A simple model to demonstrate the balance of forces at functional residual capacity.

NUMEROUS MODELS have been constructed to aid teaching respiratory mechanics. A simple model using a syringe and a water-filled bottle has been described by Thomas Sherman to explain inspiration and expiration (4). The elastic recoil of the chest wall and lungs has been described using a coat hanger (1) or by using rods and rubber bands (3). A more complex two-component model has also been described by Kuebler et al. (2). The model described below is a modification of the classical balloon and jar experiment that is simple to construct and explains the balance of forces at functional residual capacity (FRC). The model also uses the elastic nature of the materials to represent the elastic nature of the lung and chest wall.