Figure 8.2. Blood vessel cross sections: in the body (left) and in the CNS (right).
Note the gaps between the cells on the left and the tight joining of cells on the
right.The blood-brain barrier refers to how the blood vessels are constructed
within the central nervous system, or CNS (the brain and spinal cord),
to regulate the passage of material from the blood into the brain and
vice versa. Blood vessels anywhere in the body have vessel walls that
are made of cells (just as all parts of the body are made of cells) packed
together just as bricks may be packed together to form the walls of a
building. For blood vessels in most of the body, the cells forming the
walls of the vessel are not tightly joined together—there are tiny gaps
between the cells (Fig. 8.2). Although these gaps are very small, they
are large enough for many molecules to pass through. Thus, there is
free exchange of molecules between the blood and the tissues of the
body. This occurs everywhere in the body, except in the CNS. There,
cells forming the walls of the blood vessels are tightly joined together,
with no gaps, no pores, no holes between the cells.
What are the implications of this? There must be exchange of mole-
cules between the blood and the tissues of the body. Certain molecules
carried by the blood—oxygen, glucose, and essential amino acids, for
example—are necessary for cells to function. In all parts of the body
outside of the CNS, these molecules and others can readily move be-
tween the blood and body tissues through gaps between cells in blood
vessel walls. In the CNS there are no such gaps—this is the blood-brain
barrier.
There are two ways by which molecules can cross the blood-brain
barrier. One is via transporter proteins that shuttle specific molecules
across the membranes of the cells forming the blood vessel walls.
Such a transporter exists for glucose, the primary energy currency