Accessible pool and system parameters: assumptions and models.

Quantitative assessment of substrate metabolism from in vivo tracer kinetic data requires a model of the system, i.e., a hypothesis on the structure and functioning of the system. Some fundamentals of modeling important for studying intermediary metabolism in the steady state will be discussed. Accessible pool and system parameters are defined. Although the calculation of accessible pool parameters is structure-free, that of system parameters requires the use of non-compartmental or compartmental structures. Assumptions, bases for choice, and relative merits of these two modeling strategies are discussed. Glucose and leucine metabolism serve as prototypes to illustrate the theoretical points.