A rough estimate of the glucose clearance in man can be calculated from the rate of consumption, about 0.55 mol day−1 = 380 µmol min−1 [295, 296] or, for a 1400 g brain, 270 nmol g−1 min−1. For a difference between the concentrations in plasma and ISF of 5 mM this corresponds to CL ~ 54 µL g−1 min−1. In isolated perfused dog brains Betz et al. [302] measured the loss of glucose from the blood flow through the brain and found about 0.6 µmol g−1 min−1 at 6 mM from which at this concentration CL = 100 µL g−1 min−1. Hawkins [307] lists values ranging from 158 to 352 µL g−1 min−1 (at 6 mM glucose) depending on brain region (inferior colliculus the highest). Note that the first two of the estimates above are based on the net flux of glucose while the values listed by Hawkins are based on the unidirectional influx. Because all of these estimates far exceed the clearance expected for perivascular efflux, ~ 1 µL g−1 min−1 (see Sect. 3 and Table 1), the perivascular route is likely to be of minor importance.