Gas exchange

In the lungs, gas exchange takes place at tiny air sacs called alveoli, surrounded by small blood vessels called capillaries. Oxygen moves from the air in the alveoli into the blood by diffusion: it moves from a higher oxygen concentration to a lower oxygen concentration. Carbon dioxide moves in the opposite direction, from the blood into the alveoli.

The alveoli are the site of gas exchange. They are surrounded by a network of tiny blood vessels called capillaries, bringing blood close to the air inside the lungs.