Air , Lungs and the Heart takes place when you inhale and exhale. When you inhale you get O And when you exhale you get CO2 so its simple. Lungs because it makes oxygen into carbon dioxide after every exhale while the in the Heart It processes there first before it become carbon dioxide so the lungs and the heart is connected with each when you inhale and exhale but the most common answer we may say is the lungs.
