During breathing, air enters and exits the lungs. It flows in through increasingly small airways, finally filling tiny sacs called alveoli.
Blood circulates around the alveoli through capillaries.
Where the capillaries and alveoli meet, oxygen crosses into the bloodstream. At the same time, carbon dioxide crosses from the bloodstream into the alveoli to be exhaled. The lungs are continuously being exposed to particles in the air, including smoke, pollen, dust, and microorganisms. Therefore, another a major function of the lungs is to protect from diseases caused by inhaled particles. An average person continues to slowly make new alveoli until about age 20.