How do your lungs exchange oxygen for carbon dioxide?

Gas exchange happens in tiny air sacs called alveoli, of which you have about 300 million in your lungs. When you inhale, air fills these sacs, which are surrounded by a dense network of capillaries. Oxygen diffuses across the thin alveolar walls into the blood because oxygen concentration is higher in the air sac than in the blood. Simultaneously, carbon dioxide, which has built up as a waste product in the blood, diffuses in the opposite direction into the alveoli because its concentration is higher in the blood. This process is called passive diffusion. The oxygen binds to hemoglobin in red blood cells and gets carried to tissues, while the carbon dioxide gets expelled when you exhale.