The site of gas exchange of the respiratory system is/are:

The site for gas exchange of the respiratory system are the LUNGS.
The oxygen we breathe in will be diffused into the blood stream through the air sacs in the lungs, at the same time, waste such as carbon dioxide would be diffused back into the air sac and would be breathed out. That's why we breathe in more oxygen, and breathe out more carbon dioxide. This process is called gas exchange.