Transport through barriers
In this graphic, the distribution of different types of compounds is shown. The lipophilic, hydrophilic and high molecular weight compounds are all present in the plasma layer. Due to small gaps in the epithelial layer of the blood vessel, the smaller compounds can easily distribute into the interstitial fluid. However, due to their size, the larger molecular compounds cannot. (But note that compounds larger than that which can be absorbed via the GI tract can distribute to different tissues.) Once in the interstitial fluid, the lipophilic compounds can readily diffuse through the lipophilic cellular lipid bilayer into the intracellular space. However, the hydrophilic compounds cannot diffuse through the barrier and can only reach the intracellular fluid via transport through a pore or an active carrier protein.
Distribution from the systemic circulation to other compartments is dependent on several factors:
- the vascularisation of the tissue which can be represented by flow (Q)
- the integrity of the barrier, which can depend on a simple pH partition or the additional protection of the blood brain barrier
- the presence of carrier proteins or transport mechanisms in the barrier itself
See the different models for transport over membranes here.