The Blood Brain Barrier (the BBB)
- The brain receives a constant
50ml of blood per 100g of brain tissue, per minute, no matter what the conditions.
This is unlike the skeletal muscles or other organs, which have different
blood flow requirements depending on how they are being used.
- The circle of Willis
works to distribute blood at an even pressure, even if one of the major
arteries to the brain becomes occluded
- With all of this blood coming
in, what protects the brain from blood-borne drugs or other substances? The
blood-brain barrier
- The "barrier" is actually
a series of tight junctions between the endothelial cells of the tunica intima.
The blood vessels of the brain are the least permeable of all vessels due
to the intercellular connections
- Astrocytes participate in
the barrier too. They cover the capillaries, and their "feet" help the endothelium
to form the tight junctions.
- The barrier is not
absolute. Important nutrients such as glucose, amino acids and certain electrolytes
move easily via facilitated diffusion. Wastes, such as urea and large proteins,
drugs and other molecules cannot get through to the brain.
- The barrier is not effective
against smaller molecules, including fats, fatty acids, oxygen, carbon dioxide,
alcohol (ethanol) and nicotine.