Explanation

Primary bronchus of lungs divide to form secondary bronchi which divide to form tertiary bronchi. The tertiary bronchi subdivided into bronchioles. The bronchioles open to alveol through alveolar duct, atria and alveolar sacs. The alveoli have very thin wall consisting of squamous epithelium. The wall of alveoli has extensive network of blood capillaries. Due to very intimate contact of blood capillaries with the alveoli, the exchange of gases takes place easily.