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The alveoli ("many alveoli", "one alveolus") are the sites of gas exchange in the lungs. They are tiny air sacks sometimes described as being cauliflower-shaped. Oxygen diffuses across the lining of the alveoli and blood capillaries into and into red blood cells. Carbon dioxide diffuses from the blood to the alveoli. A concentration gradient is maintained by breathing as well as blood flow.
The main adaptation of the gas exchange surface are:
1. Large surface area
2. Thin wall
3. Moist lining
4. Good blood supply
5. Good ventilation
