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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]
The blood–brain barrier is formed by special tight junctions between endothelial cells lining brain blood vessels. Blood vessels of all tissues contain this monolayer of endothelial cells, however only brain endothelial cells have tight junctions preventing passive diffusion of most substances into the brain tissue. [1]
The cells of the neurovascular unit also make up the blood–brain barrier (BBB), which plays an important role in maintaining the microenvironment of the brain. [11] In addition to regulating the exit and entrance of blood, the blood–brain barrier also filters toxins that may cause inflammation, injury, and disease. [12]
Osmotic agents work by primarily affecting the blood brain barrier. [1] It's very important that the osmotic agents cannot cross the blood brain barrier because the main idea is to use osmotic agents to increase plasma osmolarity and cause an osmotic gradient to cause water from brain cells to flow into the plasma.
The blood-brain barrier protects the brain by restricting the ability of large molecules to cross the barrier between the blood, CSF, and interstitial fluid of the brain. ICV injection circumvents this barrier, to be able to deliver drugs to the CSF.
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.
The cells in this coronal section of the brain were colored with a bluish dye ("Nissl stain"). The thalamus is at the bottom of the photo. The bar at the lower right represents a distance of 200 μm (0.2mm). As noted above, capillaries in some subregions within the SFO are fenestrated, [6] and thus lack a blood–brain barrier.
The constrained intracellular pathway exacted by the tight junction barrier system allows precise control over which substances can pass through a particular tissue (e.g. the blood–brain barrier). At the present time, it is still unclear whether the control is active or passive and how these pathways are formed.