We have specialized structures called alveolar sacs. And these alveolar sacs contain many tiny balloonlike structures called alveoli. And within the alveoli is where gas exchange actually takes place. Oxygen is exchanged for carbon dioxide. Now before we actually discuss how the process of gas exchange range takes place within each individual alveoli, let's discuss what the structure of the alveolar sac is and what the individual alveolis actually looks like. Now recalling our discussion on the respiratory system, we said that when we inhale, when we breathe in air, the air enters via the nose, travels through the nasal cavity and then enters our pharynx and then connects with our larynx, the voice box, which then connects with the trachea, our windpipe.