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Tonicity Tonicity is the capability of a solution to modify the volume of cells by altering their water content. The movement of water into a cell can lead to hypotonicity or hypertonicity when water moves out of the cell. The water movement then causes the cell to swell or shrink in size through osmotic pressure differences between the intracellular compartment (IC) and the solution tested. Solutions are isotonic when the volume of cells suspended in them does not change by osmotic fluid transfers.[1] Osmosis Osmosis refers to the net movement of water, across a selectively permeable membrane, towards the location of higher osmotic concentration. Osmolarity Osmolarity is the term used for describing the concentration of solutes within a fluid. The terms isotonic, hypertonic, and hypotonic compare the osmolarity of a cell to the osmolarity of the extracellular fluid around it. Hyperosmolarity doesn't always mean hypertonicity because this depends on the solutes. Solutes such as Na+ and glucose, for example, need transporters. They contribute to serum tonicity and are termed effective osmoles (contributing to osmolarity). Meanwhile, urea and ethanol easily pass through cell membranes, contributing to serum osmolality but not tonicity.[2]

Tonicity is tightly regulated by the equilibrium between water intake and water excretion.[2] Normal conditions where water loss occurs are respiration, within gastrointestinal fluids, in urine, and through the skin. The problem occurs when patients are unable to replete those losses. When the osmoreceptors in the hypothalamus sense the increase in serum tonicity, water intake is suggested by the stimulation of thirst. In addition, the kidney's primary reaction to water loss is through concentrating the urine. Just 1% of the change in tonicity is enough to produce ADH release, but it needs a greater than 10% fall in extracellular volume to be released. ADH acts on the V2 receptors in the principal cells of the collecting tubules within kidneys and causes the expression of aquaporins for water movement from the tubules to the hypertonic interstitium. One of the cardinal manifestations of a hyperglycemic crisis is hypertonicity.[18] The excess of glucose in the extracellular fluid has a hypertonic effect and produces an osmotic diuresis that can cause water loss to exceed the losses of sodium and potassium. This results in an elevated sodium concentration within the cell and will stimulate thirst. High-glucose conditions in patients with diabetic microvascular complications, particularly with diabetic nephropathy, have shown TonEBP to upregulate the expression of AR.[19] The production of AR in cells that can produce the enzyme is desired for the enzyme's ability to catalyze the reaction of glucose to sorbitol. The inability of sorbitol to cross cell membranes and its accumulation within the cell aids in counteracting the osmotic stress placed on cells during a hyperglycemic event. AR is present in tissues such as nerves, retina, lens, glomerulus, and vascular cells.[20] During acute states of tonicity variability, the brain is also in danger. The primary defensive adaptation occurs through RVI, but astrocytes also play a major role by accepting the movement of water from the cerebrospinal fluid.