Endothelial junction, what's your function?

(Section of artery wall. Note the gap, or "fenestration" in the endothelium at the center of the image which would allow material carried along by the red blood cells on the right to escape into the surrounding tissues.)

"Blood-Brain Barrier" is one of those medical phrases that gets thrown around a lot without being defined. The general idea can be gleaned from the phrase itself: there exists a wall somewhere in or around the brain that prevents most things (chemicals, viruses and bacteria, etc) in the blood from entering the brain itself. Neat, the brain has an extra layer of security. But what, where, and how works the "Blood-Brain Barrier"?

Like most of the body's coolest features, it's amazingly simple. The innermost layer of all blood vessels are composed of endothelial cells. In the vessels supplying blood to most of the body, there are gaps between the cells (see image). The endothelial cells in the capillaries of the brain are arranged differently, in "tight junctions" where more endothelial cells are packed into a smaller surface area.

If you imagine endothelial cells as quarters, they would be arranged in the blood vessels of the body face up, overlapping, like scales. Within the capillaries of the brain, they would be arranged as in a roll of quarters, standing up, face to face.

Damage to the endothelial cells of the cerebral capillaries is now theorized to play a role in the development of Multiple Sclerosis, Alzheimer's, and other degenerative neurological diseases. Specifically, damage caused by free-oxygen that escapes from red blood cells as they're blasting through the capillaries.