Smith and Rapoport used S = 140 cm2 g−1 from [557], and a tissue volume per gram, V¯brain = 1 cm3 g−1, leading to the values of P (in cm s−1) quoted in their Table 3. (Eq. 44 shows that the PS product, which is the estimate of the clearance, does not depend on the values assumed for either V¯brain or S. For Na+ PS was 2 × 10−5 cm3 s−1 g−1 = 1.2 µL min−1 g−1. This is similar to estimates of perivascular clearance, ~ 1 µL min−1 g−1 (see Sect. 3.2).