Every cell in the human body requires nutrients and oxygen, which are transported to the cells by blood vessels. The brain is no different in that it has blood vessels that go through it to nourish the brain cells. Blood vessels are made of a bunch of cells called endothelial cells that closely associate to form a tube. It’s sort of like if you go into a stone tunnel and all the individual rectangular stones are arranged next to each other without any gaps. Usually the gaps between the endothelial cells are kind of “leaky” which helps some of the nutrients get through. Also, immune cells can squeeze through these gaps to get to the site of an infection to destroy the pathogen. However, the brain is a very sensitive organ and it would be unwise to let molecules or cells leak in between the endothelial cells. Therefore, in the brain, the endothelial cells are very tightly packed together so there are no gaps between cells and therefore no leaks. The regions of blood vessels where the blood vessels don’t leak at all are part of the blood-brain barrier. Additionally, at the blood-brain barrier, there are brain “support cells” called astrocytes which help the endothelial cells maintain the integrity of the barrier. This barrier is important because if any bacteria or toxic molecules can get through the blood vessel into the brain, they could be very damaging.

The blood brain barrier doesn’t stop all molecules from entering though; gases (like CO₂ and O₂), water, and some molecules like hormones can pass through. And other molecules that are needed, but can’t easily pass through, like sugars, are specifically transported by proteins through the barrier. The down-side of this barrier, is that if something bad happens in the brain, we can’t easily get medicine in to solve the problem. Also, some bacteria, viruses, and parasites have ways to get through the barrier, but immune cells still can’t which means the infectious organism is invulnerable to the body’s defenses. While the blood brain barrier is very important for our survival, there are many scientific efforts in trying to get past it to more effectively treat brain-related illnesses.