ICSE Class 9 Answered

Trachea divides into bronchi, which enter in each lung and divide further into secondary and tertiary bronchi and then, into bronchioles. These bronchioles ultimately end in a cluster of tiny air chambers called alveoli. There are about 700 million alveoli in the lungs of an adult human. Total surface area of the alveoli is about 70 cm². Thus, lungs have a large surface area for the exchange of gases.