1. Capillary endothelial barrier: All drugs (ionized, non-ionized) with molecular size less than 600 daltons diffuse through the capillary endothelium.
2. Cell membrane barrier: Once a drug diffuses from the capillary wall into extracellualar fluid, its further entry into cells of most tissues is limited by its permeability through membrane (Similar to lipoidal barrier).
3. Blood-brain barrier (BBB): Brain capillaries consist of endothelial cells which are joint together to one another by continuous tight intracellular junctions called as blood-brain barrier.
Promotion of crossing BBB by :
a) use of DMSO (permeation enhancer).
b) Osmotic disruption of BBB by infusing internal carotid artery with mannitol.
c) use of dihydropyridine redox system as drug carriers to the brain.
4. Blood-CSF barrier: Cerebrospinal fluid (CSF) formed by choroid plexus of lateral, third and fourth ventricles. Choroid cells are joined together by tight junction forming blood-CSF barrier.
5. Placental barrier: The maternal and fetal blood vessels are separated by number of tissue layers made of fetal trophoblast basement membrane and endothelium which together constitute the placental barrier.
6. Blood-testis barrier: This barrier is located at the sertoli-sertoli cell junction. It is tight junction between neighboring sertoli cells that act as blood testis barrier.