blood-brain barrier
Meaning of blood-brain barrier: A seal built into the brain's blood vessels that keeps most of what circulates in the blood from reaching brain tissue.
Definition of blood-brain barrier
The blood-brain barrier is the tightly sealed lining of blood vessels in the brain and spinal cord, which controls what can pass from the bloodstream into nervous tissue. The cells lining these vessels are joined edge to edge with no gaps, so substances must move through the cells rather than between them. Most large molecules, many drugs and most bacteria are excluded.
How is the seal built?
Elsewhere in the body, small vessels are leaky by design, with gaps between the lining cells that let fluid and molecules pass freely. In the brain those gaps are sealed by protein bands called tight junctions. Two further layers reinforce the arrangement: supporting brain cells wrap their endings around the vessel, and cells embedded in the vessel wall help regulate it. Pumps in the lining cells actively eject many compounds that do get in, sending them back to the blood.
What gets through and what does not?
| Crosses easily | Excluded or limited |
|---|---|
| Oxygen and carbon dioxide | Most proteins and large molecules |
| Small fat-soluble molecules, including alcohol and anesthetics | Most antibiotics and chemotherapy drugs |
| Glucose and amino acids, via dedicated transporters | Most bacteria and immune cells under normal conditions |
Why does it complicate treatment?
A drug that works well against a tumor or an infection elsewhere may not reach effective levels in the brain at all, which is a central difficulty in treating a brain tumor and in treating infections of the nervous system. Approaches to the problem include designing molecules that fit an existing transporter, attaching a drug to something that is carried across, injecting into the fluid around the spinal cord to bypass the barrier entirely, and techniques of deliberate blood-brain barrier disruption that open it briefly. Focused ultrasound with injected microbubbles is one method under active study.
What happens when it breaks down?
Inflammation, injury and tumor growth all damage the seal. Fluid then leaks into brain tissue, producing swelling within a rigid skull, and immune cells that are normally kept out gain entry. This leakage is what makes contrast dye visible on a brain scan, since dye stays inside intact vessels and escapes only where the barrier has failed. A few small regions of the brain lack a barrier by design, allowing them to sample the blood directly, which is how the area that triggers vomiting detects toxins in the circulation.
Also known as
Used in a sentence
The agent was selected for its ability to cross the blood-brain barrier at therapeutic concentrations.
Good to know
The barrier excludes many antibiotics and cancer drugs from the brain, and its breakdown is what allows contrast dye to appear on a scan at the site of a tumor or inflammation.