Fig. 1.

A simple one-pool structure for a product of interest, with the tracer entering by a synthetic pathway from a precursor whose enrichment, w(t), is known. The precursor is denoted by a square to indicate an arbitrary model structure for the precursor. The synthetic rate is S, the mass of the product pool is Q, the tracer activity or amount in the product pool is q(t), the tracer enrichment or specific activity in the product pool is y(t), and the fractional synthetic (or catabolic) rate is k.