Blood is the highway by which our immune system cells gets around our body to take care of anything that invades. Blood is also how medications, drugs, poison and toxins, hormones, etc. can get around our bodies.
But let's talk about blood and the brain. Our brain is a very special organ that deserves special protection. It's the only part of our body that is protected by a 7 mm thick covering of bone, in addition to cerebrospinal fluid cushioning and protective layers of meninges. That protects from the outside in, but we also have protection from the inside out, called the blood-brain barrier (I will abbreviate it BBB).
Blood supply is very important to the brain so it has a constant supply of energy and waste removal. Here are some diagrams showing the blood vessels supplying the brain.
|Notice the arch at the bottom of this diagram is the aorta which comes right off the heart itself|
|The Common carotid artery is the one you are feeling when you take your pulse on your neck|
This "Circle of Willis" shows the blood supply on the inferior/ ventral side of the brain. You can see in the image on the right where this is in relation to the brain.
We have special gate-keepers to protect things from getting into our brains. Here's a cross-section of what a blood vessel in the brain looks like compared to a regular one elsewhere in the body:
Not only are the capillary cells (red in the diagram) closed more tightly so things can't leak through, but the entire blood vessel is covered with the "feet" of astrocytes. (My favorite glia! Here's a post about them.)
Here's a more 3D view:
See how is it a gatekeeper? Anything in the blood must go through the astrocyte in order to get to the neuron. Astrocytes are like the bouncer, protective big brother, or best friend: "if you want to get to the neuron, you have to [quite literally] go through me first!"
Astrocytes are really integral to the chemical integrity in the brain and are a bit of the "unsung heroes" of the brain. Not only are they gatekeepers, but they act as a kind of mop-up crew and storage unit for any leftovers the neurons leave around (like ions, some neurotransmitters), and they serve to make sure the neuron stays well-fueled, like a mother who keeps snacks in her purse for her toddler. No wonder astrocytes far outnumber neurons in the brain.
How do these tight blood vessels and "feet" of the astrocytes actually protect it? They are cells, which means they are surrounded by membrane- a phospholipid bilayer, which looks like this up close:
Because of this configuration, stuff that is polar (charged) or water-soluble can't get through the membrane- it can't get past all those hydrophobic fatty tails.
YOU SHALL NOT PASS!!!
Water-soluble stuff such as nutrients (Amino Acids, Glucose, vitamins)
Chemicals & toxins
Stopping viruses and bacteria for the win. Stopping nutrients? FAIL.
So to fix that, we have special transporters to let the good stuff in. They can be super specific, so a glucose transporter will ONLY let glucose in.
Okay, you can go in...
Non-polar/ uncharged/ fat soluble stuff: this includes oxygen going in and carbon dioxide going out
Drugs that are fat-soluble
Other important stuff with special transporters embedded into the membrane to let them in, like water, glucose, amino acids, vitamins, etc. (Glucose has a wicked-awesome backstage pass, AND it knows the lead singer of the band.)
Whew! That's a big job and an important one for the BBB.