A DEMANDING ORGAN

The brain is hungry and sensitive

⚡

Hungry

The brain is about 2% of body weight but uses roughly 20% of the body's energy

🧪

Sensitive

Neurons fire on tiny changes in ions and signaling molecules, so stray chemicals cause trouble

🩸

Densely supplied

Capillaries thread through brain tissue, so almost every neuron sits close to one

The brain needs a generous supply, but a filtered one.

THE DEFENSE

The barrier is the vessel wall

In most organs, capillary walls let small molecules slip through gaps. In the brain, the cells lining the capillaries are sealed together and tightly control what crosses.

The blood-brain barrier is not a separate wall. It is the living lining of the brain's blood vessels.

SCALE

A huge barrier made of tiny pipes

Width Brain capillaries are only about 5 to 10 micrometers across, so red blood cells squeeze through in single file

Length Laid end to end, the brain's blood vessels stretch for hundreds of kilometers

Area The exchange surface is often estimated at 10 to 20 square meters, packed inside the skull

A barrier this long has to be built from cells, because nothing else can wrap that much plumbing.

WHAT IT DOES

Four jobs in one wall

🚧

Blocks

Keeps out most germs, toxins, large proteins, and circulating blood cells

🚚

Delivers

Moves in glucose and amino acids through dedicated transporters

🧹

Removes

Pumps waste and unwanted molecules back out into the blood

⚖️

Stabilizes

Keeps the chemical surroundings of neurons steady

It is a selective gate, not a solid wall.

THE DOWNSIDE

The same wall blocks medicine

A barrier that cannot tell a drug from a toxin turns away most drugs too. Treatments for brain tumors, Alzheimer's disease, and other brain conditions often struggle to reach their target in useful amounts.

That drug-delivery problem is the reason engineers want a better model of the barrier.

UNIT 1 STUDY COMPLETE

🧠

Ready for the Fab Challenge?

You've covered what the blood-brain barrier is, what it does, and why it also blocks medicine.