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 .
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 .
