Alveoli

Definition:

Thin-walled, tiny air sacs located at the ends of the smallest airways in the lungs (the bronchioles) where the exchange of oxygen and carbon dioxide takes place.

Source: GreenFacts, based on WebMD Asthma Glossary of Terms 

More:

Bronchi, bronchial tree and lungs

bronchi
  1. Trachea
  2. Pulmonary artery
  3. Pulmonary vein
  4. Alveolar duct
  5. Alveoli
  6. Cardiac notch
  7. Bronchioles
  8. Tertiary bronchi
  9. Secondary bronchi
  10. Primary bronchi
  11. Larynx

Source: Rastrojo  (public domain)