The CSR of the brain and cerebrospinal fluid (cerebrospinal fluid) washing it is little dependent on changes in the pH of other body systems. This isolation is useful because the acidity of the extra- and intracellular fluid is a factor in regulating brain metabolism and neural activity. Numerous studies have shown a high degree of reliability of the control of the pH of the cerebrospinal fluid in acute and chronic violations of the CRR in the body that accompany various human diseases. Under physiological conditions, cerebrospinal fluid has a more acidic reaction (pH 7.35) than arterial plasma. Homeostasis The pH of brain fluids is maintained through a variety of negative feedbacks. It depends, on the one hand, on the general mechanisms that regulate the pH of body fluids, and on the other hand, on mechanisms specific to the brain. Among the latter, an especially important role seems to be played by three specific systems. One of them is related to the selective BBB permeability for various substances, the second to the nature of transport mechanisms located between the blood and extracellular fluids of the brain, and the third to compensatory metabolic shifts.