A respiratory mechanics model of the human lung is developed for studying pressure-compensated breathing through a respiration mask in low ambient pressure environments encountered during high-altitude flight. The model formulation is described, using a bond graph approach to convey the key elements used to capture critical effects in lung airways and effects of dissipative and energy storing processes. Specific extensions to constitutive relations are described, along with derivation and solution of system state equations via simulation. Gas exchange effects are not incorporated in the model, with emphasis placed on developing and assessing a respiratory mechanics model for integration with breathing support systems. Results from several case studies with variations in the lung characteristics and operational conditions are presented to demonstrate the effectiveness of the model in predicting key physiological measures, reported in the form of flow-volume loops and work-of-breathing, WoB. Favorable comparisons with past results reported in the literature confirm the suitability of this model as part of a system-level model capable of guiding modifications and explaining anomalous behavior in these critical systems.