In the alveoli and lung capillaries, the exchange of carbon dioxide and oxygen occurs through diffusion. Diffusion is the process by which molecules move from an area of higher concentration to an area of lower concentration. In this case, oxygen moves from the alveoli, where its concentration is higher, to the lung capillaries, where its concentration is lower. At the same time, carbon dioxide moves from the lung capillaries, where its concentration is higher, to the alveoli, where its concentration is lower. This exchange of gases is facilitated by the thin walls and large surface area of the alveoli, allowing for efficient diffusion.