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

One reason is the body’s protective blood-brain barrier, which blocks 99% of a dose from reaching the brain.

From Barron's • Jun. 3, 2026

When the antibody plugs into the capillary’s transferrin receptor, a portal through the blood-brain barrier opens up.

From Barron's • Jun. 3, 2026

One of the biggest problems facing current therapies is getting enough medicine across the blood-brain barrier safely and efficiently.

From Science Daily • May 17, 2026

Scientists increasingly view Alzheimer's as both a neurological and vascular disease, with disrupted blood flow and blood-brain barrier damage contributing to the spread of toxic proteins.

From Science Daily • May 17, 2026

There are many layers of tissue, bone, and skin to keep it protected from the outside, but there's also a "blood-brain barrier" that keeps out many substances circulating in the blood.

From "Phineas Gage" by John Fleischman