组织液静水压。
6. Electrocardiogram (ECG) Interpretation | 心电图 (ECG) 解读
Question: The diagram of an ECG trace shows the P wave, QRS complex and T wave. Explain what electrical and mechanical events each represents. (3 marks)
题目：心电图上显示了 P 波、QRS 波群和 T 波。解释每个波所代表的电活动和机械事件。（3分）
Answer:
答案：
- P wave: represents atrial depolarisation, which leads to atrial contraction (atrial systole). P 波：代表心房去极化，导致心房收缩（心房收缩期）。
- QRS complex: represents ventricular depolarisation, initiating ventricular contraction. QRS 波群：代表心室去极化，启动心室收缩。
- T wave: represents ventricular repolarisation, which precedes ventricular relaxation. T 波：代表心室复极化，先于心室舒张发生。
Examiner’s tip: Note that atrial repolarisation is hidden within the QRS complex.
考官提示：注意心房复极化波被掩盖在 QRS 波群中。
7. Carbon Dioxide Transport Mechanisms | 二氧化碳运输机制
Question: Describe the three ways in which carbon dioxide is transported in the blood from tissues to the lungs. (4 marks)
题目：描述二氧化碳从组织运输到肺部的三种方式。（4分）
Answer:
答案：
- About 5–10% dissolves directly in the plasma; 约 5–10% 直接溶解在血浆中；
- 20–25% binds to haemoglobin to form carbaminohaemoglobin; 20–25% 与血红蛋白结合形成碳氨基血红蛋白；
- The majority (about 70%) is converted to hydrogen carbonate ions (HCO₃⁻) within red blood cells. CO₂ combines with water to form carbonic acid (H₂CO₃) catalysed by carbonic anhydrase. The acid dissociates into H⁺ and HCO₃⁻, and HCO₃⁻ diffuses out of the cell in exchange for Cl⁻ ions. 绝大多数（约 70%）在红细胞内转化为碳酸氢根离子（HCO₃⁻）。CO₂ 与水在碳酸酐酶催化下生成碳酸（H₂CO₃），然后解离为 H⁺ 和 HCO₃⁻，HCO₃⁻ 通过交换 Cl⁻ 离开细胞。
This process is reversed in the lungs where CO₂ is expelled.
在肺部该过程逆转，CO₂ 被排出。
8. Blood Components and Defence | 血液成分与免疫功能
Question: Erythrocytes and leucocytes are both found in blood. Outline their roles in transport and defence. (4 marks)
题目：红细胞和白细胞均存在于血液中。概述它们在运输和防御中的作用。（4分）
Answer:
答案：
- Erythrocytes (red blood cells): contain haemoglobin for oxygen transport. Their biconcave shape increases surface area for gas exchange, and lack of nucleus leaves more room for haemoglobin. 红细胞：含血红蛋白以运输氧气。双凹碟形增大了气体交换的表面积，无细胞核为血红蛋白腾出更多空间。
- Leucocytes (white blood cells): involved in immune defence. Phagocytes engulf pathogens via phagocytosis; lymphocytes produce antibodies and provide specific immunity. 白细胞：参与免疫防御。吞噬细胞通过吞噬作用吞食病原体；淋巴细胞产生抗体，提供特异性免疫。
Additionally, platelets are responsible for blood clotting to prevent excessive bleeding.
此外，血小板负责凝血，防止过度出血。
9. Mixed Exam-Style Question: High Altitude Adaptation | 混合真题练习：高海拔适应
Question: A person living at sea level moves to a high-altitude location where the partial pressure of oxygen is significantly lower. Describe and explain the short-term and long-term physiological changes in blood oxygen transport. (5 marks)
题目：一名海平面居民移居到氧分压显著较低的高海拔地区。描述并解释血液氧气运输的短期和长期生理变化。（5分）
Model answer:
标准答案：
Short-term: The reduced pO₂ means haemoglobin loads less oxygen in the lungs. The body responds by increasing breathing rate and heart rate to maintain oxygen delivery. The oxygen dissociation curve shifts slightly to the right due to increased CO₂ production, aiding unloading.
短期：氧分压降低，血红蛋白在肺部结合的氧气减少。身体通过加快呼吸率和心率来维持供氧。由于 CO₂ 生成增加，氧合解离曲线轻微右移，有助于氧气卸载。
Long-term: The kidneys detect low oxygen levels and release erythropoietin (EPO), which stimulates the bone marrow to produce more red blood cells. This increases the total oxygen-carrying capacity. Additionally, 2,3-bisphosphoglycerate (2,3-BPG) concentration rises in red blood cells, further decreasing haemoglobin’s oxygen affinity and promoting unloading in tissues. Over weeks, these changes improve endurance at altitude.
长期：肾脏感知到低氧水平，释放促红细胞生成素（EPO），刺激骨髓生成更多红细胞，增加了总携氧能力。同时，红细胞中 2,3-二磷酸甘油酸（2,3-BPG）浓度升高，进一步降低血红蛋白对氧的亲和力，促进组织中的氧卸载。数周内，这些变化提高高海拔下的耐力。
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