Microscopic air sacs at the end of the bronchioles where gas exchange between air and blood takes place.
Structure
The lungs contain roughly 300 to 500 million alveoli, providing a total surface area of about 70 square meters. Each alveolus is around 200 to 300 micrometers in diameter and is wrapped in a dense capillary network.
Cells
Type I pneumocytes form the thin walls across which gases diffuse. Type II pneumocytes produce surfactant, a phospholipid film that lowers surface tension and prevents the sacs from collapsing on exhalation.
Gas exchange
Oxygen and carbon dioxide cross the alveolar-capillary membrane by simple diffusion, driven by partial-pressure gradients between the air in the alveolus and the blood in the surrounding capillaries.
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