So we see that this entire section, number six is the alveolar sac that contains many of these specialized balloons shaped structures we call alveoli. And within these alveoli is where gas exchange actually takes place. So we exchange oxygen for carbon dioxide. So remember, oxygen is a very important molecule that is used by our individual cells in the process of cellular respiration to actually produce ATP, the energy molecules used by the cell. And carbon dioxide is a waste product of cellular metabolism. And so we have to actually excrete it to the outside of our body.