Grey area

In today's Boston Globe, Carolyn Y. Johnson writes that the blood-brain barrier is a "wall of tightly packed cells, which line the tiny blood vessels that permeate the brain..." She goes on to say, "The barrier is not a solid wall -- it lets in oxygen and nutrients."

The word "solid" is being used as an unfortunate shortcut for "completely impermeable". Since the blood-brain barrier discriminates on size, electric charge and solubility, there are many small, uncharged, or hydrophobic molecules that easily squish right through. Protein pumps in the cell membrane transfer other useful non-squishable substances, as Johnson points out later in her article.

And like all cells, a lot of water goes into endothelial cells of the blood-brain barrier, but the tissue itself is neither liquid, gas, nor plasma. A solid wall indeed.