The central nervous system is isolated from the circulation by the blood-brain-barrier. This barrier around the endothelium is formed by specialized nerve cells, i.e. astrocytes. In adults this barrier consists of tight connections between the processes of astrocytes and the endothelium. Lipophilic compounds can easily pass the barrier, but glucose, water and ions can only pass via carriers or specific channels (passive or active transport). Other molecules e.g. drugs are not able to cross the blood-brain-barrier.
In infants till 6 months the blood-brain-barrier is not completely mature yet; in fact a real barrier does not exist. This has important implications for pharmacokinetics: in contrast to adults, certain drugs are able to pass the blood-brain-barrier in young children. The presence of certain drugs in the brain can have severe effects on the developing neural tissue or adverse effects that are not known in adults.
The blood brain barrier of infants is complete at: