Question

practice it what substances are allowed to pass through the blood - brain barrier? important substances like oxygen and nutrients that the brain needs to function properly. the blood - brain barrier does not allow any substances to pass through it all substances can pass through the blood - brain barrier only harmful substances are allowed to pass through the blood - brain barrier substances that are not important for brain function can pass through the blood - brain barrier

Explanation:

The blood - brain barrier is a selective semi - permeable border that allows essential substances like oxygen and nutrients necessary for the brain's proper functioning to pass through while restricting harmful substances and many other substances. It is not completely impermeable (so not "no substances") and not all substances can pass through it. Also, it doesn't allow only harmful substances or substances unimportant for brain function.

Answer:

Important substances like oxygen and nutrients that the brain needs to function properly.