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. And this is what happens inside our lungs, specifically inside each alveoli. So now let's actually zoom in on one of these alveoli. And this is what a single alveolis actually looks like.