What is the blood-brain barrier?
The blood-brain barrier is a barrier between the blood and the brain substance. It is formed by the endothelial cells on the inner wall of the blood capillaries in the brain and the astrocytes (a form of glial cells) surrounding the vessels. The endothelial cells in the capillary brain vessels are so tightly linked to each other via so-called tight junctions (belt-shaped, narrow junctions) that no substances can slip between the cells in an uncontrolled manner. To enter the brain, all substances must pass through the cells, which is strictly controlled.
A comparable barrier exists between the blood and the brain’s cavity system, which contains the cerebrospinal fluid (CSF). This so-called blood-cerebrospinal fluid barrier is somewhat weaker than the blood-brain barrier. Thus, despite the barrier function, some exchange of substances between blood and CSF is possible.
What is the function of the blood-brain barrier?
Filter function of the blood-brain barrier
The blood-brain barrier also has a highly selective filtering function:
Small fat-soluble substances such as oxygen, carbon dioxide or even anesthetic gases can cross the blood-brain barrier by diffusing through the endothelial cells. Certain other substances required by brain tissue (such as blood glucose = glucose, electrolytes, some peptides, insulin, etc.) are passed through the barrier with the aid of special transport systems.
The remaining substances, on the other hand, are held back so that they cannot cause any damage in the sensitive brain. For example, neurotransmitters in the blood are not allowed to pass through the blood-brain barrier because they would disrupt the flow of information from the nerve cells in the brain. Various drugs and pathogens must also be kept away from the brain by the blood-brain barrier.
Some substances penetrate the barrier
In medicine, it is sometimes necessary to deliver drugs to the brain that cannot cross the blood-brain barrier. One example: The brains of Parkinson’s patients are deficient in the neurotransmitter dopamine. However, the patients cannot be given dopamine to compensate because it cannot cross the blood-brain barrier. Instead, patients are given the dopamine precursor levodopa (L-dopa), which can easily pass from the blood into the brain. There it is then converted by an enzyme into the effective dopamine.
For the treatment of brain tumors, the blood-brain barrier is temporarily overridden by infusing a highly hypertonic solution into the carotid artery. This allows tumor-inhibiting drugs to reach the brain.
Where is the blood-brain barrier located?
The blood-brain barrier is located in the brain. The endothelial cells on the inner wall of the fine blood vessels seal the wall of the vessels through tight junctions, providing the actual barrier function (along with the surrounding astrocytes).
What problems can the blood-brain barrier cause?
Bilirubin, a bile pigment, is normally kept out of the brain by binding to plasma proteins. In premature babies, however, the concentration of bilirubin in the blood may be so greatly increased by hemolysis (dissolution of red blood cells) and slow degradation that the ability of plasma proteins to bind bilirubin is exceeded. The free, unbound bilirubin can then cross the blood-brain barrier (baby) and enter brain tissue. This nuclear or neonatal icterus can result in irreversible brain damage.
Infections and tumors
Cytomegaloviruses from the herpes virus group use white blood cells as carriers to cross the blood-brain (baby) barrier. In a pregnant woman, infection leads to miscarriage (abortion), death of the unborn embryo, or generalized infection of the baby with inflammation of the brain (encephalitis), calcifications in the brain, convulsions, and paralysis. If the baby becomes infected after birth, the same symptoms may occur, but the course may remain inconspicuous.
Tumor metastases can also cross the blood-brain barrier. Cancer cells attach themselves to the endothelial wall of the capillaries and express their own molecules for adhesion. These then bind to special receptors, by means of which the path through the blood-brain barrier is open.