Blood Brain Barrier

Ependymal Cells

The blood brain barrier is not a structure you can touch. It’s also not a membrane you could color on a diagram.  The blood brain barrier refers to the barrier that controlsVariables that remain constant to ensure a fair test. the ability of moleculesGroups of atoms bonded together. to enter the cerebrospinal fluid(CSF) – A fluid that cushions and nourishes the brain and spinal cord.. It also regulates the movementA fundamental property of life involving motion of the body or its parts. into the interstitial fluids between neuronsThe functional cells of the nervous system that transmit signals.. 

Recall that ependymal cellsGlial cells that produce and circulate cerebrospinal fluid. were neuroglia that make the cerebrospinal fluid.  These cellsThe basic structural and functional units of life. exist in the choroid plexuses in the ventricles.  These cells take blood from a blood vessel and filter it to become cerebrospinal fluid.  Ependymal cells are only one of the neuroglia that participates in this barrier.

Astrocytes

Astrocytes also maintain the blood brain barrier.  These were the cells that take substances from the blood vessels and feed it to neurons.  By doing so, astrocytesStar-shaped glial cells in the CNS that support neurons and maintain the blood-brain barrier. are tasked with regulating the interstitial environment of the central nervous systemComposed of the brain and spinal cord; integrates and processes information. .  In the inset picture on the right, we can see the purple feet of the astrocytes. They are encircling a blood vessel.  Although not neuroglia, the simple squamous epithelial cells that line blood vessels also have very tight junctions in the brain.  This lowers the probability that substances will leak from the blood capillariesThe smallest blood vessels where gas, nutrient, and waste exchange occurs between blood and tissues. and enter into the interstitial environment.

Who gets in

This is a closer-up picture of an astrocyte interacting with a blood vessel.  You can see the purple foot of the astrocyte at the bottom of the picture. The endothelial cell lines the vessel. Blood flows through the vessel.  As you can see, there are various ways to transport a substance across the endothelial cells. These methods allow it to reach the interstitial environment.  What is allowed to enter are substances that are essential for brain function: oxygen and glucoseA simple sugar that is the main source of energy for cells..  Nonpolar gases such as oxygen are able to enter. Carbon dioxide can also enter. It can rapidly change the pHA measure of hydrogen ion concentration in a solution. of the interstitial environment.  Other, larger nonpolar substances can also enter such as ketone bodies and other fatty acids.  These, too, can lower the pH of the interstitial environment. 

What is prevented from entering are any toxins or wastes.  We can’t prevent carbon dioxide from entering. Astrocytes do a great job at limiting those nitrogen-based waste products. Your liverA large organ that produces bile, detoxifies blood, and stores nutrients. is constantly producing these from protein catabolismThe metabolic process that breaks down molecules to release energy..  The blood brain barrier treats antibiotics as toxins.  So does your kidney.  This is why you may take an antibiotic that you have to take three times a day.  Your kidney keeps clearing it out.  This is problematic for bacterial infections, such as meningitis, in the central nervous systemThe organ system that controls body functions using electrical and chemical signals..  Currently, we have few antibiotics that we can get across this barrier.  So, for good or for bad, this blood brain barrier will do its job.