Isotonic dehydration

Surgical patients are most prone to this type of dehydration; water and sodium are lost proportionally, so serum sodium is in the normal range and extracellular fluid osmotic pressure remains normal. It causes a rapid decrease in the amount of extracellular fluid (including circulating blood volume); because the lost fluid is isotonic, it does not substantially change the osmotic pressure of the extracellular fluid, and the initial intracellular fluid does not transfer to the extracellular fluid space. The amount of extracellular fluid is compensated for, so the amount of intracellular fluid does not change. However, after this kind of fluid loss lasts for a long time, the intracellular fluid will gradually move outward, and it will be lost along with the extracellular fluid, resulting in dehydration of the cells.