An overview of the multiple functions of the blood-brain barrier.

The BBB formed by brain capillary endothelial cells is now recognized as more than an impermeable wall. It has specific transport systems that facilitate the uptake of important nutrients and hormones and active pumps that help to regulate the concentrations of ions and metabolites in the brain's ISF. Enzymes present in the endothelial cell metabolize neurotransmitters, drugs, and toxins before they can enter the brain and disrupt its function. Many of these properties are likely to be under regulation either by neurotransmitters and hormones released in the brain or by those present in the systemic circulation. As a result of this constellation of diverse functions, the brain capillary endothelial cell is able to efficiently supply the brain with the metabolites that it requires while contributing to the maintenance of the brain's ionic homeostasis and protecting it from circulating toxins.