The Blood Brain Barrier

2009-01-08

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I wouldn’t be wrong if I say the Brain is the most vital organ in the human body. As we all know brain handles most of critical operations inside our body such as keeping and managing memories, organizing our activities, keep other organs perform, and so much more.

The Brain, as most of us already know, works by sending electric signals through complex meshes of neurons so the brain has to maintain a good environment around to keep the accuracy of this electrical signal flow.

The environment inside the brain is the brain fluid so the ingredients or components of the brain fluids are very critical to the functionalities of brain.

Any organ of the body gets the supplies it needs through blood and so does the brain. Ingredients of human blood vary depending on the situation, food, problems of other organs, etc. so it is obvious that the blood is not the same all the time.

Not like other organs, the brain has to think a lot before extracting anything from blood because if the chemical levels of brain fluid vary it directly affects the signals passed through neurons. If the brain fluid is not maintained in an optimum level, the environment will become too noisy for neurons and sending signals would become like talking in the middle of a party.

So there is this Blood-Brain Barrier (which is also referred to as BBB) which extracts the necessary ingredients such as Oxygen and Glucose and make sure nothing else is taken in.

Blood-Brain Barrier Diagram

Photo courtesy: Malcolm Segal

Where is Blood-Brain Barrier Located?

BBB is located at the brain blood capillaries. These capillaries are unusual in several aspects from capillaries in other organs. Those aspects are briefly described below:

• The end-points of cells which make up the walls of these capillaries are sealed together at their edges by tight junctions which are a key component in the barrier. These junctions make sure that water soluble substances in the blood don’t pass between the edges of cells.
• These capillaries are enclosed by flattened cells which altogether called the ‘end-feet’ which also work as a (partial and active) barrier.
• The only way for water soluble substances in the blood is through the walls of capillaries. These walls plays the other role in the barrier because their cell membranes are made up of a lipid/protein belayed only allowing flat-soluble molecules including those of oxygen and carbon dioxide, anesthetics and alcohol to pass through the walls of capillaries.
• In the capillary wall there are three classes of ‘efflux pumps’ which pumps various lipid-soluble molecules back in to the blood out of the brain.

However, the brain needs water soluble compounds such as Glucose for energy production and amino acids for protein synthesis so there are these Carriers in the walls of capillaries which allow those compounds to go through the wall and move waste products and unwanted molecules in the opposite direction.

The blood-brain barrier plays another key role in keeping the volume of the brain at a constant level. Since the brain is contained inside a rigid skull it is important to keep the fluid from free movement thus keeping volume of the brain static.

Problems Related to BBB

Even though BBB is made in the sole purpose of protecting the brain, it has become a barrier to medicine to access the brain. Since many medicines are water soluble, they are barricaded at the BBB. This process has made it very hard to treat brain tumors and infections such as AIDS virus.

In fact, AIDS virus uses BBB as a shield by hiding behind the BBB from body defense mechanisms.

Due to such reasons medicine has to be made as fat-soluble but then a new problem arises because then the medicine will be absorbed by most cells of the body which may be toxic.

The alternatives are making drug molecules that can ride on natural transporter proteins or use drugs that can open the Blood-Brain-Barrier.

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Cassian Menol Razeek