Question

why does the exchange of gases happen only in alveoli though arteries are present in pharynx taken bronchus

Answer 1

the lungs is to provide a very large surface area for gas exchange. The greater the surface area, the greater the amount of gas exchange capable of taking place. The surface area inside the lungs greatly exceeds that of the trachea, pharynx and bronchus, and so it is more efficient in terms of providing respiratory function to allow the gas exchange to take place in the alveoli.

Answer 2

bcoz alveoli provides larger surface area for exchange of gases