A.11 Blood–brain barrier
The blood vessels that carry substances around the body are lined with so called
endothelial tissue made up of so called endothelial cells. The gaps between these
endothelial cells that form the endothelial tissue varies depending on the func-
tion of the blood vesel. For example, in the kidney the gaps are comparatively
large, thereby allowing relatively large chemical species to diffuse through the
tissue. However, the single layer of endothelial cells that line the capillaries of
the brain forms a structure that is known as theblood–brain barrier, in which the
gaps between the endothelial cells are extremely small. This means that nearly all
the substances entering the brain have to pass through the endothelial cell
membranes, that is, through the blood–brain barrier. This makes it more diffi-
cult for polar substances to enter the brain unless they are actively transported
(see Appendix 5). Consequently, this factor must be taken into account when
designing drugs to target the brain. Another factor is the fact that the blood–
brain barrier also contains enzymes that protect the brain.
APPENDIX 11 BLOOD–BRAIN BARRIER 259