Measurements of lung complicance and pressure-volume curves of the respiratory system yield important diagnostic and prognostic information in various lung diseases. Yet, students often find it difficult to understand the theory of the combined mechanical behavior of lungs and chest wall during inspiration and expiration. Therefore, we have developed a new mechanical model to illustrate static pressure-volume relationships during spontaneous breathing and mechanical ventilation. In this model, bellows reflecting the lung are positioned within the inverted glas cylinder of a bell spirometer which is sealed by a water lock and reflects the chest wall. A counterweight fixed by springs representing the elastic properties of the chest wall lifts the glas cylinder thus creating a negative pressure inside the cylinder and inflating the bellows. "Intrapleural" and "intrapulmonary" pressures within cylinder and bellows, respectively, are visualized by dye-filled horseshoe bends. Active inspiration, passive and active expiration are mimicked by gradual weighting; mechanical ventilation by bellows inflation with positive pressure. Resulting "pulmonary" volume shifts are monitored by a gas meter, allowing for generation of typical pressure-volume curves of lungs, chest wall and respiratory system. Pathological processes such as pneumothorax or restrictive lung disease can be mimicked. This new model visualizes pressure-volume relationships of the respiratory system and may thus help students to better understand lung mechanics.