CBSE Class 10 Answered

Within the lungs, the air passage divides into smaller and smaller tubes which finally terminate in balloon like structures called alveoli. The alveoli provide a surface where the exchange of gases can take place. The 2 lungs together have about 300-500 million alveoli. The walls of the alveoli are supplied with an extensive network of blood vessels. So lungs maximise the area for gaseous exchange through the presence of large number of alveoli which are richly supplied with blood.