The lungs have special air sacs called ‘alveoli’. The presence of air sac increases the surface area inside the lungs. Alveoli are the places where exchange of gases takes place. There are about 300-350 millions of alveoli in each lung. The alveoli are filled with air and they swell up. During inspiration, the ribs move up and diaphragm flattens which increases the surface area. The increased surface area helps in maximum exchange of gases to take place.