The alveoli are called as the structural and functional unit of lungs. These are thin-walled, tiny balloon-shaped, tiny sacs like structure present within our lungs. They have a moist membrane which functions by supporting the easy movement of oxygen and carbon dioxide between the lungs and bloodstream.
Alveoli are the smallest passageway in the respiratory system and are richly supplied with a network of blood vessels. These provide maximum surface for exchange of gases.