The blood-brain barrier refers to the lining of capillaries in the brain with tightly packed endothelial cells, creating a semi-permeable membrane that blocks many substances from passing out of or into the brain through the bloodstream, explains Neuroscience for Kids. The barrier helps keep the environment in the brain stable.
Since the blood-brain barrier is semi-permeable, certain molecules are able to pass through it, Neuroscience for Kids notes. Molecules that are lipid soluble pass readily through the barrier, which explains why certain recreational and psychiatric drugs, such as barbituates, are able to affect the brain. Molecules that are highly charged, not lipid soluble or large are not able to pass the barrier.
There are several circumventricular organs, which are areas where the barrier is weak, allowing substances to enter and exit the brain more easily, explains Neuroscience for Kids. These areas of weakness make it possible for the brain to regulate the blood's makeup throughout the rest of the body. Some of the circumventricular organs are the pineal body, which is responsible for secreting melatonin, and the median eminence, which releases neurohormones.
There are several things that can break down the blood-brain barrier, notes Neuroscience for Kids. Some of these include high blood pressure, exposure to radiation, brain trauma and infections.