This barrier is a system that supports the selective flow of substances from the blood to the brain and vice versa, which creates conditions for the relative independence of metabolism in the brain. Anatomically, the BBB consists of capillary endothelium with the underlying basement membrane and neuroglia cells. There is a selective BBB permeability for various substances: low for H + and HCO 3 – and high for CO 2 .
Transport systems play a crucial role in regulating the pH of extracellular fluids in the brain. Targeted active transport of H + and HCO 3 – ions is carried out in such a way that in case of metabolic acidosis, HCO 3 is kept in the cerebrospinal fluid at a much higher level, and with metabolic alkalosis – at a much lower level than in blood plasma. According to many authors , the factor determining the passive movement of ions is not the gradient of its concentration, but the electrochemical potential of ions, which significantly affects the potential difference between the BBB . The electrochemical potential (EP) for a hydrogen ion is connected by a simple ratio with pH and the electric potential difference (E) at the BBB boundary.
The potential difference measured under experimental conditions at the BBB boundary is on average 4 mV. In most cases, this positive value of the difference in blood potentials in relation to cerebrospinal fluid is attributed to the continuous production of acids, mainly lactate and carbon dioxide, by the brain tissue. In this case, acids do not accumulate, since there is a constant leakage of hydrogen ions from the extracellular fluid of the brain into the blood. The level of the potential difference is not a passive quantity, but, in accordance with the hypothesis, plays an active role in maintaining the CRR. Fluctuations in potential with a change in the pH of arterial blood occur in such a way as to maintain the pH of the cerebrospinal fluid at a constant level.
Compensatory shifts in metabolism. A powerful mechanism affecting all the liquid media of the brain, especially with alkalosis, is a metabolic shift, leading to a change in the production of acidic metabolites, in particular lactic acid. This shift is of such a nature that a change in the production of acid tends to prevent a violation of the pH of the intracellular fluid and, apparently, is primarily associated with the activation of glycolysis in alkalosis.
In acidosis, carbonic anhydrase is activated at the blood \ brain and blood \ CSF levels, while in glial cells HCO 3 – replaces Cl – , and the level of HCO 3 – in the extracellular fluid decreases.
However, with pathology, these mechanisms may be insufficient. In particular, with cirrhosis of the liver, there is a decrease in oxygen consumption, an intensification of glycolysis, and an increase in the flow of glucose into the brain. This leads to the development of acidosis and a deterioration in brain activity, which can be seen in the analysis of EEG .