Anything transported through the blood, before it can reach the neurons of the brain must first pass through the cytoplasm of an astrocyte. Of these cells the astrocytes are the most numerous. Neurons, helpless, temperamental, and incompetent cells that they are, can't be exposed to the nasty things that the blood frequently carries, and the astrocytes see to it that they are not. A similar arrangement of astrocyte foot processes covers the inner surface of the pia mater. This effective sealing off of the brain's intercellular space from the blood compartment constitutes the glia limitans , a significant component of the blood brain barrier.
Astrocytes have other functions.
They are believed to pick up ions released to the extracellular space during neuron firing, and move them from sites of excess to sites of deficiency, thus maintaining stable charges and facilitating neuron recovery. This process of spatial buffering is important to the normal processing of signals.