The loss of body fluid exceeds the intake of fluid, and it is clinically manifested as a decrease in the amount of extracellular fluid, that is, a low-volume state of insufficient body fluid volume. Water loss is accompanied by sodium loss. The degree of loss of the two can be different. The ratio of water and sodium loss is the same, and the osmotic pressure of the extracellular fluid is maintained within the normal range, which is called isotonic water loss; water loss> sodium loss, extracellular fluid. ..