By Category Explore the best Living sites in Greenville and around the world. Search 900,000 of the world's best sites . Search The Reflector Suggest a Greenville website. 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. So, a major function of the lungs is to protect from diseases caused by inhaled particles. The lungs begin to lose some of their tissue.