📚 Trachea, Bronchi and Bronchioles | 气管、支气管与细支气管
The trachea, bronchi and bronchioles form the branching system of airways that deliver air to the alveoli. These structures are not just passive tubes; they are lined with specialised tissues that clean, warm and moisten inhaled air, while structural elements such as cartilage, smooth muscle and elastic fibres keep the airways open yet flexible. Understanding their histology is fundamental to grasping how the respiratory system protects the delicate gas-exchange surfaces and how airway diseases develop.
气管、支气管与细支气管构成了将空气输送至肺泡的分支气道系统。这些结构不仅是简单的管道，它们内衬着特化组织，能清洁、加温和湿润吸入的空气，而软骨、平滑肌和弹性纤维等结构元素则保持了气道的开放和柔韧。理解它们的组织学结构，是掌握呼吸系统如何保护脆弱的换气表面、以及气道疾病如何发生的基础。
1. Overview of the Conducting Airways | 传导气道的概述
The respiratory tract is divided into a conducting zone and a respiratory zone. The trachea, bronchi and bronchioles belong to the conducting zone, whose primary function is to transport air to the respiratory bronchioles and alveoli where gas exchange occurs. These airways do not participate in gas exchange and therefore make up the anatomical dead space, which is about 150 mL in an adult human.
呼吸道分为传导区和呼吸区。气管、支气管和细支气管属于传导区，其主要功能是将空气运送至发生气体交换的呼吸性细支气管和肺泡。这些气道不参与气体交换，因此构成了解剖无效腔，成年人的无效腔容积约为150 mL。
The conducting airways warm, humidify and filter inhaled air. As air moves through the branching tree, its speed decreases dramatically, allowing suspended particles to settle onto the mucus layer. This cleaning function is essential to protect the fragile alveolar epithelium from damage and infection.
传导气道对吸入的空气进行加温、湿化和过滤。空气在分支树中流动时，流速显著下降，使悬浮颗粒能够沉降到黏液层上。这种清洁功能对于保护脆弱的肺泡上皮免受损伤和感染至关重要。
2. Gross Anatomy of the Trachea | 气管的大体解剖
The trachea is a flexible tube approximately 10-12 cm long and 2.0-2.5 cm in diameter. It extends from the larynx (at the level of the C6 vertebra) to the carina (at T5), where it divides into the left and right primary bronchi. Sixteen to twenty C-shaped rings of hyaline cartilage reinforce the wall, preventing collapse during inspiration.
气管是一条长约 10-12 cm、直径约 2.0-2.5 cm 的柔性管道。它从喉部（约第6颈椎水平）向下延伸至气管隆嵴（第5胸椎水平），并在那里分支为左、右主支气管。16-20个C形的透明软骨环加固了管壁，防止在吸气时塌陷。
The open part of each cartilage ring faces the oesophagus at the posterior side. This gap is bridged by smooth muscle (trachealis muscle) and fibroelastic tissue, allowing the oesophagus to expand during swallowing without compromising the airway.
每个软骨环的开口朝向背侧的食管。这个缺口由平滑肌（气管肌）和纤维弹性组织桥接，允许吞咽时食管膨大而不影响气道的通畅。
3. Tracheal Wall Histology | 气管壁的组织学
From the lumen outward, the tracheal wall consists of four layers: mucosa, submucosa, cartilage/smooth muscle layer and adventitia. The mucosa is lined by pseudostratified ciliated columnar epithelium with numerous goblet cells scattered among the ciliated cells.
从管腔向外，气管壁由四层组成：黏膜、黏膜下层、软骨/平滑肌层和外膜。黏膜内衬假复层纤毛柱状上皮，纤毛细胞之间散布着大量的杯状细胞。
Cilia on the epithelial surface beat in a coordinated manner, propelling a thin layer of mucus toward the pharynx at about 1-2 cm per minute. This mucociliary escalator removes inhaled dust, pathogens and debris. Basal stem cells lying on the basement membrane replenish the epithelium.
上皮表面的纤毛协调摆动，以每分钟约1-2 cm的速度将薄层黏液推向咽部。这种黏液纤毛清洗装置清除了吸入的尘埃、病原体和碎屑。位于基膜上的基底干细胞负责更新上皮。
The submucosa contains loose connective tissue, blood vessels and mixed seromucous glands whose ducts open onto the epithelial surface. These glands supplement the goblet cells by secreting additional mucus and watery fluid.
黏膜下层含有疏松结缔组织、血管以及混合性的浆液-黏液腺，腺体导管开口于上皮表面。这些腺体补充了杯状细胞的分泌，产生额外的黏液和稀薄的液体。
4. Cartilage: C-shaped Rings and Their Function | 软骨：C形环及其功能
The hyaline cartilage rings in the trachea maintain a permanently open lumen, yet the posterior gap gives the trachea enough flexibility to accommodate the oesophagus. If the airway were a complete rigid tube, swallowing a large food bolus would be painful or impossible.
气管中的透明软骨环保持管腔永久开放，而后方的缺口则赋予气管足够的柔韧性以适应食管。若气道是完全僵硬的管道，吞食大块食团就会变得疼痛甚至不可能。
In the bronchi, the cartilage becomes arranged as irregular plates rather than complete or C-shaped rings. These plates still provide structural support but allow the bronchial walls to change calibre in response to smooth muscle contraction.
在支气管中，软骨变为不规则的板状，而不再是完整的或C形的环。这些软骨板仍能提供结构支撑，但允许支气管壁在平滑肌收缩时改变管径。
5. Bronchi: Branching from the Trachea | 支气管：从气管的分支
At the carina, the trachea bifurcates into the left and right primary (main) bronchi. The right main bronchus is wider, shorter and more vertical than the left, which is why inhaled foreign objects more frequently lodge in the right lung. Each primary bronchus enters the lung at the hilum and divides into secondary (lobar) bronchi supplying each lobe.
在隆嵴处，气管分叉为左、右主支气管。右侧主支气管较左侧更宽、更短且更垂直，因此吸入的异物更常落入右肺。左右主支气管经肺门进入肺内，并分为供应各个肺叶的次级支气管（叶支气管）。
Secondary bronchi further branch into tertiary (segmental) bronchi, each ventilating a specific bronchopulmonary segment. This hierarchical branching continues for about 23 generations in humans, with the largest portion belonging to the conducting airways.
叶支气管进一步分支为三级支气管（段支气管），每个段支气管负责一个特定的支气管肺段。人类气道大约有23级分支，其中大部分属于传导气道。
6. Bronchial Wall Adaptations | 支气管壁的适应特征
As the airways branch, their walls progressively thin. In the primary bronchi the epithelium is still pseudostratified ciliated columnar, but the number of goblet cells begins to decline. Cartilage appears as irregular plates embedded in a layer of smooth muscle that encircles the lumen.
随着气道分支，管壁逐渐变薄。在主支气管中，上皮仍为假复层纤毛柱状，但杯状细胞的数量开始减少。软骨呈不规则的板状，嵌在环绕管腔的平滑肌层中。
The smooth muscle in the bronchial wall is arranged in a helical pattern and is innervated by the autonomic nervous system. Parasympathetic stimulation (via the vagus nerve) releases acetylcholine, causing bronchoconstriction; sympathetic stimulation (and circulating adrenaline) causes bronchodilation.
支气管壁中的平滑肌呈螺旋状排列，受自主神经系统的支配。副交感神经刺激（通过迷走神经）释放乙酰胆碱，引起支气管收缩；交感神经刺激（以及循环中的肾上腺素）引起支气管扩张。
Lymphoid nodules (bronchus-associated lymphoid tissue, BALT) become more prominent in the walls of smaller bronchi, providing immunological surveillance against inhaled antigens.
在较小支气管的管壁中，淋巴小结（支气管相关淋巴组织，BALT）变得更加明显，对吸入的抗原有免疫监视作用。
7. Bronchioles: The Smaller Airways | 细支气管：更小的气道
Bronchioles are airways with a diameter of 1 mm or less. They lack cartilage entirely and have no submucosal glands. Their walls consist mainly of smooth muscle, elastic fibres and a thin lining of epithelium. The absence of cartilage means that bronchioles can constrict dramatically, which is highly relevant in conditions such as asthma.
细支气管是直径不大于1 mm的气道。它们完全没有软骨，也没有黏膜下腺体。管壁主要由平滑肌、弹性纤维和一层薄的上皮构成。软骨缺失意味着细支气管可以发生剧烈的收缩，这在哮喘等疾病中非常关键。
The epithelium transitions from ciliated simple columnar in larger bronchioles to simple cuboidal in terminal bronchioles. Goblet cells are largely absent, replaced by club cells (also called Clara cells), which secrete a surfactant-like substance and detoxify harmful inhaled compounds via cytochrome P450 enzymes.
上皮从较大细支气管中的纤毛单层柱状，逐渐过渡为终末细支气管中的单层立方上皮。杯状细胞基本消失，取而代之的是克拉拉细胞，它们分泌类似表面活性物质的分泌物，并通过细胞色素P450酶对吸入的有害化合物进行解毒。
Terminal bronchioles represent the most distal part of the conducting zone. Beyond them lie respiratory bronchioles, where occasional alveoli bud from the wall and gas exchange first becomes possible.
终末细支气管是传导区的最末梢部分。再往后就是呼吸性细支气管，其管壁开始出现散在的肺泡，气体交换首次成为可能。
8. Cellular Changes Along the Airways | 气道沿程的细胞变化
The following table summarises the key histological gradients observed when moving from the trachea to terminal bronchioles. Gradual loss of complex epithelium and protective cartilage parallels the shift from a purely conducting role to a structure ready for gas exchange.
下表总结了从气管到终末细支气管观察到的关键组织学梯度。复杂上皮和保护性软骨的逐渐丧失，与从纯粹的传导功能向准备进行气体交换的结构转变是同步的。
| Feature | Trachea | Bronchi | Bronchioles | Terminal bronchioles |
|---|---|---|---|---|
| Epithelium | Pseudostratified ciliated columnar | Pseudostratified ciliated columnar | Simple columnar, ciliated | Simple cuboidal, few cilia |
| Goblet cells | Abundant | Present, decreasing | Rare or absent | Absent |
| Club cells | Absent | Absent | Increasing | Predominant |
| Cartilage | C-shaped rings | Irregular plates | None | None |
| Submucosal glands | Present | Present (fewer) | None | None |
| Smooth muscle | Trachealis muscle only | Helical bands | Prominent, spiral | Thin layer |
| Elastic fibres | Present in lamina propria | Abundant | Very abundant | Abundant |
These gradients reflect the changing functional demands: proximal airways are robust and protective, while distal airways become increasingly adapted for dynamic regulation of airflow and eventually gas exchange.
这些梯度反映了功能需求的变化：近端气道强韧且具保护性，而远端气道则越来越适应于对气流的动态调节，并最终实现气体交换。
9. Smooth Muscle and the Control of Airflow | 平滑肌与气流的控制
Smooth muscle is a defining feature of bronchioles. Because bronchioles lack cartilage, the tone of the smooth muscle directly determines airway diameter and thus resistance to airflow. According to Poiseuille's law, resistance is inversely proportional to the fourth power of the radius, so even a small contraction can massively increase airway resistance.
平滑肌是细支气管的标志性特征。由于细支气管没有软骨，平滑肌的张力直接决定了气道直径，从而决定了气流阻力。根据泊肃叶定律，阻力与半径的四次方成反比，因此即使轻微的收缩也能极大地增加气道阻力。
The smooth muscle in bronchioles is richly supplied by parasympathetic fibres of the vagus nerve that release acetylcholine onto muscarinic receptors, causing contraction. Sympathetic nerves are sparse, but circulating adrenaline from the adrenal medulla acts on beta-2 adrenergic receptors to relax the muscle; this is the basis for beta-2 agonist inhalers used in asthma.
细支气管平滑肌受迷走神经的副交感纤维丰富支配，这些纤维在毒蕈碱受体上释放乙酰胆碱，引起收缩。交感神经则稀疏分布，但肾上腺髓质释放的循环肾上腺素作用于β₂-肾上腺素能受体，使平滑肌舒张；这就是治疗哮喘时使用的β₂-激动剂吸入剂的基础。
In addition to neural control, local factors such as histamine (released from mast cells during allergic reactions) and leukotrienes provoke powerful bronchoconstriction. In a healthy lung, resting smooth muscle tone is low, keeping the airways patent.
除了神经调控，局部因子如组胺（过敏反应期间由肥大细胞释放）和白三烯也会引发强烈的支气管收缩。在健康的肺中，静息平滑肌张力很低，使气道保持开放。
10. Elastic Fibres and Lung Recoil | 弹性纤维与肺回缩
Elastic fibres are woven throughout the walls of bronchi and bronchioles, and they are particularly dense in the lamina propria and around alveolar ducts. These fibres allow the airways to stretch during inspiration and recoil passively during expiration, thereby assisting in the expulsion of air.
弹性纤维编织在支气管和细支气管的管壁中，尤其在固有层和肺泡管周围密度很高。这些纤维使得气道在吸气时得以扩张，并在呼气时被动回缩，从而协助排出空气。
The elastic recoil of the lungs is crucial for normal quiet expiration, which is a passive process. Loss of elastic tissue, as occurs in emphysema, reduces recoil and leads to airway collapse during expiration, trapping air and increasing residual volume.
肺的弹性回缩对于正常的安静呼气（一个被动过程）至关重要。弹性组织的丧失，如肺气肿中所见，会降低回缩力并导致呼气时气道塌陷，造成空气滞留并增加残气量。
Throughout the conducting airways, elastic fibres also help maintain the structural integrity of the wall when external pressure changes, preventing the collapse of small bronchioles under the stress of rapid airflow.
在整个传导气道中，弹性纤维还有助于在外部压力变化时维持管壁的结构完整性，防止小细支气管在高速气流压力下塌陷。
11. Role of Mucus and Cilia in Defence | 黏液与纤毛的防御作用
The mucociliary escalator is a major innate defence mechanism of the conducting airways. Goblet cells and submucosal glands produce a gel-like mucus layer that traps inhaled particles, bacteria and viruses. Beneath this gel layer lies a less viscous sol layer in which cilia beat, allowing the mucus blanket to be moved mouthward.
黏液纤毛清洗装置是传导气道的主要先天性防御机制。杯状细胞和黏膜下腺体分泌的凝胶状黏液层能捕获吸入的颗粒物、细菌和病毒。在这层凝胶之下是一层黏度较低的溶胶层，纤毛在其中摆动，使黏液毯向口腔方向移动。
Each ciliated cell bears about 200-300 cilia that beat at 10-20 Hz. The coordinated metachronal rhythm ensures a continuous upward flow. Inhaled noxious substances such as cigarette smoke paralyse or destroy cilia, leading to mucus accumulation and chronic bronchitis.
每个纤毛细胞上约有200-300根纤毛，以10-20 Hz的频率摆动。协调的异相节律确保了持续向上的流动。吸入的有害物质如香烟烟雾会麻痹或破坏纤毛，导致黏液积聚和慢性支气管炎。
In the smaller bronchioles, where goblet cells are absent, club cells contribute a less viscous secretion that prevents adhesion of the opposing surfaces during expiration and may also possess antimicrobial proteins.
在较小的细支气管中，由于缺乏杯状细胞，克拉拉细胞提供了一种黏性较低的分泌物，能防止呼气时相对的管壁表面发生粘连，并且可能还具有抗菌蛋白成分。
12. Asthma and the Response of Bronchioles | 哮喘与细支气管的反应
Asthma is a chronic inflammatory disorder characterised by airway hyper-responsiveness and reversible bronchoconstriction. In an asthmatic episode, triggers such as allergens or cold air cause mast cell degranulation, releasing histamine, prostaglandins and leukotrienes that contract smooth muscle in bronchioles.
哮喘是一种以气道高反应性和可逆性支气管收缩为特征的慢性炎症性疾病。在哮喘发作时，过敏原或冷空气等触发因素引起肥大细胞脱颗粒，释放组胺、前列腺素和白三烯，这些物质收缩细支气管的平滑肌。
Chronic inflammation also leads to thickening of the basement membrane, goblet cell hyperplasia and mucosal oedema, all of which further narrow the airway lumen. Because bronchioles lack cartilage, the effects of bronchoconstriction are most severe in these small airways.
慢性炎症还会导致基膜增厚、杯状细胞增生和黏膜水肿，所有这些都进一步使气道管腔狭窄。由于细支气管没有软骨，支气管收缩的后果在这些小气道中最为严重。
Treatment aims to reduce inflammation (inhaled corticosteroids) and to rapidly dilate the airways (short-acting beta-2 agonists such as salbutamol). Understanding the histology of bronchioles explains why beta-2 agonists target smooth muscle and why chronic inflammation must be controlled to prevent permanent airway remodelling.
治疗目标是减轻炎症（吸入性皮质类固醇）和快速扩张气道（如沙丁胺醇等短效β₂-激动剂）。理解细支气管的组织学结构，就能解释为什么β₂-激动剂靶向平滑肌，以及为什么必须控制慢性炎症以防止永久性的气道重塑。
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