📚 Gas Exchange | 气体交换
Gas exchange is a vital process that supplies cells with oxygen for respiration and removes carbon dioxide, a waste product. In the CIE GCSE Biology syllabus, this topic covers the structures and mechanisms in humans and plants that facilitate efficient diffusion of these gases. Understanding how the breathing system works, how alveoli are adapted, and how plants exchange gases through stomata is essential for exam success.
气体交换是向细胞提供呼吸作用所需氧气并清除代谢废物二氧化碳的关键过程。在 CIE GCSE 生物考纲中，本主题涵盖了人类和植物促进这些气体高效扩散的结构与机制。理解呼吸系统如何工作、肺泡如何适应以及植物如何通过气孔进行气体交换，是考试成功的关键。
1. Structure of the Human Breathing System | 人体呼吸系统的结构
Air travels through the nasal cavity, where it is filtered by hairs and sticky mucus, warmed by blood capillaries, and moistened. It then passes the pharynx and larynx before entering the trachea, a tube reinforced with C-shaped cartilage rings to keep it open.
空气经过鼻腔，在此被鼻毛和粘性黏液过滤、被毛细血管加温并湿润。然后经过咽和喉，进入气管——一个由 C 形软骨环加固以保持通畅的管子。
The trachea splits into two bronchi, one entering each lung. Inside the lungs, bronchi divide into smaller bronchioles, which finally end in clusters of microscopic air sacs called alveoli. The lungs are protected by the ribcage and lie above the diaphragm, a dome-shaped muscle.
气管分成两条支气管，分别进入两侧肺部。在肺内部，支气管分支为更小的细支气管，最终终止于成簇的微观气囊——肺泡。肺由胸廓保护，并位于圆顶状的膈肌之上。
2. Alveoli: The Site of Gas Exchange | 肺泡：气体交换的场所
Alveoli are the functional units of the lungs where gas exchange takes place. Each lung contains millions of alveoli, creating an enormous total surface area of around 70 m² in an adult. Their walls are extremely thin, consisting of a single layer of squamous epithelial cells.
肺泡是肺的功能单位，气体交换在此进行。每个肺含有数百万个肺泡，成人总表面积可达约 70 平方米。它们的壁极薄，由单层扁平上皮细胞构成。
Surrounding each alveolus is a dense network of pulmonary capillaries. The barrier between the air in the alveolus and the blood in the capillary is only about 0.5 micrometres thick, minimising the diffusion distance for oxygen and carbon dioxide.
每个肺泡周围密布着肺毛细血管网。肺泡内空气和毛细血管内血液之间的屏障仅约 0.5 微米厚，极大地缩短了氧气和二氧化碳的扩散距离。
3. Adaptations of Alveoli for Efficient Gas Exchange | 肺泡适应高效气体交换的特征
Alveoli show several key adaptations for rapid diffusion. Their large combined surface area, provided by millions of tiny sacs, maximises the space available for gas exchange. The barrier is only one cell thick both for the alveolar wall and the capillary wall, so the total diffusion path is two flattened cells plus a thin basement membrane.
肺泡表现出若干促进快速扩散的关键适应特征。数百万个微小气囊提供了巨大的总表面积，最大化了气体交换可用空间。屏障仅有一层细胞厚度——肺泡壁和毛细血管壁，因此总扩散路径仅为两层扁平细胞加上一层薄的基膜。
A rich blood supply maintains a steep concentration gradient: blood arriving is low in oxygen and high in carbon dioxide, while inhaled air is high in oxygen and low in carbon dioxide. The inner surface of each alveolus is covered with a thin film of moisture and surfactant, allowing gases to dissolve before diffusing and preventing collapse.
丰富的血液供应维持了陡峭的浓度梯度：流经的血液氧气浓度低、二氧化碳浓度高，而吸入的气体氧气浓度高、二氧化碳浓度低。每个肺泡的内壁覆盖着一层薄的水分和表面活性剂膜，使气体在扩散前先溶解，并防止肺泡塌陷。
4. Mechanism of Breathing: Inhalation and Exhalation | 呼吸机制：吸气与呼气
Breathing, or ventilation, involves changing the volume and pressure of the thoracic cavity. During inhalation, the external intercostal muscles contract and the ribs move upwards and outwards. At the same time, the diaphragm contracts and flattens.
呼吸（通气）通过改变胸腔容积和压力来实现。吸气时，外肋间肌收缩，肋骨向上、向外移动。同时，膈肌收缩并变得扁平。
This increases the volume of the chest cavity, which lowers the pressure inside the lungs below atmospheric pressure. Air rushes in through the trachea to equalise pressure. Exhalation is largely passive: the intercostal muscles and diaphragm relax, the ribcage drops, the diaphragm domes upwards, volume decreases and pressure rises above atmospheric pressure, forcing air out.
这增大了胸腔容积，使肺内压降低至低于大气压，空气经气管冲入以平衡压力。呼气则主要是被动过程：肋间肌和膈肌舒张，胸廓下沉，膈肌向上隆起，容积减小，肺内压升至高于大气压，将空气压出。
The internal intercostal muscles contract only during forced exhalation (e.g. during exercise) to pull the ribs downwards more quickly.
内肋间肌仅在用力呼气时（例如运动时）收缩，以更快地将肋骨向下拉。
| Action / 动作 | Diaphragm / 膈肌 | Intercostal muscles / 肋间肌 | Thoracic volume / 胸腔容积 | Pressure in lungs / 肺内压 |
|---|---|---|---|---|
| Inhalation / 吸气 | Contracts and flattens / 收缩变平 | External intercostals contract / 外肋间肌收缩 | Increases / 增大 | Decreases below atmospheric pressure / 降至低于大气压 |
| Exhalation / 呼气 | Relaxes and domes up / 舒张隆起 | Relax (internal contract during forced) / 舒张（用力时内肋间肌收缩） | Decreases / 减小 | Increases above atmospheric pressure / 升至高于大气压 |
5. Gas Exchange Process: Diffusion of Oxygen and Carbon Dioxide | 气体交换过程：氧气与二氧化碳的扩散
Gas exchange in the alveoli relies entirely on diffusion. Oxygen diffuses from the alveolar air space, where its partial pressure is high (about 13.3 kPa), into the blood in pulmonary capillaries, where its partial pressure is low (about 5.3 kPa). Carbon dioxide moves in the opposite direction, from the blood (high partial pressure, ~6.1 kPa) into the alveolar air (low partial pressure, ~5.3 kPa).
肺泡中的气体交换完全依赖扩散。氧气从肺泡气腔（分压高，约 13.3 kPa）扩散进入肺毛细血管血液（分压低，约 5.3 kPa）。二氧化碳则相反方向移动，从血液（高分压，约 6.1 kPa）扩散进入肺泡气（低分压，约 5.3 kPa）。
Once in the blood, oxygen combines with haemoglobin in red blood cells to form oxyhaemoglobin, which is transported to body tissues. Carbon dioxide is carried in the plasma mostly as hydrogen carbonate ions (HCO₃⁻) after being converted by the enzyme carbonic anhydrase.
进入血液后，氧气与红细胞中的血红蛋白结合形成氧合血红蛋白，并被运往全身组织。二氧化碳在血浆中主要以碳酸氢根离子（HCO₃⁻）的形式运输，此过程由碳酸酐酶催化。
O₂ (high concentration) + Haemoglobin ⇌ HbO₂ (oxyhaemoglobin)
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
6. Composition of Inhaled and Exhaled Air | 吸入气与呼出气的成分比较
Inhaled air contains approximately 21% oxygen, 0.04% carbon dioxide and 78% nitrogen. Exhaled air still contains about 16% oxygen, 4% carbon dioxide and 78% nitrogen, along with increased water vapour because the respiratory surfaces are moist. The changes confirm that the body uses oxygen and produces carbon dioxide and water.
吸入的空气大约含 21% 氧气、0.04% 二氧化碳和 78% 氮气。呼出的空气仍含约 16% 氧气、4% 二氧化碳和 78% 氮气，并且水蒸气含量增加，因为呼吸表面是湿润的。这些变化证实身体消耗氧气并产生二氧化碳和水。
The composition of exhaled air can be investigated experimentally by inhaling through limewater (which remains clear) and exhaling through limewater (which turns milky due to carbon dioxide). A thermometer can show that exhaled air is warmer, and cobalt chloride paper turns pink in the presence of water vapour.
可以通过实验检测呼出气体成分：用石灰水检测，吸入端石灰水保持澄清，呼出端石灰水变浑浊（因含二氧化碳）。温度计可显示呼出空气更温暖，氯化钴试纸遇水蒸气变粉。
7. Effects of Exercise on Breathing | 运动对呼吸的影响
During muscular exercise, cells respire more rapidly, increasing the demand for oxygen and the production of carbon dioxide. The brain detects rising CO₂ levels in the blood via chemoreceptors; this triggers the medulla oblongata to send more frequent nerve impulses to the diaphragm and intercostal muscles.
肌肉运动时，细胞呼吸作用速率加快，对氧气的需求增加，二氧化碳产生增多。脑部化学感受器检测到血液中 CO₂ 水平上升，这样触发延髓向膈肌和肋间肌发送更高频的神经冲动。
As a result, both breathing rate and depth (tidal volume) increase to deliver more oxygen to the blood and remove excess carbon dioxide. Heart rate also increases simultaneously to speed up blood circulation. After exercise stops, an oxygen debt remains, and breathing stays elevated until the debt is repaid and excess lactic acid is oxidised.
结果，呼吸频率和呼吸深度（潮气量）均增加，以便向血液输送更多氧气并清除过量二氧化碳。心率也同时增快以加速血液循环。运动停止后仍存在氧债，呼吸持续加快直至氧债偿还、过量乳酸被氧化。
ar endurance training improves the efficiency of the respiratory muscles and gas exchange.
这种强力通气量以每分钟通气量（呼吸频率 × 潮气量）来衡量。规律的耐力训练可提高呼吸肌和气体交换的效率。
8. Smoking and Gas Exchange | 吸烟与气体交换
Tobacco smoke contains tar, nicotine, carbon monoxide, and other chemicals that severely damage the gas exchange system. Tar paralyses and destroys cilia lining the airways, so mucus and trapped pathogens cannot be cleared, leading to chronic bronchitis marked by inflammation and excess mucus production.
烟草烟雾含有焦油、尼古丁、一氧化碳和其他严重损害气体交换系统的化学物质。焦油麻痹并破坏气道上皮的纤毛，导致黏液和截留的病原体无法清除，从而引发以炎症和过量黏液分泌为特征的慢性支气管炎。
Over time, the alveolar walls may break down and merge, reducing surface area and causing emphysema. Sufferers feel breathless because gas exchange becomes inefficient. Carbon monoxide binds to haemoglobin more strongly than oxygen, forming carboxyhaemoglobin and reducing the blood's oxygen-carrying capacity. Nicotine constricts blood vessels, raising blood pressure and heart rate.
随着时间推移，肺泡壁可能破裂融合，减少表面积，导致肺气肿。患者因气体交换效率下降而感到呼吸困难。一氧化碳与血红蛋白的结合力强于氧气，形成碳氧血红蛋白，降低血液的携氧能力。尼古丁使血管收缩，导致血压和心率升高。
Smoking is the major cause of lung cancer, as carcinogens in tar trigger uncontrolled cell division in bronchial epithelium. Stopping smoking allows some recovery, but destroyed alveoli cannot regenerate.
吸烟是肺癌的主要诱因，因为焦油中的致癌物诱发支气管上皮细胞失控分裂。戒烟可使部分功能恢复，但已破坏的肺泡无法再生。
9. Gas Exchange in Plants: Leaf Structure | 植物气体交换：叶片结构
Plants need carbon dioxide for photosynthesis and oxygen for respiration. Gas exchange occurs mainly through the leaves, which have a large, flat surface area to capture light and facilitate diffusion. Internally, the spongy mesophyll layer has numerous air spaces that allow gases to move rapidly in and out of cells.
植物需要二氧化碳进行光合作用，也需要氧气进行呼吸作用。气体交换主要通过叶片进行，叶片具有大面积扁平表面，可捕获光能并促进扩散。叶片内部的海绵组织有大量细胞间隙，使气体能够快速进出细胞。
The leaf is covered by a waxy cuticle that reduces water loss but is impermeable to gases. Most gas exchange takes place through stomata, tiny pores mainly on the lower epidermis, surrounded by a pair of guard cells.
叶片表面覆盖着蜡质角质层以减少水分蒸发，但它不透气。大部分气体交换通过气孔进行，气孔是主要位于下表皮微小孔隙，由一对保卫细胞环绕。
10. Stomata and Guard Cells: Mechanism and Adaptations | 气孔与保卫细胞：机制与适应
Guard cells are bean-shaped and contain chloroplasts. They open and close the stomatal pore by changing their turgidity. When guard cells take up water by osmosis, they swell and curve apart, opening the pore for gas exchange. When water is lost, they become flaccid and close the pore to conserve water.
保卫细胞呈肾形，含有叶绿体。它们通过改变膨胀度来开启和关闭气孔。保卫细胞通过渗透作用吸水后膨胀并弯曲分开，打开气孔进行气体交换。失水时它们变得松弛，关闭气孔以保存水分。
Opening is often triggered by light, which stimulates potassium ion (K⁺) uptake into guard cells, lowering their water potential and drawing in water. At night, stomata usually close, reducing water loss while still allowing some diffusion for respiration. Stomatal opening balances gas exchange needs against water loss via transpiration.
光会刺激保卫细胞吸收钾离子（K⁺），降低水势并吸水，促使气孔张开。夜间气孔通常关闭，在减少水分损失的同时仍允许一些扩散进行呼吸作用。气孔的开闭在气体交换需求与蒸腾失水之间取得平衡。
Stomata are spaced across the leaf surface to avoid overlapping diffusion shells, ensuring efficient CO₂ uptake. In hydrophytes, stomata may be on the upper surface; in xerophytes, they may be sunken or protected by hairs to reduce transpiration.
气孔在叶面均匀分布，避免扩散界面重叠，确保高效吸收 CO₂。水生植物的气孔可能分布于上表皮；旱生植物的气孔可能凹陷或有茸毛保护，以减少蒸腾。
11. Comparing Human and Plant Gas Exchange | 人体与植物气体交换的比较
Both humans and plants rely on diffusion across a moist, thin, large-surface-area membrane: alveolar walls in humans, mesophyll cell surfaces in plants. Both systems maintain a steep concentration gradient by using a transport system — blood circulation in humans, and air spaces plus transpiration stream in plants.
人类和植物都依赖湿润、薄而表面积大的膜进行扩散：人体是肺泡壁，植物是叶肉细胞表面。两者都通过运输系统维持浓陡梯度——人类依靠血液循环，植物依靠细胞间隙和蒸腾流。
However, plant gas exchange is passive and regulated by stomata, while human ventilation is an active, muscular process. Plants exchange gases through the same pores for both photosynthesis and respiration, leading to a net uptake of CO₂ during the day and a net output of CO₂ at night. Humans must continuously ventilate their lungs regardless of light conditions to sustain their high metabolic rate.
然而，植物气体交换是被动的且由气孔调节，而人的通气是主动的肌肉活动。植物通过同一气孔进行光合作用和呼吸作用气体交换，导致白天净吸收 CO₂、夜晚净释放 CO₂。人类则无论光照条件如何，都必须持续通气以维持高代谢率。
The leaf’s internal network of air spaces is analogous to the branching airways in lungs; both provide a short diffusion path from the external environment to every cell involved in gas exchange.
叶片内部的细胞间隙网络与肺内的气道分支类似；两者都提供了从外界环境到每个参与气体交换细胞的短扩散路径。
Published by TutorHao | Biology Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图