📚 The Alveoli and Gas Exchange | 肺泡与气体交换
Breathing is more than just moving air in and out of the lungs. The real purpose of the respiratory system is to allow oxygen to enter the blood and carbon dioxide to leave it. This process, called gas exchange, happens in tiny air sacs known as alveoli.
呼吸不仅仅是让空气进出肺部。呼吸系统的真正功能是让氧气进入血液，并让二氧化碳排出血液。这个过程称为气体交换，发生在被称为肺泡的微小气囊中。
1. The Human Respiratory System | 人体呼吸系统
Air enters through the nose or mouth, passes down the trachea, then splits into two bronchi, one for each lung. Inside the lungs, the bronchi branch into smaller bronchioles, which finally end in clusters of alveoli.
空气从鼻腔或口腔进入，沿气管下行，然后分为两根支气管，每根通向一个肺。在肺内部，支气管进一步分支成更细的细支气管，最终末端是一簇簇肺泡。
The pathway of air is: nose → trachea → bronchi → bronchioles → alveoli.
空气的路径是：鼻 → 气管 → 支气管 → 细支气管 → 肺泡。
2. What Are Alveoli? | 什么是肺泡？
Alveoli are tiny, ball-shaped air sacs at the very end of the bronchioles. Each lung contains around 350 million of them, giving a huge total surface area for gas exchange.
肺泡是位于细支气管最末端的小球状气囊。每个肺大约含有3.5亿个肺泡，提供了巨大的总表面积用于气体交换。
If all the alveoli in both lungs were spread out flat, their combined surface area would be about 70 square metres — roughly the size of a tennis court.
如果将两肺中所有肺泡平铺展开，它们的总表面积约为70平方米，大约相当于一个网球场的大小。
3. Structural Features of Alveoli | 肺泡的结构特点
Alveoli are very well adapted for efficient gas exchange. They have several key features:
肺泡非常适合高效气体交换。它们具有几个关键特点：
• Thin walls — each alveolar wall is only one cell thick (squamous epithelium), allowing gases to diffuse over a very short distance.
• 壁薄——每层肺泡壁只有一层细胞厚（鳞状上皮），使气体可以在极短的距离内扩散。
• Large surface area — millions of alveoli provide an enormous area for diffusion.
• 表面积大——数百万个肺泡提供了巨大的扩散面积。
• Good blood supply — each alveolus is surrounded by a dense network of capillaries, which constantly carry blood away from the alveoli, maintaining a steep concentration gradient.
• 良好的血液供应——每个肺泡周围都有密集的毛细血管网，这些血管不断将血液带走，从而维持陡峭的浓度梯度。
• Moist walls — the inner surface of each alveolus is coated with a thin layer of moisture, which helps oxygen dissolve before it diffuses through.
• 湿润的壁——每个肺泡内表面覆盖着一层薄薄的液体，这有助于氧气先溶解再扩散通过。
In the table below, the three main adaptations are summarised.
下表总结了三个主要适应性特征。
| Feature | 特征 | How it helps | 如何帮助 |
|---|---|
| Thin walls | 壁薄 | Short diffusion distance | 缩短扩散距离 |
| Large surface area | 表面积大 | More gas can diffuse at once | 更多气体同时扩散 |
| Rich blood supply | 丰富的血液供应 | Maintains steep concentration gradient | 维持陡峭的浓度梯度 |
4. Diffusion of Oxygen and Carbon Dioxide | 氧气与二氧化碳的扩散
Gas exchange in alveoli happens by simple diffusion. Oxygen in the air inside the alveolus has a higher concentration than the oxygen in the deoxygenated blood arriving in the capillaries. Therefore, oxygen diffuses across the alveolar wall and the capillary wall into the blood.
肺泡内的气体交换通过简单扩散进行。肺泡空气内的氧气浓度高于到达毛细血管的脱氧血液中的氧气浓度。因此，氧气扩散穿过肺泡壁和毛细血管壁进入血液。
At the same time, carbon dioxide in the blood has a higher concentration than carbon dioxide in the alveolar air, so it diffuses in the opposite direction — out of the blood and into the alveolus, to be breathed out.
与此同时，血液中的二氧化碳浓度高于肺泡空气中的二氧化碳浓度，因此它向相反方向扩散——从血液出来进入肺泡，然后被呼出。
O₂ (alveolus) → O₂ (blood)
CO₂ (blood) → CO₂ (alveolus)
5. Role of Haemoglobin | 血红蛋白的作用
Oxygen is not very soluble in plasma, so it is carried mainly by haemoglobin inside red blood cells. Each haemoglobin molecule can bind up to four oxygen molecules.
氧气在血浆中溶解度很低，因此它主要由红细胞内的血红蛋白携带。每个血红蛋白分子最多可以结合四个氧分子。
When oxygen binds to haemoglobin, it forms oxyhaemoglobin. This reaction is reversible: in the lungs, oxygen loads onto haemoglobin, and in body tissues, it unloads.
当氧气与血红蛋白结合时，形成氧合血红蛋白。这个反应是可逆的：在肺部，氧气与血红蛋白结合；在身体组织中，氧气被释放。
Haemoglobin + O₂ ⇌ Oxyhaemoglobin
血红蛋白 + O₂ ⇌ 氧合血红蛋白
6. Transport of Carbon Dioxide | 二氧化碳的运输
Most carbon dioxide (about 85%) is carried in the plasma as hydrogencarbonate ions. A small amount binds to haemoglobin, and a little is simply dissolved in plasma.
大部分二氧化碳（约85%）以碳酸氢根离子的形式存在于血浆中。少量与血红蛋白结合，还有少量直接溶解在血浆中。
Carbon dioxide diffuses into red blood cells, where the enzyme carbonic anhydrase speeds up its reaction with water to form carbonic acid. This acid then dissociates into hydrogen ions and hydrogencarbonate ions.
二氧化碳扩散进入红细胞，在碳酸酐酶催化下与水快速反应生成碳酸。碳酸随后解离成氢离子和碳酸氢根离子。
7. Ventilation and Gas Exchange | 通气与气体交换
Ventilation is the process of moving air into and out of the lungs. It is necessary to maintain the concentration gradients of oxygen and carbon dioxide between the alveolar air and the blood.
通气是指将空气吸入和排出肺部的过程。它对于维持肺泡空气与血液之间氧气和二氧化碳的浓度梯度是必需的。
Inhalation (breathing in): the diaphragm contracts and flattens, the external intercostal muscles contract, moving the ribs up and out. The volume of the chest cavity increases, pressure decreases, and air flows in.
吸气：膈肌收缩并变平，肋间外肌收缩，使肋骨向上向外移动。胸腔容积增大，压力降低，空气流入。
Exhalation (breathing out): the diaphragm relaxes, the internal intercostal muscles contract, pulling ribs down and in. The chest volume decreases, pressure increases, and air flows out.
呼气：膈肌舒张，肋间内肌收缩，将肋骨向下向内拉动。胸腔容积减小，压力增大，空气流出。
8. The Effect of Smoking on Alveoli | 吸烟对肺泡的影响
Cigarette smoke contains harmful substances, including tar, carbon monoxide and nicotine. Tar is a sticky black substance that damages the walls of the alveoli.
香烟烟雾含有有害物质，包括焦油、一氧化碳和尼古丁。焦油是一种粘稠的黑色物质，会破坏肺泡壁的结构。
Over time, the walls of many alveoli break down, merging small sacs into larger ones. This reduces the total surface area for gas exchange. The condition is called emphysema, and it causes severe breathlessness.
随着时间推移，许多肺泡壁被破坏，小气囊合并成大气囊。这减少了气体交换的总表面积。这种疾病称为肺气肿，会导致严重的呼吸困难。
Carbon monoxide in smoke binds irreversibly to haemoglobin, reducing the amount of oxygen the blood can carry. This forces the heart to work harder.
烟雾中的一氧化碳与血红蛋白不可逆结合，降低血液携带氧气的能力，迫使心脏更加努力工作。
9. Exercise and Breathing Rate | 运动与呼吸频率
During exercise, muscles respire more quickly, producing more carbon dioxide and using more oxygen. This increases the concentration of carbon dioxide in the blood.
运动时，肌肉呼吸加快，产生更多二氧化碳并消耗更多氧气。这使血液中二氧化碳浓度升高。
The brain detects this change and sends nerve impulses to increase both the rate and depth of breathing. This supplies more oxygen and removes more carbon dioxide to meet the increased demand.
大脑检测到这一变化，发送神经冲动以增加呼吸频率和呼吸深度。这可以提供更多氧气并排出更多二氧化碳，以满足增加的需求。
10. Investigating Carbon Dioxide in Exhaled Air | 探究呼出气体中的二氧化碳
A simple laboratory test uses limewater (calcium hydroxide solution). When carbon dioxide is bubbled through limewater, it turns milky or cloudy due to the formation of calcium carbonate.
一个简单的实验使用石灰水（氢氧化钙溶液）。当二氧化碳通入石灰水时，由于生成碳酸钙，石灰水变浑浊或呈乳白色。
In a typical comparison, exhaled air turns limewater cloudy much faster than inhaled air, proving that exhaled air contains more carbon dioxide.
在典型的对照实验中，呼出的气体使石灰水变浑浊的速度比吸入的空气快得多，证明呼出气体含有更多二氧化碳。
CO₂ + Ca(OH)₂ → CaCO₃ + H₂O
二氧化碳 + 氢氧化钙 → 碳酸钙 + 水
11. Common Exam Questions on Alveoli | 关于肺泡的常见考题
Students are often asked to explain why alveoli are efficient for gas exchange, or to describe the pathway of air, or to compare inhaled and exhaled air. Marks are usually given for linking structure to function.
学生常被要求解释肺泡为何高效进行气体交换、描述空气路径、或比较吸入气和呼出气。分数通常取决于能否将结构与功能联系起来。
Another common question is about the effects of smoking on the lungs. You should mention damage to alveolar walls, reduced surface area, emphysema, and the effect of carbon monoxide on oxygen transport.
另一个常见问题是吸烟对肺的影响。你应该提到肺泡壁损伤、表面积减少、肺气肿，以及一氧化碳对氧气运输的影响。
12. Key Points to Remember | 需要记住的关键点
Here is a quick summary of the most important facts about alveoli and gas exchange:
以下关于肺泡与气体交换的最重要事实的快速总结：
• Diffusion is passive — no energy is required.
• 扩散是被动的——不需要能量。
• Oxygen moves from high concentration in alveoli to low concentration in blood.
• 氧气从肺泡中的高浓度移向血液中的低浓度。
• Carbon dioxide moves opposite to oxygen.
• 二氧化碳的运动方向与氧气相反。
• The alveolus-capillary barrier is only two cell layers thick.
• 肺泡-毛细血管屏障只有两层细胞厚。
• Breathing rate increases with exercise to maintain high oxygen supply.
• 运动时呼吸频率增加以维持充足的氧气供应。
• Smoking destroys alveolar walls and reduces gas exchange efficiency.
• 吸烟破坏肺泡壁并降低气体交换效率。
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