I'm trying to develop the compartment model to simulate a lung concentration time profile of unchanged and metabolized drugs with tissue Kp value in simbiology app. This is because these drugs have very high lung Kp values (i.e. 100). I tried that lung concentration of unchanged drugs equals to the product of peripheral concentration and the lung Kp with repeated assginment, but I was not able to cope with this equation and the differential equation for the metabolism of unchanged drugs to metabolites in lung. How can I incorporate Kp values for the simulation of tissue concentration?