A mental model for preoxygenation is mapped onto the regions of a stochastic simulation where particles representing oxygen and nitrogen molecules move randomly about two regions depicting the lungs and body tissues. The connecting passages between the two regions represent flows between the two regions that establish a dynamic balance. We consider this a stochastic simulation because it is based on random particle paths. Because of the stochastic nature of the simulation, plots from two consecutive runs with identical user-adjustable settings will be slightly different. Use the slider bars to adjust different parameters or the presets to obtain various conditions.