The deoxygenated blood reaches the fine capillaries interlacing the single-celled walls of the alveoli in each lung, and is thus in close contact with the alveolar air.

• 去氧血最后到达微细的毛细血管，后者交织于每个肺的肺泡单层细胞壁上，去氧血因而得以与肺泡内的空气密切接触。