− | These have alveoli which open on all of it's sides, they have no 'walls' as such. Openings to individual alveoli are guarded by [[Muscles - Anatomy & Physiology#Smooth Muscle|smooth muscle]]. | + | These have alveoli which open on all of its sides, they have no 'walls' as such. Openings to individual alveoli are guarded by [[Muscles - Anatomy & Physiology#Smooth Muscle|smooth muscle]]. |