Alveoli are sac-like structures present in the lungs that act as a site of gaseous exchange. Oxygen passes from the air breathed into the lungs, via the alveoli and into the tissues.
Alveoli are made up of a single layer of squamous epithelium and an extracellular matrix that is surrounded by capillaries.
The alveoli provide a large surface area for the exchange of gases, they provide a thin moist, permeable environment for gaseous exchange by simple diffusion.
The gases can move through the alveolar epithelium according to their concentration gradient.
Final Answer:
Alveoli are basically sac-like structures that permits the transport of gases across it.
It is present inside lungs, and is the main site where actually the exchange of gases takes place.
