Fig. 2.

A two-pool cascade structure with precursor feeding both pools and material exit from both pools. Two experimental designs are considered in the text, one with a bolus tracer injection into pool 1, and the other with a labeled precursor at constant enrichment as in Fig. 1. With data available on enrichments in both pools, the activity model is unidentifiable, but the enrichment model is identifiable. Next to each arrow is a definition for the corresponding rate constant in the activity model (k’s) and in the enrichment model (r’s).