📚 Lungs Structure | 肺的结构
The lungs are the main organs of the respiratory system. Their structure is adapted to take in oxygen from the air and remove carbon dioxide from the blood.
肺是呼吸系统的主要器官。它们的结构适合从空气中吸入氧气，并从血液中排出二氧化碳。
1. The Respiratory System Overview | 呼吸系统概述
The human respiratory system carries air from the outside into the lungs and back out again. It includes the nose, trachea, bronchi, bronchioles and alveoli, which work together to supply oxygen to the blood.
人体呼吸系统将空气从外界带入肺部并再次呼出。它包括鼻腔、气管、支气管、细支气管和肺泡，它们共同工作为血液供应氧气。
The lungs are located in the chest cavity and are protected by the ribcage. A large sheet of muscle called the diaphragm sits below the lungs and helps with breathing movements.
肺位于胸腔内，由肋骨保护。肺下方有一大片称为膈肌的肌肉，帮助进行呼吸运动。
Each lung is surrounded by a double membrane called the pleura. The pleura produces a small amount of fluid that reduces friction as the lungs expand and contract during breathing.
每个肺都由称为胸膜的双层膜包裹。胸膜产生少量液体，在呼吸时肺扩张和收缩过程中减少摩擦。
2. The Nose and Mouth | 鼻腔与口腔
Air usually enters the body through the nose, although it can also enter through the mouth. The nose is lined with tiny hairs and a sticky liquid called mucus.
空气通常通过鼻子进入人体，但也可以通过口腔进入。鼻腔内壁有细小的鼻毛和一种叫做黏液的粘稠液体。
The hairs and mucus trap dust, pollen and germs before they can reach the lungs. The nose also warms and moistens the air, which helps protect the delicate lung tissue.
鼻毛和黏液会在灰尘、花粉和细菌到达肺部之前将它们捕获。鼻子还会使空气变暖和湿润，这有助于保护脆弱的肺组织。
Breathing through the nose is generally better than breathing through the mouth because the nasal passages filter and condition the air more effectively. However, during heavy exercise, mouth breathing allows more air to enter quickly.
用鼻子呼吸通常比用嘴呼吸更好，因为鼻腔通道能更有效地过滤和调节空气。然而，在剧烈运动时，用嘴呼吸可以让更多空气快速进入。
3. The Trachea | 气管
The trachea, or windpipe, is a wide tube that carries air from the throat down to the chest. Rings of cartilage keep the trachea open so that air can flow freely.
气管是一根宽阔的管道，将空气从咽喉向下输送到胸腔。软骨环使气管保持张开，以便空气可以自由流动。
The inner lining of the trachea has mucus and tiny hair-like structures called cilia. The cilia move mucus upwards to remove trapped particles from the airways.
气管内壁有黏液和称为纤毛的微小毛发状结构。纤毛将黏液向上移动，以清除气道中截留的颗粒。
The cartilage rings are C-shaped rather than complete circles. This allows the oesophagus, which lies behind the trachea, to expand slightly when food is swallowed.
软骨环呈 C 形而不是完整的圆环。这使得位于气管后方的食道在吞咽食物时可以稍微扩张。
4. The Bronchi | 支气管
At its lower end, the trachea divides into two branches called the bronchi. One bronchus enters the left lung and the other enters the right lung.
气管在下端分成两个分支，称为支气管。一条支气管进入左肺，另一条进入右肺。
Like the trachea, the bronchi are supported by cartilage and lined with mucus and cilia. They carry air deeper into each lung.
与气管一样，支气管由软骨支撑，内衬黏液和纤毛。它们将空气输送到每个肺的更深处。
The right bronchus is slightly wider and more vertical than the left bronchus. Because of this, inhaled small objects are more likely to enter the right lung if they pass beyond the trachea.
右支气管比左支气管略宽且更垂直。因此，吸入的小物体如果通过气管，更有可能进入右肺。
5. The Bronchioles | 细支气管
Inside the lungs, the bronchi divide many times into smaller and smaller tubes called bronchioles. These tubes spread throughout the lungs like the branches of a tree.
在肺内部，支气管多次分成越来越小的管道，称为细支气管。这些管道像树枝一样遍布肺部。
Bronchioles are narrower than bronchi and have less cartilage. Their walls contain smooth muscle that can tighten or relax to control airflow.
细支气管比支气管更窄，软骨更少。它们的管壁含有平滑肌，可以收缩或舒张以控制气流。
The smallest bronchioles lead directly into the alveolar ducts and then into the alveoli. This branching structure greatly increases the total volume of air that can be moved into the lungs.
最小的细支气管直接通向肺泡管，然后进入肺泡。这种分支结构大大增加了可进入肺部的空气总量。
6. The Alveoli | 肺泡
At the end of each bronchiole there are clusters of tiny air sacs called alveoli. Each lung contains millions of alveoli, giving the lungs a spongy texture.
每条细支气管末端都有一簇微小的气囊，称为肺泡。每个肺含有数百万个肺泡，使肺具有海绵状质地。
The alveoli are the site of gas exchange. Their walls are extremely thin and are surrounded by a network of blood capillaries.
肺泡是气体交换的场所。它们的壁非常薄，周围布满毛细血管网。
The total surface area of all the alveoli in a healthy adult is very large, often compared to the size of a tennis court. This large area allows enough oxygen to enter the blood each minute.
健康成人所有肺泡的总表面积非常大，常常被比作一个网球场的大小。这个巨大的面积使每分钟有足够的氧气进入血液。
7. Gas Exchange in the Alveoli | 肺泡内的气体交换
Oxygen from the air inside the alveoli diffuses through the thin walls into the blood. At the same time, carbon dioxide diffuses from the blood into the alveoli.
肺泡内空气中的氧气通过薄壁扩散进入血液。同时，二氧化碳从血液扩散进入肺泡。
Diffusion happens because gas particles move from an area of higher concentration to an area of lower concentration. In the lungs, oxygen concentration is higher in the alveoli than in the blood, so oxygen moves into the blood.
扩散的发生是因为气体粒子从浓度较高的区域向浓度较低的区域移动。在肺中，肺泡内的氧气浓度高于血液中的氧气浓度，因此氧气进入血液。
Oxygen: alveoli (high concentration) → blood (low concentration)
氧气：肺泡（高浓度）→ 血液（低浓度）
Carbon dioxide moves in the opposite direction because its concentration is higher in the blood arriving at the lungs than in the air inside the alveoli.
二氧化碳则朝相反方向移动，因为到达肺部的血液中二氧化碳浓度高于肺泡内空气中的浓度。
8. The Diaphragm and Intercostal Muscles | 膈肌与肋间肌
Breathing is powered by two main sets of muscles: the diaphragm and the intercostal muscles between the ribs. These muscles change the volume of the chest cavity.
呼吸由两组主要肌肉驱动：膈肌和肋骨之间的肋间肌。这些肌肉改变胸腔的容积。
During inhalation, the diaphragm contracts and flattens. The intercostal muscles contract and lift the ribcage upwards and outwards, increasing the space inside the chest.
吸气时，膈肌收缩并变平。肋间肌收缩，将肋骨向上向外提升，增大胸腔内的空间。
When the chest volume increases, the lungs expand because they are closely connected to the chest wall. This expansion is essential for drawing air into the lungs.
当胸腔容积增大时，肺会扩张，因为肺与胸壁紧密相连。这种扩张对于将空气吸入肺部至关重要。
9. Breathing In and Breathing Out | 吸气与呼气
When the chest volume increases, the pressure inside the lungs falls below atmospheric pressure. Air rushes into the lungs to equalise the pressure.
当胸腔容积增大时，肺内压力降至大气压以下。空气迅速进入肺部以使压力平衡。
During exhalation, the diaphragm and intercostal muscles relax. The chest volume decreases, the pressure inside the lungs rises, and air is pushed out of the lungs.
呼气时，膈肌和肋间肌舒张。胸腔容积减小，肺内压力升高，空气被推出肺部。
Normal quiet breathing mainly uses the diaphragm, while deeper breathing during exercise involves stronger contractions of both the diaphragm and the intercostal muscles.
正常平静呼吸主要使用膈肌，而运动时较深地呼吸则涉及膈肌和肋间肌更有力地收缩。
10. Adaptations of the Alveoli | 肺泡的适应特征
Alveoli have several adaptations that make gas exchange fast and efficient. They provide a very large total surface area for diffusion.
肺泡具有多种适应特征，使气体交换快速而高效。它们为扩散提供了非常大的总表面积。
The walls of the alveoli and capillaries are only one cell thick, which shortens the diffusion distance. A moist lining helps gases dissolve before diffusing.
肺泡壁和毛细血管壁只有一个细胞厚，这缩短了扩散距离。湿润的内衬有助于气体在扩散前溶解。
A rich blood supply keeps the concentration gradient steep, and constant breathing brings fresh oxygen into the alveoli while removing carbon dioxide.
丰富的血液供应保持浓度梯度陡峭，而持续的呼吸将新鲜氧气带入肺泡并清除二氧化碳。
11. Lung Health and Smoking | 肺部健康与吸烟
Smoking damages the structure and function of the lungs. Tar from cigarette smoke coats the airways and paralyses the cilia, so mucus and germs are not cleared effectively.
吸烟损害肺的结构和功能。香烟烟雾中的焦油覆盖气道并使纤毛麻痹，因此黏液和细菌无法被有效清除。
Over time, smoking can cause chronic bronchitis, emphysema and lung cancer. The walls of the alveoli may break down, reducing the surface area available for gas exchange.
久而久之，吸烟可导致慢性支气管炎、肺气肿和肺癌。肺泡壁可能被破坏，减少可用于气体交换的表面积。
Regular exercise, avoiding smoking and breathing clean air all help keep the lungs healthy. A healthy lifestyle protects the delicate alveoli and keeps the airways clear.
经常锻炼、避免吸烟和呼吸清洁空气都有助于保持肺部健康。健康的生活方式保护脆弱的肺泡并保持气道通畅。
12. Summary of Lung Structure | 肺结构总结
The path of air can be summarised as: nose or mouth → trachea → bronchi → bronchioles → alveoli. Each part has a structure that matches its job in ventilation and gas exchange.
空气的路径可以概括为：鼻或口 → 气管 → 支气管 → 细支气管 → 肺泡。每个部分都有与其在通气和气体交换中的作用相匹配的结构。
The alveoli are the most important exchange surfaces, where oxygen enters the blood and carbon dioxide leaves it. Understanding lung structure helps us explain how the body gets the oxygen it needs for respiration.
肺泡是最重要的交换表面，氧气在那里进入血液，二氧化碳离开血液。了解肺的结构有助于解释人体如何获得呼吸作用所需的氧气。
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