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[Neurological](/glossary?system=Neurological)[Brain Health](/topics/brain-health)

# Blood-Brain Barrier

blood-brain bar·ri·er - blud-brayn BARE-ee-er

## Definition

The blood-brain barrier (BBB) is a highly selective semi-permeable border of specialized cells that separates the circulating blood from the brain and central nervous system. It acts as a gatekeeper, allowing essential nutrients like glucose and [amino acids](/glossary/amino-acid) to pass from the blood into the brain while blocking potentially harmful substances including toxins, [pathogens](/glossary/pathogen), and most large molecules.

The BBB is formed primarily by the endothelial cells lining the brain's blood capillaries. Unlike the relatively "leaky" endothelial cells in capillaries elsewhere in the body, brain endothelial cells are joined by extremely tight junctions that create an almost impenetrable seal. Additional cell types - astrocytes and pericytes - surround the capillaries and help maintain and regulate the barrier's function.

While the BBB is essential for protecting the brain, it also presents a major challenge in medicine: most drugs cannot cross the barrier, making it extremely difficult to deliver medications to treat brain diseases like Alzheimer's, brain tumors, and many neurological conditions. Developing strategies to get therapeutic drugs across the BBB is one of the most active areas of neuroscience research.

## Also Known As

BBB 

## Key Facts

-   • The BBB blocks approximately 98% of small-molecule drugs and virtually 100% of large-molecule drugs from reaching the brain. 
-   • The brain's capillary network spans an estimated 400 miles and provides a surface area of about 20 square meters for exchange. 
-   • BBB dysfunction has been implicated in Alzheimer's disease, multiple sclerosis, epilepsy, and brain tumors. 
-   • Inflammation, infections, and certain toxins can temporarily compromise BBB integrity. 
-   • Magnesium threonate is one of the few magnesium forms shown to effectively cross the blood-brain barrier. 
-   • Researchers are developing nanotechnology-based approaches to ferry drugs across the BBB for treating brain diseases. 

## How It Relates To Your Health

The blood-brain barrier is relevant when discussing why certain supplements, medications, or substances do (or don't) affect brain function. For example, the reason you can drink coffee and feel alert is that caffeine is one of the molecules able to cross the BBB. Conversely, many promising drugs that work well in lab tests fail as brain treatments because they cannot penetrate this barrier.

If you're interested in brain health, cognitive function, or have a neurological condition, understanding the BBB helps explain why treatment options may be limited and why research into BBB-crossing delivery methods is so important for future therapies.

## Related Terms

[Serotonin](/glossary/serotonin)[Dopamine](/glossary/dopamine)[Neuroplasticity](/glossary/neuroplasticity)[Neuropathy](/glossary/neuropathy)

## Related Conditions

[

### Multiple Sclerosis

Neurological 

Multiple sclerosis is an autoimmune disease in which the immune system attacks the protective myelin sheath covering nerve fibers, disrupting communication between the brain and body. Symptoms vary widely depending on which nerves are affected and can range from mild numbness to severe disability.

Numbness or tingling in limbs Vision problems Muscle weakness or spasms 

Chronic Learn More 

](/conditions/multiple-sclerosis)[

### Epilepsy

Neurological 

Epilepsy is a neurological disorder characterized by recurrent, unprovoked seizures caused by abnormal electrical activity in the brain. Seizures can range from brief lapses in attention to full-body convulsions, and the condition can significantly impact daily life and safety.

Recurrent seizures Temporary confusion Loss of consciousness 

Chronic Learn More 

](/conditions/epilepsy)

## Sources

1.  [Blood-Brain Barrier](https://www.ninds.nih.gov/health-information/disorders/blood-brain-barrier) - National Institute of Neurological Disorders and Stroke (NIH)
2.  [Physiology, Blood Brain Barrier](https://www.ncbi.nlm.nih.gov/books/NBK519556/) - StatPearls / PubMed

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