How do your lungs exchange oxygen and 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 is more concentrated in the blood, diffuses in the opposite direction into the alveoli to be exhaled. This process is called passive diffusion and requires no energy. The oxygen then binds to hemoglobin in red blood cells and gets carried throughout the body, while the carbon dioxide is expelled when you breathe out.