Gas exchange in the lungs occurs in the alveoli, which are tiny air sacs located at the end of the bronchioles. The alveoli are surrounded by a network of tiny blood vessels called capillaries. Oxygen from the air inside the alveoli diffuses across the thin walls of the alveoli and capillaries into the blood, while carbon dioxide from the blood diffuses into the alveoli to be exhaled. This process happens by diffusion due to concentration gradients between the alveolar air and the blood

. Key features of the alveoli that make gas exchange efficient include:

• Large surface area due to millions of alveoli
• Very thin walls (one cell thick) for a short diffusion path
• Moist lining to allow gases to dissolve and diffuse
• Dense capillary network providing a good blood supply
• Constant ventilation maintaining concentration gradients

Thus, the alveoli and their surrounding capillaries form the primary site of gas exchange in the lungs.