Gas Exchange | 气体交换

📚 Gas Exchange | 气体交换

Gas exchange is the process by which organisms take in oxygen and remove carbon dioxide. In A-Level biology, this topic links surface area, diffusion, ventilation, transport and enzyme-catalysed reactions. Understanding gas exchange is essential for explaining how animals, plants and insects meet their respiratory demands.

气体交换是生物体吸收氧气并排出二氧化碳的过程。在 A-Level 生物中，该主题将表面积、扩散、通气、运输和酶促反应联系起来。理解气体交换对于解释动物、植物和昆虫如何满足呼吸需求至关重要。

1. The Need for Gas Exchange | 气体交换的必要性

All living cells require a constant supply of oxygen for aerobic respiration. This process releases ATP from glucose and produces carbon dioxide as a waste product. If carbon dioxide accumulates, it lowers pH and can damage enzymes, so it must be removed.

所有活细胞都需要持续供氧以进行有氧呼吸。该过程从葡萄糖中释放 ATP，并产生二氧化碳作为废物。如果二氧化碳积累，会降低 pH 并损害酶，因此必须将其排出。

Small organisms such as Amoeba have a large surface area to volume ratio. Oxygen can diffuse directly across their cell membrane and carbon dioxide can diffuse out, so they do not need a specialised gas exchange system.

像变形虫这样的小型生物具有较大的表面积与体积比。氧气可以直接跨细胞膜扩散进入，二氧化碳可以扩散排出，因此它们不需要专门的气体交换系统。

Large multicellular organisms have a small surface area to volume ratio. Their cells are far from the external environment, so diffusion alone is too slow to supply oxygen and remove carbon dioxide. They need specialised exchange surfaces and, in most cases, a transport system such as blood.

大型多细胞生物的表面积与体积比较小。它们的细胞远离外部环境，仅靠扩散太慢，无法供应氧气和排出二氧化碳。它们需要专门的交换表面，并且在大多数情况下还需要血液等运输系统。

2. Features of an Efficient Exchange Surface | 高效交换表面的特征

An efficient gas exchange surface must have a large surface area relative to body volume. In humans, millions of alveoli provide about 70 m² of surface. In plants, the spongy mesophyll cells create many air spaces.

高效的气体交换表面必须具有相对于身体体积较大的表面积。人体中数百万个肺泡提供约 70 m² 的表面积。在植物中，海绵状叶肉细胞形成许多气室。

The exchange surface must be thin to give a short diffusion distance. Alveolar walls and capillary walls are each only one squamous epithelial cell thick. This keeps the diffusion pathway to a minimum.

交换表面必须很薄，以提供较短的扩散距离。肺泡壁和毛细血管壁各只有一层鳞状上皮细胞厚。这使扩散路径保持在最短。

The surface must be moist so that gases can dissolve. Oxygen and carbon dioxide diffuse across membranes in solution. A good blood supply and ventilation maintain steep concentration gradients, which increase diffusion rate.

表面必须湿润，使气体能够溶解。氧气和二氧化碳以溶解状态跨膜扩散。良好的血液供应和通气维持陡峭的浓度梯度，从而提高扩散速率。

Fick’s law summarises these factors as:

菲克定律将这些因素概括为：

Rate of diffusion = (surface area × concentration difference) ÷ thickness

3. The Human Respiratory System | 人体呼吸系统

Air enters through the nasal cavity, where it is warmed, filtered and moistened. It passes through the pharynx, larynx and trachea. The trachea branches into two bronchi, which divide repeatedly into smaller bronchioles and finally end in alveolar sacs.

空气通过鼻腔进入，在此被加温、过滤和湿润。空气经过咽、喉和气管。气管分为两根支气管，支气管再反复分成更小的细支气管，最后终止于肺泡囊。

The trachea and bronchi are supported by C-shaped rings of cartilage. These rings prevent the airways from collapsing during breathing while still allowing food to pass down the oesophagus behind the trachea.

气管和支气管由 C 形软骨环支撑。这些软骨环防止气道在呼吸时塌陷，同时仍允许食物从气管后方的食道通过。

The airways are lined with ciliated epithelium and goblet cells. Goblet cells secrete mucus that traps dust and pathogens. Cilia beat upwards and move the mucus towards the throat, where it is swallowed. Smooth muscle and elastic fibres in the walls allow bronchioles to constrict or dilate and to recoil during expiration.

Published by TutorHao | A-Level Biology Revision Series | aleveler.com

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