📚 Organisms 2.1.3: Mechanism of Breathing | 生物体 2.1.3：呼吸机制考点突破
Breathing, or pulmonary ventilation, is the process of moving air into and out of the lungs to facilitate gas exchange. It relies on pressure gradients created by the respiratory muscles and the elastic properties of the lungs. A thorough understanding of the mechanism of breathing is essential for A-level Biology, and this guide breaks down each key concept, complete with paired English-Chinese explanations to solidify your exam knowledge.
呼吸，即肺通气，是指气体进出肺部以促进气体交换的过程。这一过程依赖于呼吸肌产生的压力梯度和肺的弹性回缩力。透彻理解呼吸机制对 A-level 生物学考试至关重要；本指南将逐一分解每个核心概念，提供英中双语讲解，帮助你巩固考试要点。
1. Introduction to Pulmonary Ventilation | 肺通气简介
Pulmonary ventilation consists of inspiration (inhalation) and expiration (exhalation). Air moves from a region of higher pressure to lower pressure. During breathing, the volume of the thoracic cavity is altered by muscles, which changes the pressure inside the lungs relative to atmospheric pressure. This pressure difference drives airflow.
肺通气包括吸气（吸入）和呼气（呼出）。气体从高压区域向低压区域移动。呼吸时，胸腔容积因肌肉活动而改变，使肺内压相对于大气压发生变化。这种压力差驱动气体流动。
Boyle’s law states that at constant temperature, the pressure of a gas is inversely proportional to its volume: P₁V₁ = P₂V₂. As lung volume increases, pressure decreases, drawing air in; as volume decreases, pressure increases, pushing air out.
波义耳定律指出，在恒温条件下，气体压力与其体积成反比：P₁V₁ = P₂V₂。肺容积增大时，压力下降，将空气吸入；容积减小时，压力上升，将空气排出。
2. Anatomy of the Thoracic Cavity | 胸腔解剖
The thoracic cavity is bounded by the rib cage, the vertebral column, and the diaphragm. The pleural membranes (visceral and parietal pleura) enclose a fluid-filled pleural cavity. This arrangement couples the lung to the chest wall, allowing movements of the thorax to expand and contract the lungs.
胸腔由肋骨、脊柱和膈肌围成。胸膜（脏层和壁层）包裹着充满液体的胸膜腔。这种结构将肺与胸壁耦合，使得胸部的运动能够扩张和收缩肺部。
The intercostal muscles (external and internal) lie between the ribs. The external intercostals raise the rib cage, while the internal intercostals lower it. The diaphragm is a dome-shaped muscle separating the thorax from the abdomen.
肋间肌（肋间外肌和肋间内肌）位于肋骨之间。肋间外肌提升胸廓，而肋间内肌则降低胸廓。膈肌是一块穹隆状的肌肉，分隔胸腔和腹腔。
3. Muscles of Breathing | 呼吸肌群
The primary muscles of inspiration are the diaphragm and the external intercostals. When the diaphragm contracts and flattens, it increases the vertical dimension of the thoracic cavity. Contraction of the external intercostals lifts the ribs upwards and outwards, expanding the anteroposterior and lateral dimensions.
吸气的主要肌肉是膈肌和肋间外肌。膈肌收缩变平时，增加了胸腔的上下径。肋间外肌收缩将肋骨向上向外提拉，扩大胸腔的前后径和左右径。
Normal quiet expiration is passive and does not require muscular contraction. During forced expiration, the internal intercostals and abdominal muscles (such as the rectus abdominis) contract to depress the rib cage and increase intra-abdominal pressure, pushing the diaphragm upwards more forcefully.
正常的平静呼气是被动的，无需肌肉收缩。在用力呼气时，肋间内肌和腹部肌肉（如腹直肌）收缩，压低胸廓，增加腹内压，更有力地将膈肌上推。
4. Mechanism of Inspiration | 吸气机制
Inspiration begins with the contraction of the diaphragm and external intercostals. This increases the volume of the thoracic cavity. Because the pleural fluid holds the visceral pleura tightly against the parietal pleura, the lungs are stretched open. Lung volume expands, and alveolar pressure (Palv) drops below atmospheric pressure (Patm), typically to about -1 to -2 mmHg relative to atmosphere.
吸气始于膈肌和肋间外肌的收缩。这使得胸腔容积增大。由于胸膜液将脏层胸膜与壁层胸膜紧密贴合，肺部被牵拉扩张。肺容积增大，肺泡内压（Palv）下降到大气压（Patm）以下，通常比大气压低约1–2 mmHg。
As a result, air flows from the atmosphere through the airways into the alveoli until Palv equals Patm. The volume of air drawn in during one normal breath is the tidal volume (TV), approximately 500 mL in an adult.
因此，空气从大气经气道流入肺泡，直至肺泡内压与大气压平衡。一次正常呼吸吸入的气体量称为潮气量（Vₜ），成人约为500 mL。
5. Mechanism of Expiration | 呼气机制
At the end of inspiration, the inspiratory muscles relax. The elastic recoil of the lungs and chest wall causes the thoracic cavity to decrease in volume. As lung volume decreases, alveolar pressure rises above atmospheric pressure (usually +1 to +2 mmHg). This positive pressure gradient drives air out of the lungs.
吸气末，吸气肌松弛。肺和胸壁的弹性回缩力使胸腔容积减小。肺容积缩小时，肺泡内压升至大气压以上（通常为+1至+2 mmHg）。这种正压梯度将气体推出肺部。
Forced expiration recruits the internal intercostal muscles and abdominal muscles to further decrease thoracic volume, making expiration faster and more powerful, such as during exercise or coughing.
用力呼气动用肋间内肌和腹部肌肉，进一步缩小胸腔容积，使呼气更快、更有力，例如在运动或咳嗽时。
6. Pressure-Volume Relationships | 压力-容积关系
The intrapleural pressure (Pip) is always slightly negative relative to alveolar pressure under normal conditions, typically around -4 to -8 mmHg at rest. This negative pressure, created by the opposing elastic forces of the lungs wanting to collapse and the chest wall wanting to expand, keeps the alveoli open and prevents lung collapse.
正常情况下，胸膜腔内压（Pip）相对于肺泡内压总呈轻微负值，静息时通常约为-4至-8 mmHg。这种负压由肺的向内回缩力和胸壁的向外扩张力相互拮抗而产生，使肺泡保持开放，防止肺萎陷。
During inspiration, Pip becomes even more negative (e.g., -6 to -8 mmHg) as the lungs are stretched. During expiration, Pip returns to its resting negative value. If the pleural cavity is punctured (pneumothorax), Pip equilibrates with Patm, causing the lung to collapse.
吸气时，随着肺被牵拉，胸膜腔内压变得更负（如-6至-8 mmHg）。呼气时，Pip恢复至静息负值。如果胸膜腔破损（气胸），Pip与大气压平衡，导致肺萎陷。
Lung compliance (ΔV/ΔP) measures the ease with which the lungs expand. High compliance means less pressure is needed to achieve a given volume change; reduced compliance (e.g., in fibrosis) makes breathing harder.
肺顺应性（ΔV/ΔP）衡量肺扩张的难易程度。顺应性高意味着只需较小的压力变化即可实现一定的容积改变；顺应性降低（如肺纤维化）使呼吸更费力。
7. Lung Volumes and Capacities | 肺容积与容量
Spirometry records the volumes of air moved during breathing. Key volumes and capacities include: Tidal Volume (TV), Inspiratory Reserve Volume (IRV), Expiratory Reserve Volume (ERV), Residual Volume (RV), Vital Capacity (VC = TV + IRV + ERV), and Total Lung Capacity (TLC = VC + RV
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