The Anatomy of the Blood-Brain Barrier

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The blood-brain barrier is made up of tightly packed cells in the brain’s capillaries that prevent harmful substances from entering the brain. It protects your brain from injury and disease while also letting in substances that your brain needs, like oxygen and water. While it performs an important function in keeping your brain healthy, it can also cause challenges in treating some brain conditions when medications can’t cross the blood-brain barrier.

X-ray brain scan
Derek Berwin / Getty Images


Endothelial tissue is found in the inner part of blood vessels throughout your body. Usually these endothelial cells are loosely spaced to allow substances to pass from your blood to other tissues. However, in the brain’s capillaries, the endothelial cells are more tightly connected, creating a barrier that keeps some molecules from crossing from the blood to the brain. Astrocytes and other neural cells also surround the brain’s blood vessels to help the endothelial cells maintain this blood-brain barrier.


The blood-brain barrier serves a filter, controlling which molecules can pass from the blood into the brain. Because the endothelial cells are positioned so closely together, they keep out any harmful toxins or pathogens from reaching your brain. 

While the blood-brain barrier keeps many things out of the central nervous system, it is not impermeable. Some essential molecules, like oxygen, can get past the blood-brain barrier. Fat-soluble substances with small molecules can also pass through the barrier, including caffeine and alcohol. Other substances, like glucose, can be transported from the blood to the brain by a system of transport proteins.

Associated Conditions and Problems

The blood-brain barrier is usually effective at keeping foreign or toxic substances out of your central nervous system. Most of the time this is a good thing, but it can pose a problem when developing new drugs for the nervous system. For example, one of the major challenges in treating brain tumors is that it can be difficult to make a medication capable of getting across the blood-brain barrier to reach the cancer. Because of this problem, researchers are developing medicine to try to bypass the blood-brain barrier.

The blood-brain barrier can sometimes also be broken down by injuries and infections. Research shows that strokes and traumatic brain injury can damage the endothelial tissue and cause the blood-brain barrier to open. Researchers have also found that those with early signs of cognitive impairment have a breakdown of the blood-brain barrier. The findings could help to lead to early diagnostic tests for Alzheimer’s disease and other conditions that cause cognitive impairment. 


Researchers are developing ways to get medications past the blood-brain barrier. One method involves a “transport system,” where medications would be created with antibodies that bind to receptors on the endothelial cells to help the medication cross the barrier and get to the brain.

Another method uses an ultrasound to temporarily open portions of the blood-brain barrier. Patients are injected with microscopic bubbles that spread through the circulatory system. An ultrasound is used to vibrate the bubbles in the brain and temporarily open the blood-brain barrier. However, the impact of opening the blood-brain barrier with this method is still being researched.

A Word From Verywell

The blood-brain barrier plays an important role in keeping your brain healthy. When the blood-brain barrier breaks down, it can lead to neurological disease. Researchers are developing ways to safely bypass the blood-brain barrier to get needed medicine to the brain without any long-term effects. 

5 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  3. Shen HH. Core Concept: Circumventing the blood-brain barrier. Proc Natl Acad Sci USA. 2017;114(43):11261-11263. doi:10.1073/pnas.1716187114

  4. Sweeney MD, Zhao Z, Montagne A, Nelson AR, Zlokovic BV. Blood-Brain Barrier: From Physiology to Disease and Back. Physiol Rev. 2019;99(1):21-78. doi:10.1152/physrev.00050.2017 

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By Peter Pressman, MD
Peter Pressman, MD, is a board-certified neurologist developing new ways to diagnose and care for people with neurocognitive disorders.