Answer:
Alveoli.
Step-by-step explanation:
Respiration may be defined as the process of exchange of gases between the environment, lungs and body tissues. Energy is given off during the process of respiration.
The alveoli is a ballon like structure present in the lungs when gases are inflated in the alveoli. The alveoli increses the surface area for the exchange of gases. Oxygen is transferred in the body tissue from the alveoli.
Thus, alveoli is the main site of gas exchange in the lungs.