<prompt> Blood brain barrier is permeable to all exceptWaterGasLipophilic drugProtein  <response>: 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 bodys 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. 