Pericardial pressure is based on a curvilinear pressure volume relationship for the intact pericardium. The normal pericardium becomes very stiff as the volume approaches its maximum volume. The transpericardial pressure is added to the current instantaneous pleural pressure to give the total pressure outside the heart. Be aware that sudden changes in this parameter can create physically impossible situations, thereby producing impossible pressures and program crashes. If you change it, do so slowly.
Valve function is based on an electrical circuit analogy.
There is a separate forward and reverse flow circuit. Inertia is potentially important for the forward flow, especially in the atrioventricular valves. The regurgitant flow resistor is linear, but the forward flow resistor is not. Under normal circumstances, valvular resistance is negligible. The gas exchange simulator is described as a steady state solution of a 50 compartment ventilation-perfusion model.