Functions of the Blood–Brain Barrier

Water, gases that are water or lipid soluble (example, O² or volatile general anesthetics correspondingly), and lipophilic molecules (example, steroids) passively diffuse across the plasma membranes of the endothelial cells. The transport of ions or all but the smallest polar molecules should be carrier-mediated through ion channels and transporters.

The selective permeability of the blood–brain barrier assures that the crucial molecules, like glucose—brain relies nearly exclusively on glucose as a metabolic energy substrate (plus ketones in infancy or starvation)—and amino acids, are taken up by the brain. It protects neurons from the actions of neuroactive molecules in the blood, like circulating catecholamines or glutamate. Also the blood–brain barrier is able to actively exclude a wide range of potentially neurotoxic lipophilic compounds, most part of a natural diet. This is achieved by a transport protein in the plasma membrane of the endothelial cells, P glycoprotein. Lipophilic toxins that diffuse into the endothelial cell are rapidly pumped back out into the blood. Most of the brain tumor cells also express P-glycoprotein and therefore are able to exclude the chemotherapeutic agents. Accordingly chemotherapy is not usually successful in treating brain tumors.