The blood-brain barrier consists of two parts: 1) the barrier between plasma and brain cells: it is composed of capillary walls and glial cells of the brain; 2) the barrier between plasma and cerebrospinal fluid: it is composed of the choroid plexus. The main function of the blood-brain barrier is to prevent harmful substances in the blood from entering the brain cells and producing a series of adverse reactions. There are many solutes in the blood that can pass through the capillary wall and the glial cell layer to enter the brain tissue, but their ability to pass through is strong or weak, some can pass quickly, some more slowly, and some cannot pass at all, and this situation is called selective permeability phenomenon. This structure allows the brain tissue to be less damaged by the harmful substances circulating in the blood and maintains the relative stability of the internal environment of the brain tissue.