The alveoli are tiny air sacs in the lungs where gas exchange takes place. Oxygen moves into the blood, to be used for respiration, and carbon dioxide is removed from the blood. Alveoli are surrounded by a network of capillaries, which are small blood vessels that transport these gases.
In the diagram of an alveolus below, we can see that the capillary is close to the wall of the alveolus. The close distance makes the exchange of gases between the alveoli and the blood more efficient.
The alveoli are specialised structures in the lungs that are adapted for efficient and effective gas exchange. These features help the alveoli to maximise gas exchange: