blood-brain barrier

American  

[bluhd-breyn] / ˈblʌdˈbreɪn /

noun

Physiology.

1. a layer of tightly packed cells that make up the walls of brain capillaries and prevent substances in the blood from diffusing freely into the brain: passage across the cell membranes is determined by solubility in the lipid bilayer or recognition by a transport molecule.

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blood-brain barrier Scientific  

1. A physiological mechanism that alters the permeability of capillaries in the brain, so that some substances, such as certain drugs, are prevented from entering brain tissue, while other substances are allowed to enter freely.

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blood-brain barrier Cultural  

1. The separation of the brain, which is bathed in a clear cerebrospinal fluid, from the bloodstream. The cells near the capillary beds external to the brain selectively filter the molecules that are allowed to enter the brain, creating a more stable, nearly pathogen-free environment.

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Discover More

Oxygen, glucose, and white blood cells are molecules that are able to pass through this barrier. Red blood cells cannot.

Etymology

Origin of blood-brain barrier

First recorded in 1940–45

Example Sentences

In animals treated with 4e, the compound crossed the blood-brain barrier efficiently and produced a lower but longer lasting level of psilocin in the brain compared with psilocybin.

From Science Daily • Mar. 8, 2026

Some scientists were sceptical because this would require the particles to cross the powerful defences of the blood-brain barrier.

From Barron's • Jan. 27, 2026

The drug is unable to cross the blood-brain barrier and so does not help with cognitive symptoms.

From BBC • Nov. 23, 2025

To overcome this, the inserted gene is modified so that the enzyme it produces crosses the blood-brain barrier more efficiently.

From BBC • Nov. 23, 2025

The hamsters have hopped off their wheels and are gnawing at the blood-brain barrier, trying to get out of my skull.

From Eastern Standard Tribe by Doctorow, Cory