Tiny, delicate air sacs deep within the lungs, where the gas/blood exchange occurs. Oxygen from inhaled air passes through the walls of aveoli and enters the bloodstream while carbon dioxide passes out in the same way when air is exhaled.
Toxins from cigarette smoke can break the thin walls of alveoli, leaving larger, less efficient air sacs. In a smoker, this process signals the beginning of emphysema, a form of COPD.
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