Alveoli are small sac like structures where the exchange gases between the lungs and the circulatory system occurs.

 

 

 

 

 

 

 

 

 

 

Gas exchange occurs at the capillary beds of the alveolus. Oxygen and carbon dioxide cross the alveolar membrane down their concentration gradients. Oxygen crosses into the circulation while CO₂ is removed.

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A human lung contains about 480 million alveoli, with a total surface area of about 70-90 square meters (m²)