📚 Gas Exchange in Humans | IGCSE OCR 生物 气体交换考点精讲
Gas exchange is a vital process that supplies oxygen to cells for respiration and removes carbon dioxide. Understanding the structure and function of the human respiratory system is essential for IGCSE OCR Biology. This article covers key exam points, from breathing mechanisms to the effects of smoking, ensuring you are fully prepared.
气体交换是为细胞提供氧气并排出二氧化碳的关键生命过程。理解人体呼吸系统的结构和功能对 IGCSE OCR 生物学至关重要。本文涵盖从呼吸机制到吸烟影响的核心考点，帮助你全面备考。
1. Structure of the Respiratory System | 呼吸系统结构
The human respiratory system consists of the nasal cavity, trachea, bronchi, bronchioles, and alveoli. The trachea branches into two bronchi, which further divide into smaller bronchioles ending in clusters of air sacs called alveoli. The ribs, intercostal muscles, and diaphragm support the mechanics of breathing.
人体呼吸系统由鼻腔、气管、支气管、细支气管和肺泡组成。气管分为两个支气管，继续分支成更小的细支气管，末端是成群的气囊——肺泡。肋骨、肋间肌和膈肌支撑着呼吸运动。
The air enters through the nasal cavity where it is filtered, warmed, and moistened. It then passes down the trachea, which is supported by C-shaped cartilage rings to keep it open. The trachea divides into two bronchi (singular bronchus) that enter the lungs. Inside the lungs, bronchi branch into smaller bronchioles. The bronchioles end in tiny air sacs called alveoli, which are surrounded by a dense network of capillaries.
空气通过鼻腔进入，在此被过滤、加温和湿润。然后向下进入气管，气管有C形软骨环支撑，以保持通畅。气管分为两个支气管进入肺部。在肺内，支气管分支成更小的细支气管。细支气管末端是微小的气囊——肺泡，肺泡周围密布毛细血管网。
The trachea and bronchi are lined with ciliated epithelial cells and goblet cells that secrete mucus. The mucus traps dust and pathogens, and the cilia beat to move the mucus upwards toward the throat to be swallowed or expelled. This self-cleaning mechanism helps protect the delicate alveoli from infection.
气管和支气管内壁衬有纤毛上皮细胞和分泌黏液的杯状细胞。黏液能捕获灰尘和病原体，纤毛摆动将黏液向上推向咽喉，然后被吞下或咳出。这种自洁机制有助于保护脆弱的肺泡免受感染。
2. Differences Between Inhaled and Exhaled Air | 吸入与呼出空气的差异
Inhaled air contains approximately 21% oxygen, 0.04% carbon dioxide, and a small amount of water vapour. Exhaled air has reduced oxygen (around 16%) and increased carbon dioxide (about 4%), with saturated water vapour. The nitrogen content remains largely unchanged at about 78%.
吸入空气含约21%的氧气、0.04%的二氧化碳和少量水蒸气。呼出空气中氧气减少（约16%），二氧化碳增加（约4%），且水蒸气饱和。氮气含量基本不变，约占78%。
These differences arise because oxygen diffuses from the alveoli into the blood, while carbon dioxide diffuses from the blood into the alveoli to be exhaled. The air is also warmed and gains moisture as it travels through the respiratory passages. The body does not use nitrogen, so its concentration stays constant.
这些差异是因为氧气从肺泡扩散入血液，而二氧化碳从血液扩散入肺泡被呼出。空气在呼吸道中还得到加热和湿润。人体不利用氮气，因此其浓度保持不变。
3. Gas Exchange at the Alveoli | 肺泡内的气体交换
Gas exchange occurs in the alveoli by diffusion. Oxygen diffuses from the alveolar air space into the blood capillaries because the oxygen concentration in the alveoli is higher than in the deoxygenated blood. Simultaneously, carbon dioxide diffuses from the blood (where its concentration is higher) into the alveoli. The gases move down their concentration gradients without requiring energy.
气体交换通过扩散在肺泡进行。由于肺泡内的氧气浓度高于缺氧血，氧气从肺泡气腔扩散入毛细血管。同时，二氧化碳从血液（浓度较高）扩散入肺泡。气体沿浓度梯度移动，不耗能。
As oxygen dissolves in the moist lining and crosses the thin alveolar and capillary walls, it enters red blood cells and binds to haemoglobin. The resulting oxyhaemoglobin is then transported to body cells. Carbon dioxide produced during respiration travels in plasma and is released into the alveoli to be exhaled.
氧气在湿润的内壁溶解，穿过极薄的肺泡壁和毛细血管壁，进入红细胞并与血红蛋白结合。形成的氧合血红蛋白随后被运送到身体细胞。呼吸作用产生的二氧化碳在血浆中运输，并释放入肺泡被呼出。
4. Adaptations of Alveoli for Gas Exchange | 肺泡适应气体交换的特征
Alveoli are adapted for efficient gas exchange in several ways:
肺泡通过多种方式适应高效气体交换：
1. Large surface area – millions of alveoli provide a combined surface area of approximately 70 m² in an adult, greatly increasing the rate of diffusion.
1. 表面积大——成年人约有数百万个肺泡，总表面积约70平方米，大大加快扩散速率。
2. Thin walls – each alveolus wall is only one cell thick (made of squamous epithelium), minimizing the diffusion distance for gases.
2. 壁薄——每个肺泡壁仅一个细胞厚（由扁平上皮构成），最大程度缩短气体扩散距离。
3. Moist surface – the inner alveolar surface is covered with a thin film of moisture, allowing gases to dissolve before diffusing through the membrane.
3. 湿润内表面——肺泡内壁覆盖一层薄液膜，使气体在穿过膜之前先溶解。
4. Rich blood supply – a dense network of capillaries surrounds each alveolus, maintaining a steep concentration gradient by constantly carrying away oxygenated blood and bringing deoxygenated blood.
4. 丰富的血液供应——每个肺泡周围有致密的毛细血管网，不断运走含氧血并带来缺氧血，维持了陡峭的浓度梯度。
5. Elastic fibres – the alveolar walls contain elastic fibres that enable the alveoli to stretch during inhalation and recoil during exhalation, helping to force air out.
5. 弹性纤维——肺泡壁含有弹性纤维，使肺泡在吸气时能扩张，在呼气时能回缩，有助于排出空气。
5. Mechanism of Inhalation (Breathing In) | 吸气机制
Inhalation is an active process. The external intercostal muscles contract, pulling the ribcage upwards and outwards. At the same time, the diaphragm muscle contracts and flattens. These movements increase the volume of the thoracic cavity. As volume increases, the pressure inside the lungs decreases below atmospheric pressure. Air is drawn into the lungs to equalize the pressure.
吸气是一个主动过程。肋间外肌收缩，将肋骨向上向外提起。同时，膈肌收缩变平。这些运动增大了胸腔容积。容积增大时，肺内压力降低，低于大气压。空气被吸入肺中以平衡压力。
During deep or forced inhalation, additional muscles such as the sternocleidomastoid and pectoral muscles may also help raise the ribs further, increasing the inspiratory volume. The abdominal muscles relax to allow the diaphragm to move downwards easily.
在深吸气或用力和吸气时，如胸锁乳突肌和胸肌等额外肌肉也可能协助将肋骨提得更高，增加吸入气体量。腹肌放松，使膈肌能够轻松下移。
6. Mechanism of Exhalation (Breathing Out) | 呼气机制
Normal exhalation at rest is mostly passive. The external intercostal muscles and diaphragm relax. The ribcage moves down and in, and the diaphragm returns to its dome shape. This decreases thoracic volume, increasing the pressure inside the lungs above atmospheric pressure. Air is forced out of the lungs.
静息时正常呼气大多为被动过程。肋间外肌和膈肌放松。胸腔向下向内移动，膈肌恢复穹顶状。这减小了胸腔容积，使肺内压高于大气压，空气被压出肺。
During exercise or forced exhalation, the internal intercostal muscles contract to pull the ribs down more forcibly, and abdominal muscles contract to push the diaphragm up further. This produces a more rapid and complete expulsion of air.
运动或用力呼气时，肋间内肌收缩，更有力地下拉肋骨；腹肌收缩，进一步上推膈肌。这样能更迅速、更彻底地排出空气。
7. Effects of Exercise on Breathing | 运动对呼吸的影响
During exercise, muscle cells respire faster, consuming more oxygen and producing more carbon dioxide. The increased CO₂ level in the blood is detected by chemoreceptors in the medulla oblongata. These receptors send impulses to increase the breathing rate and depth, leading to higher minute ventilation. This helps meet the oxygen demand and remove excess CO₂.
运动时，肌肉细胞呼吸加快，消耗更多氧气并产生更多二氧化碳。血液中升高的二氧化碳浓度被延髓的化学感受器探测到。这些感受器发出神经冲动，加快呼吸频率和深度，使每分通气量增加，从而满足氧气需求并清除多余的CO₂。
Breathing rate can rise from about 12 breaths per minute at rest to over 30 during vigorous exercise. Tidal volume also increases, and the extra air intake boosts the oxygen supply to muscles while removing metabolic waste. After exercise, breathing remains elevated for a period to repay the oxygen debt and clear lactic acid.
呼吸频率从静息时的约12次/分可升至剧烈运动时的30次/分以上。潮气量也增加，额外吸入的空气增加了对肌肉的氧气供应，同时带走了代谢废物。运动后一段时间内，呼吸仍保持较高频率，以偿还氧债并清除乳酸。
8. Lung Volumes and Spirometry | 肺容积与肺活量测定
A spirometer trace shows different lung volumes. Tidal volume (TV) is the volume of air moved in and out during a normal breath (about 0.5 dm³). Inspiratory reserve volume (IRV) is the extra air that can be inhaled after a normal inhalation. Expiratory reserve volume (ERV) is the extra air that can be exhaled after a normal exhalation.
肺活量计描记图可显示不同的肺容积。潮气量（TV）是正常呼吸时出入肺的气量（约0.5 dm³）。补吸气量（IRV）是在正常吸气后还能额外吸入的气量。补呼气量（ERV）是正常呼气后还能额外呼出的气量。
(RV) remains in the lungs to prevent collapse and keeps alveoli slightly inflated; it cannot be measured by a simple spirometer.
肺活量（VC）是最大吸气后能呼出的最大气量，计算公式为 VC = TV + IRV + ERV。残气量（RV）留在肺中以防止肺塌陷，并使肺泡保持轻微膨胀；它无法用普通肺活量计测得。
During exercise, the trace shows increased tidal volume and breathing rate, resulting in a higher minute ventilation. Exam questions often ask you to calculate TV or breathing rate from a given trace.
运动期间，描记图显示潮气量和呼吸频率增加，导致每分通气量升高。考试常要求根据给定的描记图计算潮气量或呼吸频率。
9. Effects of Smoking on the Respiratory System | 吸烟对呼吸系统的影响
Cigarette smoke contains harmful chemicals such as tar, nicotine, and carbon monoxide. Tar destroys the cilia lining the airways, so mucus and trapped pathogens cannot be swept away. This leads to a persistent smoker’s cough and chronic bronchitis, where the airways become inflamed and narrowed.
香烟烟雾含有焦油、尼古丁和一氧化碳等有害物质。焦油破坏气道内壁的纤毛，使黏液和截留的病原体无法被清除。这导致吸烟者持续性咳嗽和慢性支气管炎，即气道发炎变窄。
Tar also damages the alveolar walls, causing them to break down and merge into larger, irregular sacs. This reduces the total surface area for gas exchange – a condition called emphysema. Sufferers become breathless even after minimal exertion because their lungs cannot oxygenate blood efficiently.
焦油还损害肺泡壁，使其破裂并融合成较大的不规则囊泡。这
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