Alveoli are present in a large amount at the end of terminal bronchioles. They act as a site for gas exchange between lungs and blood, where oxygen and carbon dioxide are exchanged. There are numerous adaptations in the alveolus due to which it is efficient at gas exchange.
1. Firstly, the alveolar walls have elastic fibers that can stretch when inhalation happens and can recoil when exhalation happens so that air is pushed out. They are elastic in nature and this helps the alveoli to expand according to volume of air breathed in and out.
2. There are many alveoli present which increases the rate of diffusion of gases. Also, alveoli have a large surface area to volume ratio for an efficient movement.
3. They also have very thin walls and have a single layer of epithelial cells. Hence, the gas exchange happens quickly as the distance for diffusion decreases.
4. Along the wall of alveoli, there are many capillaries present and between them and alveolus the oxygen and carbon dioxide diffuse in and out. For an efficient gas exchange to happen there should be a steep concentration gradient. When the blood comes from the heart to the lungs, it has less oxygen and more carbon dioxide, and at the same time the lungs have more oxygen and less carbon dioxide. Hence, oxygen diffuses down its concentration gradient into the blood while carbon dioxide diffuses into the alveolus where it is breathed out. The blood flow is constant so that this gradient is maintained.