BLOOD-BRAIN BARRIER: STRUCTURE, FUNCTIONS, DISEASES - GEOGRAFÍA - 2025

The blood-brain barrier is a semipermeable wall between the blood and the brain. It is made up of the cells that make up the walls of the cerebral blood capillaries. This barrier allows neurons in the central nervous system to be chemically isolated from the rest of the body.

The brain is the only organ that has its own security system. Thanks to the blood-brain barrier, essential nutrients can reach it while blocking the entry of other substances.

Type 1 atrocytes around capillaries in the brain

This barrier serves to maintain the proper functioning of neurons by controlling the entry and exit of chemicals in the brain. Unfortunately, however, this barrier works so effectively by blocking the passage of foreign substances into the brain that it usually also prevents medications from reaching the brain.

In any case, research continues to design drugs that have the necessary requirements to penetrate this barrier. However, there are some regions of the body where there is no blood-brain barrier; they are known as circuventricular organs.

Finally, there are certain conditions that produce an opening of the blood-brain barrier. This allows the exchange of substances freely, so that brain function can be altered. Some of them are inflammation, trauma, or diseases such as multiple sclerosis.

Structure of the blood-brain barrier

Blood-brain barrier between brain and blood

Some substances can pass through this barrier, but others cannot, which means that it is a selectively permeable barrier.

In much of the body, the cells that make up the blood capillaries do not stick together tightly. These are called endothelial cells, and they have gaps between them through which various substances can enter and exit. Thus, elements are exchanged between the blood plasma and the fluid surrounding the cells of the body (extracellular fluid).

However, in the central nervous system, capillaries do not have these clefts. Rather, cells are tightly knit. This prevents many substances from leaving the blood.

It is true that there are some specific substances that can cross this barrier. They do it by means of special proteins that transport them from through the walls of the capillaries.

For example, glucose transporters allow glucose to enter the brain to provide fuel. Additionally, these transporters prevent toxic waste products from remaining in the brain.

Glial (support) cells called astrocytes cluster around blood vessels in the brain and appear to play an important role in the development of the blood-brain barrier. These also appear to contribute to the transport of ions from the brain to the blood.

On the other hand, there are areas of the nervous system that have a more permeable blood-brain barrier than in others. The next section explains what this is for.

Features

In order for the brain to function properly, it is essential that a balance be maintained between substances within the neurons and in the extracellular fluid that is around them. This allows messages to be properly transmitted between cells.

If the components of the extracellular fluid change, even slightly, this transmission will be altered, leading to alterations in brain function.

Therefore, the blood-brain barrier acts by regulating the composition of this fluid. For example, many of the foods we eat contain chemicals that could modify the exchange of information between neurons. The blood-brain barrier prevents these substances from reaching the brain, maintaining good functioning.

Importantly, the blood-brain barrier does not have a uniform structure throughout the nervous system. There are places where it is more permeable than others. This is useful to allow the passage of substances that in other places are not welcome.

An example is the brainstem area postem. This region controls vomiting, and has a much more permeable blood-brain barrier. Its purpose is that the neurons in that area can quickly detect toxic substances in the blood.

Thus, when some poison that comes from the stomach reaches the circulatory system, it stimulates the post-cerebral area, causing vomiting. In this way, the body can expel poisonous contents from the stomach before it begins to be harmful.

In summary, the three main functions of the blood-brain barrier are:

- Protects the brain from potentially dangerous foreign substances or that could alter brain function.

- Protects and separates the central nervous system from hormones and neurotransmitters that are in the rest of the body, avoiding unwanted effects.

- Maintains a constant chemical balance in our brain.

What substances cross the blood brain barrier?

Some substances are more susceptible than others to cross the blood-brain barrier. Substances with the following characteristics enter more easily than others:

- Small molecules pass the blood-brain barrier much more easily than large ones.

- Fat-soluble substances easily cross the blood-brain barrier, while those that are not do so more slowly or are unable to cross it. One type of fat-soluble drug that easily reaches our brain are barbiturates. Other examples are ethanol, nicotine, caffeine, or heroin.

- Molecules with less electrical charge pass the barrier more quickly than those with a high charge.

Some substances can cross the blood-brain barrier. Above all, glucose, oxygen and amino acid molecules pass through it, which are essential for the proper functioning of the brain.

Amino acids such as tyrosine, tryptophan, phenylalanine, valine, or leucine enter the blood-brain barrier very quickly. Many of these are precursors to neurotransmitters that are synthesized in the brain.

However, this barrier excludes practically all large molecules and 98% of all drugs that are composed of small molecules.

That is why there are difficulties in treating brain diseases, since drugs usually do not cross the barrier or do not do so in the necessary amounts. In certain cases, therapeutic agents can be injected directly into the brain to bypass the blood-brain barrier.

At the same time, it prevents the entry of neurotoxins and lipophilic substances through a transporter regulated by the so-called P-glycoprotein.

Circuventricular organs

As mentioned, there are several regions of the brain where the blood-brain barrier is weaker and more permeable. This causes substances to reach these regions easily.

Thanks to these areas, the brain can control blood composition. Within the circuventricular organs are:

- Pineal gland: it is a structure located inside our brain, between the eyes. It is related to our biological rhythms and important hormonal functions. Releases melatonin and neuroactive peptides.

- Neurohypophysis: it is the posterior lobe of the pituitary gland. It stores substances from the hypothalamus, mainly neurohormones such as oxytocin and vasopressin.

- Aftermath area: as mentioned above, it produces vomiting to prevent us from becoming intoxicated.

- Subfornical organ: it is essential in the regulation of body fluids. For example, it has an important role in the feeling of thirst.

- Vascular organ of the terminal lamina: also contributes to thirst and fluid balance through the release of vasopressin. Detects peptides and other molecules.

- Median eminence: it is an area of ​​the hypothalamus that regulates the anterior pituitary through interactions between stimulating and inhibiting hypothalamic hormones.

Conditions that affect the blood-brain barrier

It is possible that the blood brain barrier is disturbed due to different diseases. Also, when this barrier is weakened, it may increase the likelihood or accelerate the onset of neurodegenerative disorders.

- Hypertension or high blood pressure: it can cause this barrier to be altered, becoming permeable, which can be dangerous for our body.

- Radiation: long exposure to radiation can weaken the blood-brain barrier.

- Infections: inflammation of some part of the central nervous system makes this barrier weaker. An example is meningitis, a disease in which the cerebral meninges (layers that surround the brain and spinal cord) become inflamed by various viruses and bacteria.

- Trauma, ischemia, stroke… can cause direct damage to the brain, affecting the blood-brain barrier.

- Brain abscess. It is due to inflammation and accumulation of pus inside the brain. The infection usually comes from the ear, mouth, sinuses, etc. Although it can be a consequence of trauma or surgery. In most cases, 8 to 12 weeks of antibacterial therapy are necessary.

- Multiple sclerosis: it seems that people with this disease have leaks in the blood-brain barrier. This causes too many white blood cells to reach the brain, where they mistakenly attack the myelin.

Myelin is a substance that covers nerve cells and allows nerve impulses to travel quickly and efficiently. If it is destroyed, progressive cognitive and motor deterioration appears.

References

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4. Carlson, NR (2006). Physiology of behavior 8th Ed. Madrid: Pearson.