Figure 4.

(A) Cells association at and molecular organization of the neurovascular unit (NVU). (B) The steric (up) and interactive (down) barrier properties of mucus. The mucus forms a filter, which can prevent diffusion across the gel according to size (up) or surface properties (down) of the diffusing compound. (C) Schematic representation of GIT sections and their pH, transit time, and relevant parameters for drug delivery. (D) Schematic diagram illustrating glucose absorption by transcellular pathways and paracellular pathways. Reprinted and adapted with permission from: (A) Kadry et al. [89], (B) Lieleg et al. [108], (C) Alonso et al. [129], and (D) Karasov et al. [130].