 |
The
Blood-Brain-Barrier (BBB)
"Keep Out"
|
Related Links
|
| Over 100 years ago it was discovered that if a blue dye was
injected into the bloodstream of an animal, that tissues of the whole
body EXCEPT the brain and spinal cord would turn blue. To explain this,
scientists thought that a "Blood-Brain-Barrier" (BBB) which
prevents materials from the blood from entering the brain existed. More
recently, scientists have discovered much more about the structure and
function of the BBB. |
Anatomy of the BBB
The BBB is semi-permeable; that is, it allows some materials to cross,
but prevents others from crossing. In most parts of the body, the
smallest blood vessels, called capillaries, are lined with endothelial
cells. The endothelial tissue has small spaces between each individual
cell so substances can move readily between the inside and the outside
of the vessel. However, in the brain, the
endothelial cells fit tightly together and substances cannot pass out of
the bloodstream. (Some molecules, such as glucose, are transported out
of the blood by special methods.)
You may have heard that glial cells (astrocytes) form a layer around
brain blood vessels. However, it appears that astrocytes do NOT
contribute to the BBB. Rather, the astrocytes may be important for the
transport of ions from the brain to the blood.
|
 |
Functions of the BBB
The BBB has several important functions:
- Protects the brain from "foreign
substances" in the blood that may injure the brain.
- Protects the brain from hormones and
neurotransmitters in the rest of the body.
- Maintains a constant environment for the
brain.
|
 |
General Properties of the BBB
- Large molecules do not pass through the
BBB easily.
- Low lipid (fat) soluble molecules do not
penetrate into the brain. However, lipid soluble molecules,
such as barbituate drugs, rapidly cross through into the brain.
- Molecules that have a high electrical charge to
them are slowed.
|
The BBB can be broken down by:
- Hypertension (high blood pressure): high
blood pressure opens the BBB
- Development: the BBB is not fully formed
at birth.
- Hyperosmolitity: a high concentration of
a substance in the blood can open the BBB.
- Microwaves:
exposure to microwaves can open the BBB.
- Radiation: exposure to radiation can open
the BBB.
- Infection: exposure to infectious agents
can open the BBB.
- Trauma, Ischemia, Inflammation, Pressure:
injury to the brain can open the BBB.
|
 |
Circumventricular Organs
There are several areas of the brain where the BBB is weak. This allows
substances to cross into the brain somewhat 6.00ly. These areas are
known as "circumventricular organs". Through the
circumventricular organs the brain is able to monitor the makeup of the
blood. The circumventricular organs include:
- Pineal body
-
- Secretes melatonin and neuroactive peptides. Associated with
circadian rhythms.
- Neurohypophysis (posterior pituitary)
- Releases neurohormones like oxytocin and vasopressin into the
blood.
- Area postrema
- "Vomiting center": when a toxic substance enters the
bloodstream it will get to the area postrema and may cause the
animal to throw up. In this way, the animal protects itself by
eliminating the toxic substance from its stomach before more harm
can be done.
- Subfornical organ
- Important for the regulation of body fluids.
- Vascular organ of the lamina terminalis
- A chemosensory area that detects peptides and other molecules.
- Median eminence
- Regulates anterior pituitary through release of neurohormones.
|
|
