Adaptations of the human lungs for gas exchange

The lungs are made up of tiny air sacs called alveoli. Gas exchange takes place between air in alveoli and blood in capillaries. There are millions of alveoli to increase the surface area of the lungs for gas exchange. The walls of the alveoli are one cell thick to create a short diffusion distance for gas exchange. The alveoli are surrounded by capillaries carrying blood, to maintain a concentration gradient for gas exchange.