Question

practice it
what is the blood - brain barrier?
a membrane that connects the lungs and heart.
a protective mechanism that shields the brain from harmful substances.
a type of medication used to treat migraines.
a network of veins in the brain.
a condition where the brain is unable to receive oxygen.

Explanation:

The blood - brain barrier is a protective system made up of tightly - packed cells in blood vessels that line the brain. It prevents harmful substances like toxins and pathogens from entering the brain while allowing essential nutrients to pass through.

Answer:

A protective mechanism that shields the brain from harmful substances.