Blood-brain barrier. Illustration of the haematoencephalic barrier between the blood flow and the central nervous system. This system regulates the environment (homeostasis) in the brain by separating it from the blood. The capillary is shown in cutaway view with red blood cells seen inside the blood vessel. Both the endothelial cells (red) that line the capillaries and the surrounding astrocytes (blue) are essential components of this barrier. The blood-brain barrier stops harmful chemicals and bacteria from reaching the brain. The pink cells are the neurons in the brain. Viruses and special types of drugs are able to pass the blood-brain barrier.