Blood Brain Barrier (BBB):
CNS is chemically a very stable environment (so it can function normally), so needs to be isolated from the variations that occur in the blood of the systemic circulation. This barrier inhibits the entry of drugs and other potentially harmful substances into the brain, due to the impermeability of epithelium of capillaries of brain (due to tightly joined endothelial cells of the blood capillaries). Prevents passage of potentially harmful substances.
Only allows passage of water, respiratory gases, nutrients and lipid-soluble molecules. Only drugs that can cross the blood-brain barrier are those that are lipid soluble or have specific transport system.