Structure and Function of the Lungs | 肺的结构与功能

📚 Structure and Function of the Lungs | 肺的结构与功能

The lungs are the primary organs of the respiratory system in mammals, responsible for gas exchange between the air and the blood. Their intricate structure — from the branching airways to the thin alveolar walls — is perfectly adapted to maximise oxygen uptake and carbon dioxide removal. In this article, we will explore the anatomy of the lungs, the mechanics of ventilation, and the physiological principles that underpin efficient gas exchange, all tailored to the CIE A-Level Biology syllabus.

肺是哺乳动物呼吸系统的主要器官，负责空气与血液之间的气体交换。其精细的结构——从分支的气道到薄薄的肺泡壁——完美地适应了最大限度摄取氧气和排出二氧化碳的需求。在本文中，我们将结合CIE A-Level生物学考纲，深入探讨肺的解剖结构、通气机制以及支撑高效气体交换的生理学原理。

1. Overview of the Respiratory System | 呼吸系统概览

The human respiratory system consists of a series of conducting airways and the gas-exchange surfaces. Air enters through the nasal cavity, passes through the pharynx, larynx, trachea, bronchi, bronchioles, and finally reaches the alveoli, where gas exchange occurs. The entire pathway from the trachea to the terminal bronchioles is known as the conducting zone, while the respiratory bronchioles and alveoli form the respiratory zone.

人体呼吸系统由一系列传导气道和气体交换表面组成。空气经鼻腔进入，依次通过咽、喉、气管、支气管、细支气管，最终到达肺泡，在此进行气体交换。从气管到终末细支气管的整个通路称为传导区，而呼吸性细支气管和肺泡则构成呼吸区。

The conducting airways serve to warm, humidify, and filter the incoming air. Mucus secreted by goblet cells traps dust and pathogens, while cilia on the epithelial surface beat rhythmically to move mucus upward toward the pharynx, where it is swallowed or expectorated. This mucociliary escalator is a vital defence mechanism.

传导气道的作用是加温、加湿和过滤进入的空气。杯状细胞分泌的黏液可捕获灰尘和病原体，上皮表面的纤毛有节律地摆动，将黏液向上推向咽部，随后被吞咽或咳出。这种黏液纤毛清除系统是重要的防御机制。

2. Gross Anatomy of the Lungs | 肺的大体解剖

The lungs are paired, cone-shaped organs located in the thoracic cavity, protected by the rib cage. The right lung has three lobes (superior, middle, inferior), while the left lung has two lobes (superior, inferior), accommodating space for the heart. Each lung is enclosed in a double-layered membrane called the pleura. The visceral pleura adheres to the lung surface, and the parietal pleura lines the inner chest wall. The pleural cavity between them contains a thin film of fluid that reduces friction and couples the lungs to the chest wall mechanically.

肺是成对的锥形器官，位于胸腔内，受肋骨笼保护。右肺有三个叶（上叶、中叶、下叶），左肺有两个叶（上叶、下叶），为心脏留出空间。每个肺被称为胸膜的双层膜包绕。脏胸膜紧贴肺表面，壁胸膜衬于胸壁内侧。两层之间的胸膜腔含有薄层液体，可减少摩擦并在机械上使肺与胸壁耦联。

The trachea bifurcates into the left and right primary bronchi at the level of the sternal angle (around T4–T5). Each primary bronchus enters the lung at the hilum, along with pulmonary arteries and veins. The bronchi divide into secondary (lobar) bronchi, then tertiary (segmental) bronchi, eventually branching into progressively smaller bronchioles. Cartilage gradually disappears as airways get smaller, and smooth muscle becomes relatively more prominent, allowing bronchoconstriction and bronchodilation.

气管在胸骨角水平（约T4–T5）分叉为左、右主支气管。每条主支气管与肺动脉和肺静脉一起经肺门进入肺。支气管依次分为次级（叶）支气管、三级（段）支气管，最终分支为逐渐变小的细支气管。随着气道变小，软骨逐渐消失，平滑肌相对更加突出，从而允许支气管收缩和舒张。

3. Histology of the Airways | 气道的组织学

The trachea and bronchi are lined by pseudostratified ciliated columnar epithelium containing goblet cells. Beneath the epithelium lies a lamina propria rich in elastic fibres, followed by the submucosa with seromucous glands. The C-shaped cartilage rings in the trachea prevent collapse while allowing the oesophagus to expand during swallowing. As the airways branch, cartilage changes from rings to irregular plates and then disappears entirely in the bronchioles.

气管和支气管内衬假复层纤毛柱状上皮，其中含有杯状细胞。上皮下方是富含弹性纤维的固有层，再往下是含有浆液黏液腺的黏膜下层。气管中的C形软骨环防止气管塌陷，同时允许吞咽时食道扩张。随着气道分支，软骨由环状变为不规则片状，最终在细支气管中完全消失。

Bronchioles are lined by simple ciliated columnar or cuboidal epithelium, and they lack cartilage and glands. They possess a relatively thick layer of smooth muscle, making them the primary sites of airway resistance regulation. The terminal bronchioles, the last of the conducting airways, branch into respiratory bronchioles, which have thin-walled outpocketings called alveoli emerging from their walls.

细支气管内衬单层纤毛柱状或立方上皮，无软骨和腺体。它们具有相对较厚的平滑肌层，是气道阻力调节的主要部位。终末细支气管是传导气道的最后部分，分支为呼吸性细支气管，其壁上有称为肺泡的薄壁小囊泡向外突出。

4. The Alveoli: Structure for Gas Exchange | 肺泡：气体交换的结构

Alveoli are tiny, polyhedral air sacs with a diameter of approximately 200–300 µm. A single human lung contains about 300 million alveoli, providing a total surface area of roughly 70 m² — about the size of a tennis court. This enormous surface area is the first key adaptation for efficient gas exchange.

肺泡是微小的多面体状气囊，直径约为200–300微米。单个人肺约有3亿个肺泡，总表面积约为70平方米——大约相当于一个网球场的大小。如此巨大的表面积是高效气体交换的第一个关键适应。

The alveolar wall is composed of two types of epithelial cells. Type I pneumocytes are extremely thin, flattened squamous cells that cover about 95% of the alveolar surface, forming a barrier of only 0.1–0.5 µm thickness for gas diffusion. Type II pneumocytes are cuboidal cells that secrete pulmonary surfactant, a phospholipid-protein mixture that reduces surface tension, preventing alveolar collapse (atelectasis) at the end of expiration.

肺泡壁由两种上皮细胞组成。I型肺泡细胞是极薄的扁平鳞状细胞，覆盖约95%的肺泡表面，形成仅0.1–0.5微米厚的气体扩散屏障。II型肺泡细胞是立方状细胞，分泌肺表面活性物质，这是一种磷脂-蛋白质混合物，可降低表面张力，防止呼气末肺泡塌陷（肺不张）。

Surrounding each alveolus is a dense network of pulmonary capillaries. The respiratory membrane — through which gases must diffuse — consists of the alveolar epithelium, a fused basement membrane, and the capillary endothelium. In many regions, the basement membranes of the alveolar and capillary walls are fused, minimising diffusion distance to less than 1 µm.

每个肺泡周围环绕着密集的肺毛细血管网。气体必须扩散通过的呼吸膜由肺泡上皮、融合的基膜和毛细血管内皮组成。在许多区域，肺泡壁和毛细血管壁的基膜融合在一起，使扩散距离缩短至不足1微米。

5. Ventilation Mechanics | 通气机制

Ventilation is the process of moving air into and out of the lungs. Inspiration is an active process: the diaphragm contracts and flattens, and the external intercostal muscles contract, lifting the rib cage upward and outward. These movements increase the thoracic cavity volume, decreasing the intrapleural pressure, which in turn expands the lungs. Air flows in because alveolar pressure falls below atmospheric pressure (Boyle’s law).

通气是空气进出肺的过程。吸气是一个主动过程：膈肌收缩并变平，肋间外肌收缩使肋骨笼向上向外抬起。这些运动增加胸腔容积，降低胸膜腔内压，从而使肺扩张。由于肺泡压低于大气压（玻意耳定律），空气流入肺内。

Expiration is normally a passive process. The diaphragm relaxes and moves upward, the elastic recoil of the lungs and chest wall decreases the thoracic volume, and intra-alveolar pressure rises above atmospheric pressure, forcing air out. During forced expiration, the internal intercostal muscles and abdominal muscles contract to further compress the thoracic cavity.

呼气通常是一个被动过程。膈肌松弛并向上移动，肺和胸壁的弹性回缩减少胸腔容积，肺泡内压升至高于大气压，迫使空气排出。在用力呼气时，肋间内肌和腹肌收缩，进一步压缩胸腔。

The pressure changes in the pleural cavity are essential to this process. Intrapleural pressure is always sub-atmospheric (about −4 mmHg at rest, falling to about −6 mmHg during inspiration). This negative pressure keeps the lungs against the chest wall. If air enters the pleural space (pneumothorax), the lung collapses due to its elastic recoil.

胸膜腔的压力变化对这一过程至关重要。胸膜腔内压始终低于大气压（静息时约为−4 mmHg，吸气时降至约−6 mmHg）。这种负压使肺紧贴胸壁。如果空气进入胸膜腔（气胸），肺会因弹性回缩而塌陷。

6. Gas Exchange across the Respiratory Membrane | 呼吸膜上的气体交换

Gas exchange occurs by simple diffusion, driven by partial pressure gradients. According to Fick’s law, the rate of diffusion (V) is proportional to the surface area (A), the diffusion coefficient (D), and the partial pressure difference (ΔP), and inversely proportional to the diffusion distance (T):

气体交换通过简单扩散进行，由分压梯度驱动。根据菲克定律，扩散速率（V）与表面积（A）、扩散系数（D）和分压差（ΔP）成正比，与扩散距离（T）成反比：

V = A × D × ΔP / T

Oxygen diffuses from the alveoli (PO₂ ≈ 100 mmHg) into the deoxygenated blood in the pulmonary capillaries (PO₂ ≈ 40 mmHg). Carbon dioxide diffuses in the opposite direction, from the blood (PCO₂ ≈ 46 mmHg) into the alveoli (PCO₂ ≈ 40 mmHg). Although CO₂ diffuses about 20 times faster than O₂ through the respiratory membrane, it is still critical to maintain the correct ventilation–perfusion ratio.

氧气从肺泡（PO₂约100 mmHg）扩散进入肺毛细血管中的去氧血（PO₂约40 mmHg）。二氧化碳沿相反方向扩散，从血液（PCO₂约46 mmHg）进入肺泡（PCO₂约40 mmHg）。虽然CO₂通过呼吸膜的扩散速度约为O₂的20倍，但维持正确的通气/血流比仍然至关重要。

Haemoglobin in red blood cells binds oxygen, forming oxyhaemoglobin. This binding removes free O₂ from the plasma, maintaining a steep concentration gradient and accelerating the diffusion process. The oxygen–haemoglobin dissociation curve is sigmoidal, reflecting cooperativity: the binding of the first O₂ molecule increases the affinity for subsequent molecules. Factors such as low pH (Bohr effect), increased temperature, and high 2,3-BPG shift the curve to the right, promoting oxygen unloading to tissues.

红细胞中的血红蛋白结合氧气，形成氧合血红蛋白。这种结合清除了血浆中的游离O₂，维持陡峭的浓度梯度并加速扩散过程。氧–血红蛋白解离曲线呈S形，反映了协同效应：第一个O₂分子的结合提高了对后续分子的亲和力。低pH（玻尔效应）、温度升高和高2,3-BPG等因素使曲线右移，促进氧气在组织中的释放。

7. Lung Volumes and Capacities | 肺容积与肺容量

Pulmonary function is quantified using spirometry. The following values are typical for a healthy adult:

肺功能通过肺量计进行量化。以下是健康成人的典型数值：

Volume / Capacity 容积/容量	Typical Value (mL) 典型值	Definition 定义
Tidal volume (TV) 潮气量	500	Volume of air inhaled or exhaled per breath during quiet breathing 平静呼吸时每次吸入或呼出的气体量
Inspiratory reserve volume (IRV) 补吸气量	3000	Additional air that can be forcibly inhaled after a normal inspiration 正常吸气后还能再用力吸入的气量
Expiratory reserve volume (ERV) 补呼气量	1200	Additional air that can be forcibly exhaled after a normal expiration 正常呼气后还能再用力呼出的气量
Residual volume (RV) 残气量	1200	Air remaining in the lungs after a maximal forced expiration 最大用力呼气后肺内残留的气量
Vital capacity (VC) 肺活量	4700	TV + IRV + ERV; maximum air exhaled after maximal inspiration 潮气量+补吸气量+补呼气量；最大吸气后最大呼气量
Total lung capacity (TLC) 肺总量	5900	VC + RV; total air held in the lungs at maximal inspiration 肺活量+残气量；最大吸气时肺内总气量
Minute ventilation 每分通气量	7500 (500 × 15)	TV × respiratory rate per minute 潮气量×每分钟呼吸频率
Alveolar ventilation 肺泡通气量	5250 (350 × 15)	(TV − dead space) × respiratory rate; air reaching gas-exchange surfaces per minute (潮气量−无效腔)×呼吸频率；每分钟到达气体交换表面的气量

The anatomical dead space, approximately 150 mL, represents the volume of air that fills the conducting airways and does not participate in gas exchange. Alveolar ventilation is therefore always less than minute ventilation. Certain lung diseases, such as emphysema, increase physiological dead space by damaging alveoli, reducing the efficiency of gas exchange.

解剖无效腔约150 mL，代表填充传导气道且不参与气体交换的气量。因此肺泡通气量总是小于每分通气量。某些肺部疾病（如肺气肿）会通过损伤肺泡而增加生理无效腔，降低气体交换效率。

8. Transport of Oxygen and Carbon Dioxide in the Blood | 血液中氧气和二氧化碳的运输

Oxygen is transported in the blood in two forms: a small dissolved fraction (about 1.5%) and a large bound fraction (about 98.5%) attached to haemoglobin as oxyhaemoglobin. Each haemoglobin molecule can carry up to four O₂ molecules. The reversible reaction is:

氧气以两种形式在血液中运输：一小部分溶解于血浆（约1.5%），绝大部分（约98.5%）以氧合血红蛋白的形式与血红蛋白结合。每个血红蛋白分子最多可携带四个O₂分子。其可逆反应为：

Hb + 4O₂ ⇌ HbO₈

（血红蛋白 + 4O₂ ⇌ 氧合血红蛋白）

In the lungs, high PO₂ drives the reaction to the right; in the tissues, low PO₂ and high CO₂ drive it to the left. The Bohr effect describes how increased H⁺ concentration (lower pH) reduces haemoglobin’s affinity for oxygen, facilitating O₂ release in metabolically active tissues. Meanwhile, oxygen binding to haemoglobin enhances CO₂ uptake in the tissues — the Haldane effect — because deoxygenated haemoglobin (HbH⁺) binds CO₂ more effectively as carbaminohaemoglobin.

在肺中，高PO₂使反应向右进行；在组织中，低PO₂和高CO₂使其向左进行。玻尔效应描述了H⁺浓度升高（pH降低）如何降低血红蛋白对氧的亲和力，促进O₂在代谢活跃组织中的释放。同时，氧与血红蛋白的结合增强了组织中CO₂的摄取——即霍尔丹效应——因为去氧血红蛋白（HbH⁺）作为氨甲酰血红蛋白能更有效地结合CO₂。

Carbon dioxide is transported in three ways: approximately 5–7% dissolved in plasma, about 23% bound to haemoglobin as carbaminohaemoglobin, and roughly 70% converted to bicarbonate ions (HCO₃⁻) via the action of carbonic anhydrase in red blood cells:

二氧化碳以三种方式运输：约5–7%溶解于血浆，约23%以氨甲酰血红蛋白的形式与血红蛋白结合，约70%在红细胞中经碳酸酐酶作用转化为碳酸氢根离子（HCO₃⁻）：

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

二氧化碳 + 水 ⇌ 碳酸 ⇌ 氢离子 + 碳酸氢根离子

The H⁺ produced is buffered by haemoglobin, preventing a large pH change. The chloride shift (Hamburger phenomenon) maintains electrical neutrality as HCO₃⁻ leaves the red blood cell in exchange for Cl⁻. At the alveolar capillaries, these reactions reverse, releasing CO₂ for exhalation.

产生的H⁺由血红蛋白缓冲，防止pH大幅变化。氯离子转移（汉堡现象）在HCO₃⁻离开红细胞时与Cl⁻进行交换，以维持电中性。在肺泡毛细血管处，这些反应逆转，释放出CO₂以供呼出。

9. Control of Breathing | 呼吸的调控

Breathing is controlled by the respiratory centres in the medulla oblongata and pons. The dorsal respiratory group (DRG) drives inspiration, while the ventral respiratory group (VRG) is involved in forced breathing. The pneumotaxic centre in the pons fine-tunes the transition between inspiration and expiration. Motor neurons send impulses via the phrenic nerve to the diaphragm and via intercostal nerves to the intercostal muscles.

呼吸由延髓和脑桥中的呼吸中枢控制。背侧呼吸组（DRG）驱动吸气，腹侧呼吸组（VRG）参与用力呼吸。脑桥中的呼吸调整中枢微调吸气与呼气之间的转换。运动神经元通过膈神经向膈肌、通过肋间神经向肋间肌发送冲动。

The primary stimulus for breathing is the concentration of CO₂ in the blood, monitored by central chemoreceptors on the ventrolateral surface of the medulla. These are sensitive to H⁺ concentration in the cerebrospinal fluid, which changes in response to CO₂ diffusing across the blood–brain barrier. Peripheral chemoreceptors in the carotid and aortic bodies respond mainly to a decrease in arterial PO₂, and to a lesser extent to increased H⁺ and PCO₂.

呼吸的主要刺激因素是血液中的CO₂浓度，由延髓腹外侧表面的中枢化学感受器监测。这些感受器对脑脊液中的H⁺浓度敏感，而H⁺浓度随CO₂扩散通过血脑屏障而变化。颈动脉体和主动脉体中的外周化学感受器主要对动脉PO₂降低作出反应，其次对H⁺和PCO₂升高作出反应。

When PCO₂ rises, the respiratory rate increases; when PCO₂ falls below normal, breathing is inhibited. This explains why hyperventilation (breathing too rapidly) causes dizziness: excessive CO₂ washout reduces the drive to breathe. Stretch receptors in the lungs (Hering–Breuer reflex) protect against over-inflation by inhibiting inspiration when the lungs are distended.

当PCO₂升高时，呼吸频率增加；当PCO₂低于正常水平时，呼吸受到抑制。这解释了为什么过度通气（呼吸过快）会导致头晕：CO₂过度排出减少了呼吸驱动力。肺中的牵张感受器（黑-伯反射）在肺过度扩张时抑制吸气，以防止肺过度充气。

10. Lung Diseases and Clinical Correlations | 肺部疾病与临床联系

Asthma is a chronic inflammatory disease characterised by bronchoconstriction, excess mucus secretion, and airway oedema, resulting in increased airway resistance and difficulty in expiration. It is often triggered by allergens, exercise, or stress. Treatment includes bronchodilators (β₂-agonists such as salbutamol) to relax smooth muscle and corticosteroids to reduce inflammation.

哮喘是一种慢性炎症性疾病，以支气管收缩、黏液过度分泌和气道水肿为特征，导致气道阻力增加和呼气困难。常见诱因包括过敏原、运动或压力。治疗包括支气管扩张剂（如沙丁胺醇等β₂受体激动剂）以松弛平滑肌，以及皮质类固醇以减轻炎症。

Emphysema, a form of COPD (chronic obstructive pulmonary disease), involves destruction of alveolar walls, reducing surface area for gas exchange and causing air trapping. The primary cause is cigarette smoking, which triggers protease (elastase) release from neutrophils and macrophages, overwhelming α₁-antitrypsin, the natural protease inhibitor. The result is loss of elastic recoil, hyperinflation, and inefficient gas exchange.

肺气肿是COPD（慢性阻塞性肺疾病）的一种形式，表现为肺泡壁破坏，减少气体交换表面积并导致气体滞留。主要病因是吸烟，吸烟触发中性粒细胞和巨噬细胞释放蛋白酶（弹性蛋白酶），超越α₁-抗胰蛋白酶（天然蛋白酶抑制剂）的抑制能力。结果是弹性回缩力丧失、肺过度充气和气体交换效率低下。

Pulmonary fibrosis is the thickening and scarring of the alveolar interstitium, which increases the diffusion distance across the respiratory membrane and reduces lung compliance. Fibrotic lungs are stiff and difficult to inflate, leading to decreased vital capacity and hypoxaemia. This condition illustrates the critical importance of both a thin respiratory membrane and compliant lung tissue.

肺纤维化是肺泡间质的增厚和瘢痕化，增加呼吸膜的扩散距离并降低肺顺应性。纤维化肺僵硬且难以扩张，导致肺活量下降和低氧血症。这种疾病说明了薄呼吸膜和顺应性良好的肺组织都至关重要。

11. Adaptations for Efficient Gas Exchange | 高效气体交换的适应性特征

The lungs exhibit a perfect structure–function relationship. The following adaptations collectively ensure that oxygen delivery to the blood occurs rapidly and efficiently:

肺展现了完美的结构-功能关系。以下适应性特征共同确保氧气快速高效地进入血液：

Large surface area: Millions of alveoli provide a total gas-exchange surface of approximately 70 m², ensuring a high rate of diffusion.

巨大的表面积：数百万个肺泡提供约70平方米的气体交换总面积，确保高扩散速率。
Thin respiratory membrane: Type I pneumocytes and capillary endothelium are only 0.1–0.5 µm thick, minimising the diffusion distance to under 1 µm.

薄的呼吸膜：I型肺泡细胞和毛细血管内皮仅0.1–0.5微米厚，使扩散距离缩短至不足1微米。
Steep partial pressure gradients: Continuous ventilation refreshes alveolar air (PO₂ ≈ 100 mmHg), and continuous blood flow removes O₂, maintaining a large ΔP.

陡峭的分压梯度：持续通气更新肺泡气（PO₂约100 mmHg），持续血流移除O₂，维持较大的分压差。
Extensive capillary network: A dense plexus of capillaries around each alveolus maximises blood exposure to the respiratory surface.

丰富的毛细血管网：每个肺泡周围密集的毛细血管丛使血液最大程度地接触呼吸表面。
Moist surface: The alveolar lining is coated with a thin layer of surfactant-containing fluid, allowing gases to dissolve before diffusion.

湿润的表面：肺泡内壁覆盖含表面活性物质的薄层液体，使气体在扩散前先溶解。
Effective ventilation–perfusion matching: Local vasoconstriction in poorly ventilated alveoli diverts blood to well-ventilated regions, optimising overall exchange.

有效的通气/血流匹配：通气不良的肺泡局部血管收缩，将血流引向通气良好的区域，优化整体交换。

12. Key Takeaways for Exams | 考试要点总结

For CIE A-Level Biology, candidates must be able to draw and label the alveolus, explain the significance of each structural feature, and interpret diagrams of oxygen dissociation curves. The following points are frequently tested:

对于CIE A-Level生物学，考生必须能够绘制并标注肺泡结构，解释各结构特征的意义，并解读氧解离曲线图。以下要点经常出现在考题中：

Know the pathway of air: trachea → bronchi → bronchioles → alveoli, and the structural changes along the conducting zone.

记住空气通路：气管→支气管→细支气管→肺泡，以及传导区内结构的逐级变化。
Explain how the alveoli maximise gas exchange: thin cells, large surface area, good blood supply, steep gradients.

解释肺泡如何最大化气体交换：细胞薄、表面积大、血流充足、梯度陡峭。
Describe the mechanisms of inspiration and expiration using pressure changes and muscle actions.

利用压力变化和肌肉动作描述吸气和呼气的过程。
Understand the oxygen–haemoglobin dissociation curve and the effects of pH, temperature, and 2,3-BPG.

理解氧–血红蛋白解离曲线以及pH、温度和2,3-BPG的影响。
Compare the three methods of CO₂ transport and explain the chloride shift.

比较CO₂运输的三种方式并解释氯离子转移。
Use spirometry data to calculate tidal volume, vital capacity, and minute ventilation.

利用肺量计数据计算潮气量、肺活量和每分通气量。

Mastering these concepts will not only prepare you for exam questions but also deepen your understanding of how a healthy respiratory system sustains life and how diseases disrupt its function.

掌握这些概念不仅能帮助你应对考试题目，还能加深你对健康呼吸系统如何维持生命以及疾病如何破坏其功能的理解。

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Structure and Function of the Lungs | 肺的结构与功能