So this is a sac like structure that contains these tiny sacs known as alveoli. And these alveoli are specialized structures where gas exchange actually takes place. So the blood is oxygenated and it basically deposits our waste product, the carbon dioxide, back into the bronchioles. And that eventually travels out of our system and to the outside environment. And we'll discuss the structure and the function of the alveolis in much more detail in the next several electrodes. So, as I mentioned earlier, our lungs are based inside the thoracic cavity.