**4. A bit of pathophysiology and clarity**

The brain has a special type of structure and interaction of cells, and SIR (which was given great attention in the previous section) manifests itself in it as a neuroinflammatory response, which determines all the disorders that are clinically manifested in a patient [57]. If we want to understand the essence of this process, then we need to look at the theory of the neurovascular unit of the brain (NVU).

The fact is that regulatory processes in the brain are a special type of cell interaction that requires the creation of an isolated microenvironment. For this purpose, there is the blood-brain barrier (BBB) and its components, the functional unit of which is NVU. NVU consists of microvessels that are associated with astrocytes, which in turn are associated with neurons and their axons. The BBB contains special carrier proteins for the selective transport of substances from blood plasma to neurons. All this in combination ensures the coordinated work of the brain through intercellular communications and metabolic coupling of cells of the central nervous system [58]. Cerebral injury leads to the activation of microglia and astrocytes and the sequential

production of inflammatory mediators in the brain [59]. Mediators damage the BBB and further stimulate cell death and gliosis [60]. Moreover, when the integrity of the BBB is violated, the NVU can be influenced not only by local but also by systemic cytokines (recall the notorious SIR). They stimulate the expression of adhesion molecules, potentiate the adhesion and extravasation of neutrophils and monocytes into ischemic tissues [61]. Local production of chemokines enhances leukocyte extravasation in these tissues [62]. All of these processes occur within the NVU, producing certain pathological changes in the tissues of the brain, followed by its dysfunction. Summarizing the above, we can conclude that not only destructive factors such as hypoxia have a damaging effect on the brain, but also any other factors that can cause SIR [63, 64]. In addition, one should not forget about the physiological features of children, because the first year of life in them is characterized by increased permeability of the vascular wall and hydrophilicity of the interstitial space, which, of course, further contributes to the accumulation of excess fluid in it and increased SIR [65].

As you may have noticed, almost all etiological factors realized by such complex pathophysiological mechanisms are modifiable, and they can and should be combated! However, in some cases, even after making every effort, you will still encounter delirium after the operation. But you still need to be able to recognize it.
