The Blood Brain Barrier

The blood brain barrier protects the brain.

The blood brain barrier is created by an extremely tight-knit wall of cells that line the capillaries of the brain. These cells are called endothelial cells, and they allow some materials to pass, but keep out other materials.

Surrounding the capillaries are brain cells called astrocytes, which provide support to the blood brain barrier. The blood brain barrier (BBB) is considered semi-permeable, because it allows beneficial fluids and nutrients to pass, while keeping out other compounds that might be harmful to the brain.

The fluids and nutrients that can pass through the blood brain barrier are essential to keeping brain cells alive. For example, glucose (sugar) and amino acids give cells energy and help cells repair themselves. Both sugar and amino acids are recognized by the endothelial cells, and are allowed across the barrier. Water and certain fat-soluble molecules also diffuse on their own across the wall of cells.

Functions of the BBB

The blood brain barrier helps keep the brain safe. For example, it is nearly impossible for bacteria to pass across the blood brain barrier. This means that if someone develops a serious infection and has bacteria in their blood stream, the bacteria will not invade and infect the brain as long as the BBB is working properly and not broken.

If the body is exposed to or ingests a poison that cannot cross the BBB, the brain is able to keep functioning while the body eliminates that toxin through actions such as vomiting, sweating and diarrhea.

Essential cells and processes of the brain are protected from that toxin.

The brain is also protected from other chemical processes happening in the body, such as hormone spikes and circulating neurotransmitters that could confuse the very specific neurologic processes that occur in the brain. The role of the BBB is to ensure that the brain maintains a relatively constant environment so it can direct the overall functions of the body and help it survive.

The Blood Brain Barrier and Head Trauma

The blood brain barrier may be damaged during a traumatic accident. The endothelial cells that line capillaries may be torn, cut or stretched. This allows unwanted and harmful fluids, materials or organisms to have a route directly into the brain. When this happens, the brain can develop something called Vasogenic Edema.

The blood brain barrier also makes treating certain injuries more difficult. When there is damage to nerve cells in the brain, large amounts of toxic neurotransmitters may spill out from the cells. While there are medications to treat and remove those neurotoxins, they can’t reach the affected areas because the blood brain barrier won’t let the medication through.

This is also true if there is a penetrating injury to the brain which has introduced bacteria and infection. There may be antibiotics proven to treat the infection, but they are not able to pass through the barrier and reach the bacteria.

Bypassing the BBB to Treat Trauma

There are many ongoing research efforts focused on finding ways to bypass the BBB after infection, trauma or to prevent other diseases.

Researchers have successfully created a mucosal skin graft in the nose through which they can deliver some medications to treat a limited number of specific conditions. There is ongoing research to determine how this can be extended to a variety of other conditions such as cellular injury after traumatic brain injury.

Another, more recent treatment is brain microdialysis. A catheter is placed into a specific region of brain injury. Toxins and fluids are filtered out and a more normal cellular environment is slowly replaced. This helps prevent neurotoxin and other damage from conditions such as cytotoxic edema and glutamate storm.

Fortunately, some antibiotics due to their small size and other physical properties are able to cross the blood brain barrier and be helpful in those circumstances.

Sources:

Serlin, Y., Shelef, I., Knyazer, B., & Friedman, A. (2015). Review: Anatomy and physiology of the blood–brain barrier. Seminars In Cell And Developmental Biology, doi:10.1016/j.semcdb.2015.01.002

Carpenter, K. L., Jalloh, I., Gallagher, C. N., Grice, P., Howe, D. J., Mason, A., & ... Hutchinson, P. J. (2014). Review: 13C-labelled microdialysis studies of cerebral metabolism in TBI patients. European Journal Of Pharmaceutical Sciences, 57 87-97. doi:10.1016/j.ejps.2013.12.012

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