Sacs in the lungs where oxygen diffuses into the blood and carbon dioxide diffuses into the air

Diffusion of oxygen into the blood takes place at the alveoli (air sacs). The air in the alveoli contains a lot of oxygen. The alveoli are very close to capillaries, which are small blood vessels. The blood in the capillaries has very little oxygen and has a lot of carbon dioxide.

The answer is areoles