Fluctuations of protein levels as a physiological response to drug treatment are of concern in pharmacology. Coupled compartment models may be used to predict concentration levels of targeted proteins in physiological subsystems. Here we use a simply coupled, two-compartment model, with step forcing as a model for amyloid-beta concentrations in the blood plasma and cerebrospinal fluid (CSF). We examine experimental results and explain them in the context of the model. The analysis leads to necessary conditions on transport and decay parameters for capturing the experimental results.