Introduction
The blood–brain barrier – many people have probably heard this term before and have a rough idea of what it is and what it serves. Because the name already gives it away, it is a barrier between the blood circulation and the brain, or more precisely the cerebrospinal fluid (also called nerve fluid, Latin: liquor). But what exactly is this barrier made of, how does it work, what holds it back and what do we need it for anyway?
These questions will be answered in the following. The blood–brain barrier is a barrier between the small blood vessels in the brain and the cerebrospinal fluid. The cerebrospinal fluid (lat.
Liquor) is formed by the plexus choroidei and surrounds the central nervous system (CNS), consisting of brain and spinal cord. These are surrounded by three meninges. Between the inner and the middle meninges, in the so-called subarachnoid space, the clear cerebrospinal fluid flows.
It is produced in more inner areas of the brain. Here there is a system of cavities, the so-called ventriculi, in which the cerebrospinal fluid is formed by filtering it out of the blood. However, the cerebrospinal fluid ultimately contains far fewer cells and proteins than the blood.
Every day new cerebrospinal fluid is produced and at the same time old cerebrospinal fluid is reabsorbed via veins or lymphatic vessels. The main function of the cerebrospinal fluid is to cushion the CNS well and thus protect it from external mechanical influences. In addition, the fact that the brain virtually floats in the cerebrospinal fluid significantly reduces its weight.
It also plays a role in the nutrition of the nerve cells. The function of the blood-brain barrier is to keep the composition of the cerebrospinal fluid constant so that the environment of the nerve cells is subject to as few fluctuations as possible. This is possible because the barrier controls the exchange of substances between blood and cerebrospinal fluid.
It does not allow harmful substances such as toxins, pathogens and hormones to pass through. Nutrients, however, such as sugar, are let in, while stoff metabolism products of the nerve cells are let out and can be transported via the blood to the liver and finally disposed of. However, the blood-brain barrier is not present in all areas of the brain.
Certain organs depend on contact with the blood. For example, there is an area in the brain that measures the components of the blood and, if necessary – namely if there are toxic substances in the blood – triggers a reflex of vomiting. Other organs produce hormones that need to enter the blood in order for them to be distributed throughout the body and have an effect elsewhere.
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