The blood-brain barrier acts as an important filter that only allows certain substances to pass through and protects the brain with a kind of barrier. This allows our brain cells to function normally. The blood-brain barrier prevents harmful substances from entering the brain and at the same time allows important nutrients to pass through. If necessary, it also allows degradation products - such as metabolic waste products - to pass through.

Did you know that there is a barrier between your blood and your brain?

It is called the blood-brain barrier and is present in all vertebrates (tetrapods). This barrier enables the brain to maintain its own environmental conditions (homeostasis) by keeping out toxins and other things that could harm the nervous system.

The blood-brain barrier is formed by endothelial cells that are closely connected to each other in the capillary blood vessels via tight junctions.

The blood-brain barrier is a protective mechanism for the brain. It prevents harmful substances from entering the brain and protects it from pathogens, toxins and messenger substances circulating in the blood.

It is a highly selective filter through which nutrients required by the brain are supplied and metabolic products are removed. Supply and disposal is ensured by a series of special transport processes.

On the other hand, this protective function of the brain makes it difficult to treat many neurological diseases with medication, as many active substances are unable to cross the blood-brain barrier. Overcoming this barrier is currently an area of research in order to be able to treat these diseases as well. Only very few - extremely rare - diseases are directly related to the blood-brain barrier itself, whereas it can be affected in a much higher number of common diseases. A disorder or damage caused by a disrupted or damaged blood-brain barrier is a complication to be taken seriously.

The first experiments indicating the existence of this barrier were carried out by Paul Ehrlich in 1885, but he misinterpreted his experimental results. The final proof was provided in 1967 by electron microscopic investigations.

The human brain accounts for around 2% of the body's mass, but its nutritional requirements are 20%. In contrast to other organs of the body, the brain has extremely low nutrient or oxygen reserves. The neurons cannot meet their energy requirements without elemental oxygen. Thus, an interruption of the blood supply to the brain leads to unconsciousness after just ten seconds and to the death of the nerve cells just a few minutes later.

Depending on the activity of an area of the brain, its energy requirements and reserves can vary greatly. These areas regulate their blood supply automatically in order to be able to adapt the supply to the respective requirements.

The brain is an organ that is very susceptible to external influences and must therefore be protected from them. The blood-brain barrier is the main reason why the brain is so difficult to influence. It only lets certain molecules through, while others cannot penetrate.

The blood-brain barrier consists of tight junctions between cells lining the capillaries. These junctions consist of proteins called occludins and claudins. They form a complex arrangement with other proteins in the cell membrane that regulates which substances can pass through the membrane into or out of the brain (1).

The blood-brain barrier also prevents certain substances from entering or leaving the brain by shielding it from harmful substances in our bloodstream such as toxins and pathogens (2).

There are two main types of cells that form this barrier: Endothelial cells and astrocytes. Endothelial cells line all blood vessels and control what gets in and out of the vessels by regulating their permeability (3).

Compared to other organs in the body, the brain has a high energy requirement and produces many metabolic waste products. These have to be transported away again via the blood-brain barrier.

In order to ensure all these functions (supply, disposal and homeostasis), the cerebral vascular system of vertebrates differs from the peripheral vascular system. This differentiation largely separates the brain from the surrounding extracellular space and is essential for protecting the sensitive neuronal tissue and maintaining a constant internal environment.

Changes in the function of the blood-brain barrier lead to changes in the state of the central nervous system, which in turn can lead to functional disorders or diseases of the CNS. Accordingly, a number of neurological diseases are indirectly or directly related to a disruption of the blood-brain barrier.