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You've got a busy brain. There's a lot of thinking to do. And there's a bunch of multi-multi-multi-tasking that's buzzing around in that complex brain of yours. No time or energy for unwanted distractions. When they come, just call security! That's what the blood-brain barrier is about.
I'm not exactly a night life kind of guy. But we're all familiar with how a bouncer will stand with intimidation at the back entrance of a hot, happening nightclub and carefully scrutinize who will be among the chosen few from the large crowd of anxious nightlifers. The lucky ones will be allowed to enter because their names are on the list, or they're good looking, dressed really hot, or in some way look cool enough that their presence inside the club will boost its reputation as the hangout of all the beautiful people. Despite the way it seems on the surface, we all know the bouncer doesn't do this arbitrarily. He really has to be selective. And if you're ever left standing outside, your only option is to go get some lessons in being cool.
But we really can't blame the bouncer or the nightclub owner, can we? After all, the glad happenings and atmosphere inside the club are of primary importance for it to stay in business. If the bouncer lets in just any loser, the success of the club is at stake, because its inner environment would be compromised. Nobody has the gumption to try and squeeze past a big nightclub bouncer.
What does this have to do with your brain? You should be glad that there are tiny particles and even life forms that can't get past the well-designed band of thugs in your brain. If they did sneak into the active, "happening" disco that is your central nervous system, your party could be over.
The inner layer of most of your body's blood vessels is call the endothelium. When pesky germs enter your bloodstream, your body can afford to let a lot of them penetrate through endothelial cells and enter into other tissue. Your body will just dispatch troubleshooters like antibodies and crime fighting cells to go snuff out those invaders. Yes, your body can afford to stay busy 24-7 killing off microscopic pathogens with relative ease and without you even being conscious of it. But of course a virus or two does slip through the immune system every once in a while (Ah-choo!).
The immune system usually works great for most of your body, but when it comes to that most complex machine in the whole universe, that brain of yours, special security forces must stand their ground. What would be the repercussions if the endothelial blood vessel cells in your brain allowed germs and bacteria to sneak through like they do in the rest of your body? Well, such unwanted guests would certainly disrupt the complex disco dancing of neural activity that must never stop in your brain. And what if you had to put up with the cops constantly barging in to arrest them? That would be doubly bad for business, wouldn't it?
The business of the brain just isn't the same as other organs; there's just too much at stake for your life. So the special endothelial cells near your brain act like a tightened up bunch of bouncers. Inside a healthy head, these bad dudes are crammed so close together that only the party goers on the list can get into the dance; only those who are really wanted and needed to keep the party hoppin' at the right level. So bacteria and viruses are too big and ugly to squeeze through, but welcome guests whom the brain really needs, like oxygen and carbon dioxide molecules and hormones are allowed to jump out of the back alley of your bloodstream and into the party inside the brain.
Isn't it great to have cool, connected friends who vouch for you so the bouncer lets you in? Business for the brain will literally die overnight without admitting glucose molecules into the club. No problem. Glucose has special connections with certain proteins who know their way around the bouncers, escorting them right in.
So the nightclub owner hires a barrier of bouncers to keep out unwanted party crashers. And the intelligent designer of your life has set up a blood-brain barrier to keep your central nervous system free from unwanted pathogens. This leaves the energy in our brains free to impress the world with wonderful deeds and think brilliant thoughts. Well, duh, at least some of us.
