Cells in the brain require a very stable environment to ensure controlled and selective stimulation of neurons. As a result, only certain materials are allowed to pass from blood vessels to the brain. Substances such as O2, glucose, H2O, CO2, essential amino acids, and most lipid‐soluble substances enter the brain readily.
Other substances, such as creatine and urea (wastes transported in the blood), most ions (Na+, K+, Cl–), proteins, and certain toxins either have limited access or are totally blocked from entering the brain. Unfortunately, most antibiotic drugs are equally blocked from entering, while other substances such as caffeine, alcohol, nicotine, and heroin readily enter the brain (because of their lipid solubility). This blood‐brain barrier is established by the following:
Brain capillaries are less permeable than other capillaries because of tight junctions between the endothelial cells in the capillary walls.
The basal lamina (secreted by the endothelial cells) that surrounds the brain capillaries decreases capillary permeability. This layer is usually absent in capillaries found elsewhere.
Processes from astrocytes (a type of neuroglial cell) cover brain capillaries and are believed to influence capillary permeability in some way.