sentence2_structure: Smooth muscle<>regulates<>air flow in lungs
sentence1: The constriction of the smooth muscle surrounding the bronchioles and the accumulation of mucus cause periods of airway obstruction and subsequent difficulty in moving air in and (to a greater degree) out of the lungs.
sentence2: Smooth muscle regulates air flow in lungs.
question: Which kind of muscle regulates air flow in lungs?