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| style="height:20px; width:400px; text-align:center;" |A [[diagram]] showing a single [[alveolus]] with the [[Oxygen]] going into the [[blood]] and [[Carbon Dioxide]] leaving the [[blood]].
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==Key Stage 4==
===Meaning===
[[Alveoli]] are small air sacs in the [[lungs]] which are specially [[Adaptation|adapted]] for gas exchange.
===Adaptations of Alveoli===
: The walls of the [[alveoli]] separating the [[air]] from the [[blood]] are very thin allowing [[gas]]es to diffuse quickly.
: The [[blood]] supply to the [[alveoli]] is very good to constantly supply of [[blood]] with little [[Oxygen]] and a lot of [[Carbon Dioxide]] keeping a high [[Concentration Gradient|concentration gradient]].
: The walls of the [[alveoli]] are kept moist to allow gases to [[dissolve]] into to the fluid for gas exchange with the [[blood]].