Protect the alveoli from damage by having mucus that traps dirt, and cilia that move this dirt mucus away from the alveoli.
The muscle allows them to constrict, and to contro! the flow of air in and out of the alveoli.
The elastic fibres allow the alveoli to stretch as they are filled with air during inhalation and then recoil during exhalation in order to expel the carbon dioxide air. The collagen fibres add strength to the thin-walled alveoli and so prevent them from bursting during inhalation.