Alveoli consist of very thin and flattened cells of epithelial
layer. Epithelial line is a part of alveolar membrane, otherwise
they are also known as Respiratory membrane that lead a major role
in exchange of gases from lungs to the blood stream.
Also extra cellular matrix is covered or sorrounded by
capillaries.
Thin walled polyhedral sacs that are covered and seperated
another by a thin and minute connective tissue layer with numerous
blood capillaries are also present.