Solubility transcellular pathway

In summary, nasal epithelial intercellular junctions are less restrictive compared to the gastrointestinal tract. Such polar pathways will mainly be responsible for the transport of water-soluble compounds, providing a relatively slow, but significant route which is dependent on the molecular weight of the diffusing species. Secondly, transcellular (lipoidal) pathways permit extremely rapid absorption of lipophilic drugs with a rate dependency based on cell membrane partitioning. 

Figure 15.1. Absorption pathways. The first common step for all pathways in GIT absorption is the diffusion throught the unstirred layers. (1) Transcellular (most common pathway for drugs), (2) paraceUular (small, water soluble compounds), (3) facihtated transport, (4) active tranport (saturable, requires ATP), and (5) efflux pumps (i.e.,pGp. MRP).

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