Cough clears off foreign bodies from the airways and maintains bronchial clearance when mucociliary transport is defective. Cough requires intact inspiratory muscles to inflate the lung up to 80 % of vital capacity and sound expiratory muscles to generate pressure against a closed glottis. Two mechanisms are required to facilitate proximal mucus transport: 1. Constriction of the cartilage containing airways caused by anterior bulging of the pars membranacea due to high air-flow velocity. 2. Thixotropia of the sputum. This feature changes mechanical properties in a way that sputum viscosity decreases with motion. Thixotropia is caused by the complex mucin-structure of the mucus.