Answer:
Alveoli can be described as small air filled sacs that are present at the end of the respiratory tree in the lungs.
They have single layer of epithelial cell, which allows rapid gaseous exchange.
Moreover, alveoli are lined by surfactant that is mixture of proteins and lipids that reduces the surface tension within alveoli, increasing the overall surface area for the exchange of gases.
Thus, structure of alveoli ( numerous tiny, round air filled sacs, increasing overall surface area for the exchange) allows for its function.
Alveoli play important role in the respiratory system as they are involved in the gaseous exchange (oxygen and carbon dioxide) to and from the blood that is coming to the lungs.
By doing so, they help in the purification of blood (by taking carbon dioxide from deoxygenated blood and giving it oxygen to make it oxygenated/ pure).
