What is hypovolemia? Is it different from dehydration?
Hypovolemia is generally synonymous with ECF volume contraction. Physicians should distinguish between ECF volume contraction and dehydration. ECF volume contraction implies a loss of sodium and water. Dehydration implies a loss of water. For example, a loss of 1 L of fluid containing 150 mmol sodium will cause the ECF volume to fall by 1 L. Because the lost fluid is isotonic, plasma osmolality will not change. A loss of 1 L water, however, will cause much less ECF volume contraction. Water will be lost proportionately from all body fluid compartments. Because two-thirds of total body water is intracellular and one-third is extracellular, a loss of 1 L water will cause the ECF volume to fall by only 0.33 L. Plasma osmolality will rise. A loss of 1 L water is more properly called dehydration. Losses of water, then, are less likely to cause manifestations of ECF contraction than are losses of sodium and water ( Fig. 71.3 ). Losses of water, however, may cause manifestations of hypernatremia.
