Human brain
The blood–brain barrier

The larger arteries throughout the brain supply blood to smaller capillaries. These smallest of blood vessels in the brain, are lined with cells joined by tight junctions and so fluids do not seep in or leak out to the same degree as they do in other capillaries; this creates the blood–brain barrier. Pericytes play a major role in the formation of the tight junctions. The barrier is less permeable to larger molecules, but is still permeable to water, carbon dioxide, oxygen, and most fat-soluble substances (including anaesthetics and alcohol). The blood-brain barrier is not present in the circumventricular organs — which are structures in the brain that may need to respond to changes in body fluids — such as the pineal gland, area postrema, and some areas of the hypothalamus. There is a similar blood–cerebrospinal fluid barrier, which serves the same purpose as the blood–brain barrier, but facilitates the transport of different substances into the brain due to the distinct structural characteristics between the two barrier systems.
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