📚 Gas Exchange and Smoking | 气体交换与吸烟
The human respiratory system is elegantly adapted to extract oxygen from the air and expel carbon dioxide. Gas exchange occurs in the alveoli of the lungs, where air meets a vast capillary network. However, smoking introduces a cocktail of toxic chemicals that severely compromise this vital process, leading to diseases like chronic bronchitis, emphysema and lung cancer. Understanding both the normal physiology and the pathological effects of smoking is essential for A‑level Biology.
人体呼吸系统经过精妙演化，能高效地从空气中获取氧气并排出二氧化碳。气体交换在肺部的肺泡中进行，那里空气与广阔的毛细血管网紧密接触。然而，吸烟带来的大量有毒化学物质会严重损害这一关键过程，引发慢性支气管炎、肺气肿和肺癌等疾病。理解正常生理机制以及吸烟的病理性影响是A‑Level生物学的重要内容。
1. Structure of the human gas exchange system | 人体气体交换系统的结构
The human gas exchange system consists of a series of airways that conduct air to the alveoli. Air enters through the nasal passages and mouth, passes the pharynx and larynx, and moves into the trachea. The trachea splits into the left and right primary bronchi, which further branch into secondary and tertiary bronchi, then bronchioles, and finally alveolar ducts and alveolar sacs. This branching network ensures that air is delivered deep into the lungs where gas exchange occurs.
人体气体交换系统由一系列导气通路组成，将空气运输到肺泡。空气从鼻腔和口腔进入，经过咽和喉，进入气管。气管分为左右主支气管，再分支为次级和三级支气管，然后是细支气管，最后到达肺泡管和肺泡囊。这种分支网络确保空气被输送到肺部深处，在那里进行气体交换。
2. The trachea and bronchial tree | 气管与支气管树
The trachea and larger bronchi are supported by C‑shaped rings of cartilage, preventing collapse during pressure changes. Their inner lining is a pseudostratified ciliated columnar epithelium with mucus‑secreting goblet cells. The cilia beat rhythmically to move mucus, trapping dust and pathogens, up towards the pharynx to be swallowed. This mucociliary escalator is a crucial defence mechanism, keeping the conducting airways clean.
气管和较大的支气管由C形软骨环支撑，防止在压力变化时塌陷。其内衬为假复层纤毛柱状上皮，其中含有分泌黏液的杯状细胞。纤毛有节奏地摆动，将捕获了灰尘和病原体的黏液向上推动到咽部并吞下。这种黏液‑纤毛运输系统是关键的防御机制，能保持传导气道清洁。
3. Bronchioles and alveolar ducts | 细支气管与肺泡管
As bronchi branch into smaller bronchioles, the cartilage rings are gradually replaced by smooth muscle. The contraction and relaxation of this smooth muscle regulates the diameter of the airways, controlling airflow. Terminal bronchioles lead into respiratory bronchioles, which have some alveoli budding from their walls, and then into alveolar ducts. The ducts widen into alveolar sacs – clusters of alveoli where the vast majority of gas exchange takes place.
随着支气管分支成更小的细支气管，软骨环逐渐被平滑肌替代。平滑肌的收缩和舒张调节气道直径，从而控制气流。终末细支气管通向呼吸细支气管，其壁上长有一些肺泡芽，然后再通向肺泡管。肺泡管扩大形成肺泡囊——这是大量肺泡聚集的簇状结构，绝大部分气体交换在此发生。
4. Alveoli: the site of gas exchange | 肺泡：气体交换的场所
Alveoli are ideally suited for efficient diffusion. Their walls consist of a single layer of flattened type I pneumocytes, fused with the similarly thin endothelial cells of the surrounding capillaries across a shared basement membrane. This creates an extremely short diffusion distance of approximately 0.5 µm. The lungs contain around 300 million alveoli, providing a total surface area of about 70 m². Type II pneumocytes secrete pulmonary surfactant, a phospholipid that lowers surface tension and prevents alveolar collapse, while elastic fibres allow recoil during expiration. The interior surface is kept moist to dissolve gases before diffusion.
肺泡的结构极为适合高效扩散。其壁由单层扁平的I型肺泡细胞构成，与周围毛细血管同样薄的内皮细胞共用一个基膜融合在一起。这形成了仅约0.5 µm的极短扩散距离。肺内含有大约3亿个肺泡，所提供的总表面积约为70 m²。II型肺泡细胞分泌肺表面活性物质，这是一种磷脂，能降低表面张力并防止肺泡塌陷，而弹性纤维则使呼气时能够回缩。肺泡内表面保持湿润，以便气体在扩散前溶解。
5. Ventilation: mechanics of breathing | 肺通气：呼吸的力学机制
Quiet inspiration is an active process. The diaphragm contracts and flattens, while the external intercostal muscles contract, lifting the ribcage upwards and outwards. These actions increase the volume of the thoracic cavity, causing the intra‑pulmonary pressure to drop below atmospheric pressure. Following Boyle's law (P ∝ 1/V), air flows into the lungs down the pressure gradient. Quiet expiration is passive: the diaphragm and external intercostals relax, the ribcage falls, and elastic recoil of the lungs forces air out. Forced expiration involves contraction of the internal intercostal muscles and abdominal muscles, further reducing thoracic volume.
平静吸气是一个主动过程。膈肌收缩并变平，同时外肋间肌收缩，将胸廓向上和向外提拉。这些动作增加了胸腔容积，使得肺内压力下降到低于大气压力。根据波义耳定律(P ∝ 1/V)，空气顺着压力梯度流入肺部。平静呼气是被动的：膈肌和外肋间肌松弛，胸廓回落，肺的弹性回缩将空气压出。用力呼气则需要内肋间肌和腹肌收缩，进一步减小胸腔容积。
6. Gas exchange at the respiratory membrane | 呼吸膜上的气体交换
Diffusion of oxygen and carbon dioxide occurs passively across the alveolar– al pressure gradients. In the alveolar air, the partial pressure of oxygen (PO₂) is high (about 13.3 kPa) and the partial pressure of carbon dioxide (PCO₂) is low (about 5.3 kPa). Deoxygenated blood entering the pulmonary capillaries has a low PO₂ (about 5.3 kPa) and a high PCO₂ (about 6.1 to the blood and CO₂ diffuses into the alveoli. The rate of diffusion is described by Fick's law: diffusion rate ∝ ference) / thickness of the exchange surface. The large alveolar surface area, steep concentration gradients and minimal membrane thickness ensure rapid equilibration.
氧气和二氧化碳藉由分压梯度，被动地穿过肺泡‑毛细血管膜进行扩散。肺泡气体的氧分压(PO₂)较高(约13.3 kPa)，二氧化碳分压(PCO₂)较低(约5.3 kPa)。进入肺毛细血管的脱氧血液PO₂低(约5.3 kPa)、PCO₂高(约6.1 kPa)。O₂扩散进入血液，CO₂扩散进入肺泡。扩散速率遵循菲克定律：扩散速率 ∝ (表面积 × 分压差) / 交换面厚度。巨大的肺泡表面积、陡峭的浓度梯度与极薄的膜厚度确保了气体的快速平衡。
7. Oxygen and carbon dioxide transport | 氧气和二氧化碳的运输
n bind up to four O₂ molecules. Carbon dioxide is transported in three main ways: about 85% e HCO₃⁻ diffuses into plasma and is exchanged for chloride ions (the chloride shift). About 10% of CO₂ binds to haemoglobin to form carbaminohaemoglobin, and a small fraction (around 5% ssues, while the Haldane effect enhances CO₂ loading in the lungs when oxygen is released.
大多数氧气通过可逆地与红细胞内的血红蛋白(Hb)结合，形成氧合血红蛋白来运输。每个Hb分子最多可结合4个O₂分子。二氧化碳主要通过三种方式运输：约85%在红细胞内通过碳酸酐酶催化转化为碳酸氢根离子(HCO₃⁻)，反应式为CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻；HCO₃⁻扩散进入血浆并与氯离子交换(氯离子转移)。约10%的CO₂与血红蛋白结合形成氨基甲酸血红蛋白，小部分(约5%)直接溶解于血浆。波尔效应促进活跃组织中的氧气卸载，而霍尔丹效应则在肺部氧气释放时增强CO₂的结合。
8. Smoking: harmful substances in tobacco smoke | 吸烟：烟草烟雾中的有害物质
Tobacco smoke contains over 4000 chemicals, many of which are toxic or carcinogenic. Nicotine is highly addictive, stimulates the release of adrenaline, and causes vasoconstriction, increased heart rate and elevated blood pressure. Tar is a thick, sticky mixture of chemicals that coats the airways and contains numerous carcinogens such as benzo[a]pyrene and other polycyclic aromatic hydrocarbons. Carbon monoxide (CO) is a colourless, odourless gas that binds to haemoglobin with an affinity over 200 times greater than that of oxygen, forming carboxyhaemoglobin and significantly reducing the oxygen‑carrying capacity of the blood. Other harmful agents include formaldehyde, hydrogen cyanide and free radicals that damage lung tissue.
烟草烟雾中含有超过4000种化学物质，其中许多有毒或致癌。尼古丁极易成瘾，刺激肾上腺素释放，引起血管收缩、心率加快和血压升高。焦油是一种黏稠的化学混合物，覆盖在气道上，含有苯并[a]芘等多种多环芳烃类致癌物。一氧化碳(CO)是一种无色无味的气体，与血红蛋白的亲和力是氧气的200多倍，形成碳氧血红蛋白，显著降低血液的携氧能力。其他有害物质还包括甲醛、氰化氢和破坏肺组织的自由基。
9. Effects of tar on the airways and cilia | 焦油对气道和纤毛的影响
Tar paralyses and progressively destroys the ciliated epithelium that lines the trachea and bronchi. Without effective ciliary beating, mucus accumulates, and the airways become clogged with sticky secretions laden with pathogens and irritants. Goblet cells over‑produce mucus in response to the chronic irritation. This results in the characteristic 'smoker's cough', particularly upon waking, as the body attempts to clear the airways. The loss of the mucociliary escalator removes a key defence, making smokers far more susceptible to respiratory infections such as bronchitis and pneumonia. Infections further scar and narrow the airways, impairing ventilation and gas exchange.
焦油会使气管和支气管内衬的纤毛上皮麻痹并逐渐被破坏。没有有效的纤毛摆动，黏液便积聚起来；气道被充满病原体和刺激物的粘稠分泌物堵塞。为响应持续刺激，杯状细胞过度分泌黏液，由此产生典型的“吸烟者咳嗽”——尤其在睡醒时，身体试图清理气道。黏液‑纤毛运输系统的丧失使关键防御缺失，导致吸烟者极易患支气管炎和肺炎等呼吸道感染。感染会进一步形成瘢痕并使气道变窄，损害通气与气体交换。
10. Chronic bronchitis and its impact on gas exchange | 慢性支气管炎及其对气体交换的影响
Chronic bronchitis is defined clinically as a persistent productive cough lasting at least three months per year for two consecutive years. Pathologically, it is characterised by hypertrophy of submucosal glands and an increase in goblet cell numbers, leading to excessive mucus production. The mucus plugs the airways and, together with inflammation‑induced thickening of the bronchial wall and smooth muscle contraction, narrows the lumen. This airflow obstruction reduces alveolar ventilation, causing a fall in blood oxygen levels (hypoxaemia) and eventual carbon dioxide retention (hypercapnia). As part of chronic obstructive pulmonary disease (COPD), chronic bronchitis progressively limits the efficiency of pulmonary gas exchange and places strain on the right side of the heart, potentially leading to cor pulmonale.
临床上，慢性支气管炎定义为连续两年、每年持续咳嗽咳痰至少三个月的疾病。病理学上，其特征是黏膜下腺体肥大和杯状细胞数量增加，导致黏液分泌过多。黏液堵塞气道，叠加炎症引起的支气管壁增厚和平滑肌收缩，使管腔变窄。这种气流阻塞减少了肺泡通气，导致血氧水平下降（低氧血症）并最终引起二氧化碳潴留（高碳酸血症）。作为慢性阻塞性肺疾病(COPD)的一部分，慢性支气管炎会逐渐限制肺气体交换的效率，并对右心造成压力，可能发展为肺源性心脏病。
11. Emphysema: destruction of alveolar walls | 肺气肿：肺泡壁的破坏
Emphysema results from the breakdown of elastin and other structural proteins in the alveolar walls. Normally, the lung is protected by α₁‑antitrypsin, which inhibits the enzyme elastase released by neutrophils and macrophages. Tobacco smoke inactivates α₁‑antitrypsin and increases the number of inflammatory cells, leading to uncontrolled elastase activity. Alveolar septa are digested, and individual alveoli merge into large, inefficient bullae. The total surface area for gas exchange is drastically reduced. Moreover, the loss of elastic recoil makes expiration difficult, trapping air in the lungs (air‑trapping) and increasing residual volume. Patients with emphysema develop barrel‑shaped chests and experience severe breathlessness because they cannot exchange sufficient oxygen. Ventilation–perfusion mismatch further worsens gas exchange.
肺气肿是由于肺泡壁中的弹性蛋白和其他结构蛋白被破坏所致。正常时，肺部受α₁‑抗胰蛋白酶的保护，该酶可抑制中性粒细胞和巨噬细胞释放的弹性蛋白酶。烟草烟雾会使α₁‑抗胰蛋白酶失活，并增加炎症细胞数量，导致弹性蛋白酶不受控制地发挥作用。肺泡隔被消化，孤立的肺泡融合成巨大的、无效的肺大疱。用于气体交换的总表面积急剧减少。此外，弹性回缩力的丧失使得呼气困难，造成空气滞留在肺内（空气陷闭），残气量增加。肺气肿病人会出现桶状胸，并因无法充分摄入氧气而感到极度呼吸困难。通气/灌注比例失衡进一步恶化了气体交换。
12. Lung cancer and the loss of respiratory surface | 肺癌与呼吸表面积的丧失
Lung cancer is strongly associated with smoking, responsible for approximately 85% of cases. Carcinogens in tar, particularly benzo[a]pyrene, cause mutations in the DNA of bronchial epithelial cells. Uncontrolled cell division produces a tumour that may be squamous cell carcinoma, adenocarcinoma or small‑cell carcinoma. As the tumour grows, it occupies lung tissue, compresses adjacent airways and directly destroys alveoli, thereby reducing the effective gas exchange surface area. Airflow obstruction and collapse of lung regions (atelectasis) further compromise ventilation and oxygenation. Metastasis to other organs can occur through the lymphatic and blood systems. Common symptoms include a persistent cough, haemoptysis, chest pain and progressive breathlessness, ultimately leading to respiratory failure.
肺癌与吸烟密切相关，约85%的病例由吸烟引起。焦油中的致癌物，特别是苯并[a]芘，会导致支气管上皮细胞的DNA发生突变。不受控制的细胞分裂形成肿瘤，可能是鳞癌、腺癌或小细胞癌。肿瘤生长时会占据肺组织，压迫邻近气道并直接破坏肺泡，从而减少有效的气体交换面积。气道阻塞和肺组织塌陷（肺不张）进一步损害通气与氧合。癌细胞可通过淋巴和血液系统转移至其他器官。常见症状包括持续性咳嗽、咯血、胸痛和进行性呼吸困难，最终导致呼吸衰竭。
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