The capillaries are encircled by astrocytic foot processes. This lies at the level of the endothelium of the cerebral capillaries. These features act as a barrier to macromolecules. The endothelial cells are very metabolically active. As mentioned above, many of the metabolic functions of these cells are due to the presence of astrocytes which surround the capillaries.
Endothelial cells possess carrier systems to actively transport certain substances into the brain.
These include basic, acidic, and neutral amino acids- and hexoses, especially glucose.