Within Human Lungs,the Apical Surface of the Epithelium Produces a Growth

Within human lungs,the apical surface of the epithelium produces a growth factor called heregulin.The receptors for heregulin are located on the basolateral surface of the same cells.When the lung epithelial lining is intact,the tight junctions create a barrier that separates the heregulin from its receptors.However,when the epithelial lining is compromised,the heregulin is able to reach its receptors,where it stimulates mitosis and thereby leads to wound healing.Based upon this information,explain what happens with smoking cigarettes,which is known to increase the permeability of airway epithelium.