Alveoli are tiny air sacs in the lungs that allow the exchange of oxygen and carbon dioxide. They have very thin walls and are surrounded by tiny blood vessels called capillaries. Oxygen from the air we breathe moves through the walls of the alveoli into the blood, and carbon dioxide, a waste gas from the body, moves from the blood into the alveoli to be exhaled.
The alveoli are perfectly designed for this process:
- Their cauliflower-like shape and large number (millions in the lungs) give them a huge surface area to make gas exchange as efficient as possible.
- Their moist surfaces help gases dissolve and pass through easily.
- They have a good blood supply provided by many blood capillaries, helping maintain a steep diffusion gradient.