Side by side comparison between in vitro capillary and venule vascular beds. Panel A: Note how the capillary system allows for the formation of a very stringent vascular bed (high TEER) in comparison to venules (low TEER). (Panel B). Note also that capillary segments established under venules level of shear stress and venules module exposed to capillary shear stress levels formed a comparable low stringent barrier suggesting that both abluminal astrocytes and high shear stress levels are necessary to develop a tight vascular bed (Panel C) TEER and sucrose permeability correlation in capillaries and venules modules. The sigmoid curve symbolize the ideal correlation between TEER and permeability previously determined by us . Note the difference of ≈ 2 order of magnitude between capillaries (less permeable) and venules (most permeable). The more stringent vascular bed formed in the capillary module can discriminate drug permeability based on the octanol-water partition coefficient (XlogP) with a significantly higher degree of selectivity than venules (Panel D). The asterisk “*” indicates a statistically significant difference (n=4; p<0.05).