Fortunately, Smith and Rapoport designed their experiments in such a way that the calculation of the transfer constants and permeabilities for the blood–brain barrier does not depend on how the model describes diffusion within the parenchyma. Their Eq. 4 for the transfer constant, taken from Fenstermacher and Rapoport [159] yields constants with units cm3 s−1 g−1. Their actual calculations leading to the values in their Table 2 were equivalent to using an equation,41 kbr=cbrT/V¯brain∫0Tcplasmadt,which incorporates a conversion factor between the mass and volume of the brain, V¯brain assumed to be 1 cm3 g−1. In this equation the units of kbr are s−1; cbr(T), units dpm g−1, is the total concentration per gram of tissue; T, units s, is the period of time during which influx occurs; and cplasma is the concentration in plasma, units dpm cm−3. Smith and Rapoport assumed that cplasma was constant so that the integral becomes the product cplasma × T and42 kbr=cbrTcplasmaTV¯brain.