Mechanical analogy of the respiratory system. This mechanical model is equivalent to the ventilatory mechanics when inertance is neglected. The alveolar compartment is separated from the mouth by a resistance R. Palv represents the pressure in the alveoli and Pm the pressure at the mouth. The respiratory muscles (both inspiratory and expiratory) are combined into a single muscle component M which produces a negative pressure Pmus. is the elastance (inverse of compliance C) of the respiratory system (redrawn from ).