3. Describe the features of alveoli that adapt them to gas exchange

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1. Each alveolus is very small. However, the lungs contain hundreds of millions of alveoli, which gives a huge surface area for gas exchange. 

2. As the capillary wall, the wall of the alveolus is made of a single layer of extremely thin cells, creating a short diffusion distance for the gases. 

3. Alveoli are covered by a dense network of blood capillaries with low oxygen concentration and high carbon dioxide concentration. This allows diffusion of oxygen  into the blood and carbon dioxide to diffuse out of the blood. 

4. In the alveolus wall, cells secrete a fluid that keeps the inner surface of the alveolus moist, which allows gases to dissolve. the fluid also contains a natural "detergent" that prevents the sides of one alveolus from sticking to another alveolus.