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As a rule, patients with normal blood pressure retain normal alertness with ICP of 25–40 mmHg (unless tissue shifts at the same time). Only when ICP exceeds 40–50 mmHg does CPP and cerebral perfusion decrease to a level that results in loss of consciousness. Any further elevations will lead to brain infarction and brain death. [citation needed]
Cerebrospinal fluid (CSF) is a clear, colorless transcellular body fluid found within the meningeal tissue that surrounds the vertebrate brain and spinal cord, and in the ventricles of the brain. CSF is mostly produced by specialized ependymal cells in the choroid plexuses of the ventricles of the brain, and absorbed in the arachnoid granulations .
The blood–brain barrier (BBB) is a highly selective semipermeable border of endothelial cells that regulates the transfer of solutes and chemicals between the circulatory system and the central nervous system, thus protecting the brain from harmful or unwanted substances in the blood. [1]
Blood levels of oxygen become important in hypoxia. These levels are sensed by central chemoreceptors on the surface of the medulla oblongata for decreased pH (indirectly from the increase of carbon dioxide in cerebrospinal fluid ), and the peripheral chemoreceptors in the arterial blood for oxygen and carbon dioxide.
Failure of the barrier may occur in a pulmonary barotrauma.This can be a result of several possible causes, including blast injury, swimming-induced pulmonary edema, and breathing gas entrapment or retention in the lung during depressurization, which can occur during ascent from underwater diving or loss of pressure from a pressurized vehicle, habitat or pressure suit.
The blood–CSF boundary at the choroid plexus is a membrane composed of epithelial cells and tight junctions that link them. [14] There is a CSF-brain barrier at the level of the pia mater, but only in the embryo. [15] Similar to the blood–brain barrier, the blood–CSF barrier functions to prevent the passage of most blood-borne substances ...
There is a similar blood–cerebrospinal fluid barrier, which serves the same purpose as the blood–brain barrier, but facilitates the transport of different substances into the brain due to the distinct structural characteristics between the two barrier systems. [44] [63]
An increase in carbon dioxide causes tension of the arteries, often resulting from increased CO 2 output (hypercapnia), indirectly causes the blood to become more acidic; the cerebrospinal fluid pH is closely comparable to plasma, as carbon dioxide easily diffuses across the blood–brain barrier.