The blood-brain barrier, as the name suggests, is a physical barrier composed of an endothelial layer or a thin layer of cells which separates the blood in the circulation from the cells in the brain. As the picture below shows, unlike in normal circulation, the arteries within the brain contain tight junctions which prevent bacteria and hydrophilic (water-loving) molecules from crossing the blood vessels into the extracellular fluid (extracellular= outside of cells) where the brain cells are located. This barrier is the reason why infections of the brain itself are quite rare compared to other nearby structures such as the meninges (meningitis is far more common). When the brain is inflamed, however, the barrier becomes compromised and more harmful substances can pass through. I’ll discuss them in more detail in Brain Pathology: Infections of the Brain. 

Pic: http://www.rsc.org/images/FEATURE-Blood-brain-barrier-410_tcm18-202395.jpg

Despite the fact that hydrophillic substances and bacteria cannot cross the blood-brain barrier, as the picture above shows, hydrophobic (water-hating) substances can still cross it by means of diffusion (transportation through the cell either by itself or attached to proteins). This is essential for two reasons: for the cycle of obtaining oxygen and removing carbon dioxide from the brain as well as obtaining glucose, the brain’s only energy source. Other components of the blood-brain barrier are the basement membrane, which is quite thick in diameter, and astrocytes or glial cells.

Most of the brain’s structures are encompassed by the blood-brain barrier, however there are a couple of notable exceptions: the pineal gland, which secretes melatonin directly into the bloodstream, and the roof of the third and fourth ventricles.