The false vocal cords then function as a one-way valve or sphincter preventing egress but not ingress of air. The second, contractive stage of cough involves contraction of the muscles of respiration against the closed glottic and supraglottic sphincters. The third stage of cough is the compressive stage where muscular contraction leads to elevation of bronchiolar and alveolar airway pressures. In the final, or expulsive stage of cough, there is a sudden expulsion of air and secretions from the lower airways. It is the rapid velocity, not the air pressure, that affects removal of secretions.
Secretions are necessary for an effective cough.