Surgical patients are most susceptible to this type of dehydration; water and sodium are lost in proportion, so serum sodium is within the normal range and the osmotic pressure of the extracellular fluid is also maintained normal. It causes a rapid decrease in the amount of extracellular fluid (including circulating blood volume); because the lost fluid is isotonic and basically does not change the osmotic pressure of the extracellular fluid, the intracellular fluid does not initially move to the extracellular fluid gap to compensate for the decrease in extracellular fluid, so the amount of intracellular fluid does not change. However, after this type of fluid loss lasts for a long time, the intracellular fluid will gradually move outward and be lost along with the extracellular fluid, causing cellular dehydration.