The alveoli are where the crucial gas exchange takes place .

肺泡是空气相互交换的地方。

There are enough alveoli in just one lung to cover an area roughly the size of a tennis court .

一个肺叶里的肺泡就足够覆盖住网球场大小的面积。

Oxygen passes through the incredibly thin walls of the alveoli into the capillaries and is then carried back to the heart via the pulmonary veins .

氧气穿过无数肺泡的薄壁进入毛细血管中，随后经肺静脉带到心脏。

Airway cells that line the bronchus , from the trachea all the way to the tiny alveoli deep in the lungs , are the first cells that confront cigarette smoke , whether it is inhaled directly from a cigarette or secondhand from the environment .

支气管一直从气管延伸到肺部深处的微小肺泡处，其表面的气道细胞是最先接触到烟雾的细胞，无论这些烟雾是来自于直接吸烟还是被动吸烟。

Within the lungs , the mucus-lined bronchi split like the branches of a tree into tens of thousands of ever smaller tubes ( bronchioles ) , which connect to tiny sacs called alveoli .

在肺部，布满粘液的支气管像树一样又分为成千上万个更小的管（细支气管），他们连接到叫做肺泡的小囊。