What is the blood-brain barrier?
The blood-brain barrier (BBB) separates the blood circulating through our bodies from the central nervous system (CNS), which includes the brain. This acts as a control, ensuring that substances that are needed for healthy brain function, such as glucose, amino acids and electrolytes, are present in appropriate levels. It also protects our brain from harmful substances in the blood, such as disease-causing bacteria and toxins.
So how does it do this? Like blood vessels in the rest of the body, those in the CNS are lined with a thin layer of so-called endothelial cells, which are in direct contact with the blood inside the vessels. But CNS blood vessels differ in that their endothelial cells are packed tightly side by side, with their membranes zipped together by specialised protein structures that form tight junctions. These restrict the flow of substances from the blood to the CNS, allowing only small molecules such as water, some gases, and compounds that can pass unaided through cell membranes (called lipid-soluble molecules) to pass freely into the brain.
The membranes also contain efflux pumps, which kick potentially harmful compounds and drugs back out of the brain. Other channels, transporters and receptors in the membranes help essential nutrients and molecules get into the brain.
CNS blood vessels are also surrounded by a lot more supporting cells (pericytes) than blood vessels found elsewhere in the body. In addition they are surrounded by neural cells called astrocytes. The pericytes and astrocytes provide an extra barrier and influence the expression of genes that play a role in the selectivity of the BBB.
You can see that the BBB is not just a simple barrier, but rather a sophisticated and carefully managed border control mechanism.
Illustration by Rick Kollath; kollathdesign.com