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. Now the trachea ultimately bifurcates, it divides into two bronchi. And each one of these bronchies subdivides into very tiny bronchioles that permeate through our lungs. And of each bronchio at the end of this very tiny air passageway are the alveolar sacs.