Blood brain barrier is permeable to all except
D i.e. Protein In the brain, with the exception of choroid plexus, pituitary and the area postrema, cerebral blood vessels are unique in that the junctions between vascular endothelial cells are nearly fused (8 A pore size in comparison to 65 A of body's capillary). The paucity of pores is responsible for blood brain barrier (BBB). The movement across BBB is influenced by the size, charge, lipid solubility and degree of protein binding (in blood) of substance. So carbondioxide (CO2), oxygen (02), lipid soluble substances (as most anesthetics) and water freely enter brain ( BBB) whereas most ions (even small), protein and large substance such as mannitor penetrate poorlyQ.
Water
Gas
Lipophilic drug
Protein