📚 A-Level Biology: Structure of the Trachea, Bronchi and Bronchioles | A-Level 生物：气管、支气管与细支气管的结构
The respiratory system consists of a series of branching tubes that conduct air from the external environment to the gas exchange surfaces of the lungs. The trachea, bronchi and bronchioles form the conducting airways, a structural and functional continuum that conditions inspired air and distributes it to the alveoli. Understanding their precise histology and anatomy is a core requirement for CIE A-Level Biology.
呼吸系统由一系列分支管道组成，这些管道将空气从外界环境引导至肺部的气体交换表面。气管、支气管和细支气管构成传导气道，这是一个结构和功能的连续体，负责调节吸入的空气并将其分配到肺泡。理解它们精确的组织学和解剖学结构是 CIE A-Level 生物学的核心要求。
1. Overview of the Conducting Airways | 传导气道概述
The conducting airways extend from the trachea through the primary bronchi, secondary bronchi, tertiary bronchi and into the bronchioles, terminating at the respiratory bronchioles. These structures do not participate in gas exchange; their primary roles are to warm, humidify and filter inhaled air. The walls of these tubes share a common basic plan but show progressive structural modification as the airways branch deeper into the lungs.
传导气道从气管延伸至初级支气管、次级支气管、三级支气管，并进入细支气管，最终止于呼吸性细支气管。这些结构不参与气体交换；它们的主要作用是温暖、湿润和过滤吸入的空气。这些管道的管壁具有共同的基本构造，但随着气道向肺深部分支，其结构发生渐进性改变。
Structurally, the airway wall consists of a mucosa (epithelium and underlying lamina propria), a submucosa, and an outer layer that contains cartilage and smooth muscle. The relative proportions of these layers change dramatically along the airway tree, reflecting the mechanical and physiological demands of each region.
从结构上讲，气道壁由黏膜（上皮及其下方的固有层）、黏膜下层以及包含软骨和平滑肌的外层组成。这些层的相对比例在气道树中发生显著变化，反映了每个区域的力学和生理需求。
2. The Trachea: Macroscopic Anatomy | 气管：肉眼解剖学
The trachea is a flexible tube approximately 10-12 cm in length and 2-2.5 cm in diameter, extending from the larynx at the level of the sixth cervical vertebra to the level of the fifth thoracic vertebra, where it bifurcates into the left and right primary bronchi at the carina. The trachea is reinforced by 15-20 C-shaped rings of hyaline cartilage that are incomplete posteriorly, where the trachealis muscle (smooth muscle) bridges the gap.
气管是一根柔韧的管道，长约10-12厘米，直径2-2.5厘米，从第六颈椎水平的喉部延伸至第五胸椎水平，在隆嵴处分为左、右初级支气管。气管由约15-20个透明软骨的C形环加强，这些软骨环在后部不完整，由气管肌（平滑肌）填补该缺口。
The C-shaped cartilage rings serve a dual purpose: they prevent the trachea from collapsing during inspiration when intrathoracic pressure becomes negative, while the flexible posterior region allows the oesophagus to expand anteriorly during swallowing. The open part of the C faces posteriorly, permitting this oesophageal bulge. The trachealis muscle adjusts the airway diameter during coughing and regulates airflow.
C形软骨环具有双重作用：它们防止吸气时因胸内压变为负压而导致气管塌陷，而柔软的后部区域则允许食道在吞咽时向前扩张。C形的开口朝向后，正是为了方便食道的膨出。气管肌在咳嗽时调节气道直径并控制气流。
3. Histology of the Tracheal Wall | 气管壁的组织学
The tracheal wall consists of three principal layers: the mucosa, submucosa and adventitia. The mucosa is lined by a pseudostratified ciliated columnar epithelium, approximately 200-300 µm thick. This epithelium contains several distinct cell types, including ciliated columnar cells, goblet cells, basal cells and brush cells. The term “pseudostratified” indicates that the nuclei are arranged at different levels, creating a false impression of multiple layers, although every cell touches the basement membrane.
气管壁由三个主要层次组成：黏膜、黏膜下层和外膜。黏膜的内表面是假复层纤毛柱状上皮，厚约200-300微米。这种上皮包含几种不同的细胞类型，包括纤毛柱状细胞、杯状细胞、基底细胞和刷细胞。“假复层”一词表示细胞核排列在不同水平上，造成多层结构的假象，而实际上每个细胞都与基底膜接触。
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Goblet cells secrete mucin, a glycoprotein that hydrates to form mucus. The mucus traps inhaled particles and bacteria, a process called mucociliary clearance.
杯状细胞分泌黏蛋白，这是一种糖蛋白，水合后形成黏液。黏液捕获吸入的颗粒物和细菌，这一过程称为黏液纤毛清除。
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Ciliated cells possess approximately 300 cilia per cell, each about 2 µm in length, beating in a coordinated metachronal rhythm to propel mucus upward toward the pharynx.
纤毛细胞每个细胞约有300根纤毛，每根长约2微米，以协调的异相节律摆动，推动黏液向上移向咽部。
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Basal cells serve as stem cells that differentiate to replace damaged epithelial cells.
基底细胞作为干细胞，可分化为新的上皮细胞以替代受损细胞。
Beneath the epithelium lies the lamina propria, a layer of loose connective tissue rich in elastic fibres. The elastic fibres allow the trachea to stretch during inspiration and recoil during expiration. Deep to the lamina propria is the submucosa, which contains seromucous glands whose ducts penetrate the epithelium to release additional mucus and serous fluid.
上皮下方是固有层，由富含弹性纤维的疏松结缔组织构成。弹性纤维使气管在吸气时能伸展，呼气时弹性回缩。固有层深部是黏膜下层，含有浆液黏液腺，其导管穿过上皮释放额外的黏液和浆液。
4. The Bronchi: Division and Structure | 支气管：分支与结构
At the carina, the trachea bifurcates into the left and right primary bronchi. The right primary bronchus is wider, shorter and more vertical than the left, meaning inhaled foreign bodies are more likely to enter the right lung. The primary bronchi divide into secondary (lobar) bronchi – three on the right and two on the left – each supplying a lung lobe, and further into tertiary (segmental) bronchi supplying bronchopulmonary segments.
在隆嵴处，气管分为左、右初级支气管。右初级支气管较宽、较短且更垂直，因此吸入的异物更容易进入右肺。初级支气管再分为次级（肺叶）支气管——右侧三支，左侧两支——各供应一个肺叶，进一步分为供应支气管肺段的三级（肺段）支气管。
As the bronchi divide, the structural characteristics of the wall undergo gradual modification. The C-shaped rings of the trachea are replaced by irregular plates of hyaline cartilage in the primary bronchi. These plates become smaller and more discontinuous in subsequent generations. The cartilage provides support while allowing the bronchi to bend and twist during breathing movements.
随着支气管的分支，管壁的结构特征发生逐渐改变。气管的C形软骨环在初级支气管中变为不规则的透明软骨板。这些软骨板在后续分支中变小且更不连续。软骨提供支撑，同时允许支气管在呼吸运动中弯曲和扭转。
5. Bronchial Wall Composition | 支气管壁的组成
The bronchial wall contains the same basic layers as the trachea but with important quantitative differences. The epithelium becomes progressively thinner: tall pseudostratified columnar epithelium in the primary bronchi changes to a shorter columnar, then cuboidal epithelium in smaller bronchi. Goblet cells become less numerous, while the proportion of ciliated cells remains relatively high.
支气管壁含有与气管相同的基本层次，但在数量上存在重要差异。上皮逐渐变薄：初级支气管中的高假复层柱状上皮在较小的支气管中变为较短的柱状上皮，进而变为立方上皮。杯状细胞数量减少，而纤毛细胞的比例仍然相对较高。
Smooth muscle in the bronchi forms two discrete layers: an inner circular layer and an outer longitudinal layer. This arrangement is more prominent than in the trachea, reflecting the increasing role of smooth muscle in regulating regional airflow. The submucosa contains glands whose number declines with branching; smaller bronchi have few or none. The adventitial layer gradually thins, and the cartilaginous plates eventually disappear.
支气管中的平滑肌形成两个独立的层：内环层和外纵层。这种排列比在气管中更为明显，反映了平滑肌在调节局部气流中日益重要的作用。黏膜下层含有腺体，其数量随分支而减少；较小的支气管几乎没有腺体。外膜层逐渐变薄，软骨板最终消失。
| Feature | 特征 | Trachea | 气管 | Primary Bronchus | 初级支气管 | Bronchioles | 细支气管 |
| Cartilage | 软骨 | C-shaped rings | C形环 | Irregular plates | 不规则软骨板 | Absent | 无 |
| Smooth muscle | 平滑肌 | Trachealis only | 仅气管肌 | Circular & longitudinal layers | 环层与纵层 | Increasingly prominent spiral layer | 逐渐显著的螺旋层 |
| Epithelium | 上皮 | Pseudostratified ciliated columnar | 假复层纤毛柱状 | Pseudostratified / columnar | 假复层/柱状 | Simple columnar → cuboidal | 单层柱状→立方 |
| Goblet cells | 杯状细胞 | Many | 多 | Moderate | 中等 | Absent in terminal bronchioles | 终末细支气管中无 |
| Glands | 腺体 | Abundant in submucosa | 黏膜下层丰富 | Present, fewer | 存在，较少 | Absent | 无 |
6. Bronchioles: Structural Simplification | 细支气管：结构的简化
Bronchioles are airways with an internal diameter of approximately 1 mm or less, lacking any supportive cartilage entirely. In the absence of cartilage, the patency of these small airways depends entirely on the elastic recoil of surrounding lung tissue and the tension generated by smooth muscle. The bronchiolar epithelium is simple: ciliated columnar cells in larger bronchioles give way to ciliated cuboidal cells in smaller ones.
细支气管是内径约1毫米或更小的气道，完全没有任何软骨支撑。在没有软骨的情况下，这些小气道的通畅完全依赖于周围肺组织的弹性回缩以及平滑肌产生的张力。细支气管上皮为单层结构：较大细支气管中的纤毛柱状细胞逐渐变为较小细支气管中的纤毛立方细胞。
Goblet cells disappear from the bronchiolar epithelium, being replaced by Clara cells (also called club cells). These dome-shaped, non-ciliated cells project slightly into the lumen and perform several essential functions. Clara cells secrete a proteinaceous secretion containing surfactant-like proteins that prevent the bronchioles from collapsing and protect against oxidative damage and inflammation.
杯状细胞从细支气管上皮中消失，被克拉拉细胞（也称为棒状细胞）替代。这些圆顶状的非纤毛细胞略微突向管腔，执行多项重要功能。克拉拉细胞分泌一种含类表面活性蛋白的蛋白质分泌物，可防止细支气管塌陷，并保护其免受氧化损伤和炎症的影响。
Smooth muscle in bronchioles forms a complete, spirally arranged layer whose contraction directly controls the diameter of the airway. This is the major site of bronchoconstriction in conditions such as asthma. Bronchioles also lack submucosal glands; the lamina propria is thin with a dense network of elastic fibres that maintain structural integrity.
细支气管中的平滑肌形成一个完整的螺旋状排列层，其收缩直接控制气道直径。这是哮喘等疾病中支气管收缩的主要部位。细支气管也没有黏膜下腺体；固有层较薄，含有致密的弹性纤维网络以维持结构完整性。
7. Terminal and Respiratory Bronchioles | 终末细支气管与呼吸性细支气管
The terminal bronchioles represent the last generation of purely conducting airways. Their diameter is about 0.5 mm and their epithelium, composed of ciliated cuboidal cells and Clara cells, is the thinnest of the conducting zone. Beyond the terminal bronchioles lie the respiratory bronchioles, which are unique because their walls bear occasional alveoli – the first sites of gas exchange.
终末细支气管代表纯传导气道的最后一代。其直径约0.5毫米，上皮由纤毛立方细胞和克拉拉细胞组成，是传导区中最薄的上皮。在终末细支气管之后是呼吸性细支气管，其独特之处在于管壁上存在零星的肺泡——这是气体交换的第一个部位。
The respiratory bronchioles transition gradually into the alveolar ducts and alveolar sacs. The epithelium of respiratory bronchioles is simple cuboidal, with ciliated cells becoming scarce. The smooth muscle layer becomes discontinuous and then disappears entirely at the alveolar level. This structural progression demonstrates a clear functional gradient: structural support decreases as gas exchange capacity increases.
呼吸性细支气管逐渐过渡为肺泡管和肺泡囊。呼吸性细支气管的上皮是单层立方上皮，纤毛细胞变得稀少。平滑肌层变得不连续，并在肺泡水平完全消失。这种结构递进展示了清晰的功能梯度：结构支撑减少，而气体交换能力增加。
8. Functional Correlations and Exam Focus | 功能关联与考试重点
Several structural-functional relationships are consistently tested in CIE A-Level Biology. The cartilage progressively decreases from trachea to bronchioles, correlating with reduced mechanical stress on narrower airways. The smooth muscle proportionally increases in relative importance, enabling precise regulation of airflow distribution to different lung regions.
在CIE A-Level生物学考试中，若干结构-功能关系是常考的内容。软骨从气管到细支气管逐渐减少，这与较窄气道承受的机械应力减少相关。平滑肌的相对重要性按比例增加，从而能够精确调节不同肺区域的气流分配。
The mucus-secreting and ciliated cells are abundant where large volumes of air flow over large surface areas (trachea and bronchi), but diminish distally as the need for particle clearance decreases. Students should be able to describe how inhaled particles are trapped in mucus and swept upward by cilia to the pharynx for swallowing or expectoration – the mucociliary escalator. Damaged ciliated epithelium, as seen following smoking or infection, compromises this defence mechanism.
黏液分泌细胞和纤毛细胞在大面积气流经过的部位（气管和支气管）数量丰富，但随着远端分支对颗粒清除需求的减少而逐渐减少。学生应能描述吸入颗粒物如何被黏液捕获，并由纤毛向上扫至咽部进行吞咽或咳出——即黏液纤毛 escalator 机制。受损的纤毛上皮（如吸烟或感染后所见）会损害这一防御机制。
A common examination question requires comparison of the structure of the trachea and a bronchiole. To answer such questions, organise your response around the key differences in cartilage, smooth muscle, epithelium type, goblet cells, glands and overall wall thickness. Include functional explanations: for example, the presence of cartilage in the trachea prevents collapse despite negative pressure changes during ventilation, while the absence of cartilage in bronchioles allows their diameter to be regulated by smooth muscle contraction.
一个常见的考试问题要求比较气管和细支气管的结构。回答此类问题时，应围绕软骨、平滑肌、上皮类型、杯状细胞、腺体和整体管壁厚度的关键差异进行组织。同时要包括功能性解释：例如，气管中存在软骨可防止通气过程中负压变化导致的塌陷，而细支气管缺乏软骨则允许其直径通过平滑肌收缩来调节。
Students often confuse the terms “bronchiole” and “respiratory bronchiole”. A simple bronchiole conducts air only; a respiratory bronchiole conducts air and also participates in gas exchange because it possesses alveoli in its walls. Similarly, remember that alveolar ducts and alveolar sacs are entirely respiratory structures with no conducting function, and no mucus-producing cells are found there.
学生常常混淆“细支气管”和“呼吸性细支气管”这两个术语。单纯的细支气管仅传导空气；呼吸性细支气管既传导空气也参与气体交换，因为其管壁上有肺泡。同样要记住，肺泡管和肺泡囊是完全的呼吸结构，没有传导功能，也没有产生黏液的细胞。
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