Analysis of the pharmacokinetic body transport function

The pharmacokinetic body transport function which determines the output concentration in response to some dosage input is analysed by computer simulation. The subsystems are the tissues and organs of the pulmonary and systemic circulation which are characterized by transit time distributions. The subsystems are arranged according to a positive feedback loop with a steady state error equal to the inverse of the extraction ratio. The total body transport function is in fact a density function of residence times. The mean transit time equals the product of the mean transit time (single pass) and the number of recirculations. Information on the profile of transit time distribution of the subsystem is laregly lost in the overall residence time distribution. Computer simulation shows that irrespective of the nature of the subsystem functions the overall system function is essentially exponential. The time constant of only one exponential increases with decreasing extraction.