Where does gas exchange take place?

After you inhale, the air goes through a series of tubes in the respiratory system. First, the air goes through the trachea, which divides two tubes that are leading to the lungs. Inside the lungs, the tubes branch out into bronchi. At the end of the bronchi, there are alveoli that are surrounded by capillaries.

The walls of the alveoli are so thin, that gases like carbon dioxide and oxygen could go through the walls by diffusion.

The capillaries are also very thin. There, the blood cells trade the carbon dioxide for the oxygen. There is less oxygen in the alveoli than air. So the oxygen diffuses from air to blood cells. The cells in capillaries have more carbon dioxide than air also, so the carbon dioxide diffuses from blood cells to air in the alveoli. Finally, the carbon dioxide is exhaled through the organs of the respiratory system.