A multipath model of the mechanical behaviour of healthy lungs subject to a plethysmographic test (close to quiet breathing conditions) has been developed, which includes the main physiological nonlinearities. This model is built on a symmetric branching scheme based on Weibel's data, and uses non-linear fluid equations for the upper and lower airways. The alveolar gas compression, the changes in airway dimensions related to lung volume and/or transmural pressure, and the respiratory swings in glottic aperture have been taken into account. As clinically observed, the behaviour of the lungs, taken as a whole, seems linear, but it is confirmed by simulation that this linearity is only apparent. Simplifications and linearisations therefore need to be made carefully, only after their impact on the global behaviour of the lung is evaluated.