Figure 5.

Schematic illustration of analysis of diffusion transport in 2D and 3D. In 2D: c_in is the solute concentration on the input side in a transwell assay, A is the area of the cell monolayer (cm²), N is the number of moles of solute measured on the output side in volume V (cm³), and k_{in, 2D} is the 2D rate constant (cm³ s⁻¹). As long as c_in = constant then k_{in, 2D} = P_2DA. In 3D: c_pl is the solute concentration in plasma (g_s cm⁻³), S is the normalized surface area of the lumen (cm² g_br⁻¹), and F is the normalized flow rate (cm³ g_br⁻¹ s⁻¹), Q_br is the amount of solute transported to the brain (g_s g_br⁻¹), and k_{in, 3D} is the 3D rate constant (cm s⁻¹ g_br⁻¹).