When fluid loss exceeds fluid intake, clinical manifestations are as a decrease in extracellular fluid volume, i.e., a low-volume state of insufficient body fluid volume. Water loss is accompanied by sodium loss, and the degree of loss of the two may vary. The ratio of water and sodium loss is consistent, and the extracellular fluid osmotic pressure is maintained within the normal range, which is called isotonic dehydration; water loss> sodium loss, extracellular fluid osmotic pressure> 320mmol/L, blood sodium> 150mmol/L, called hypertonic dehydration; sodium loss> water loss, extracellular fluid osmotic pressure< 280mmol/L, blood sodium< 130mmol/L, called hypotonic dehydration. According to the amount of fluid loss, dehydration is divided into three degrees. Water loss of 3% to 6% of body weight is moderate dehydration, and water loss of more than 6% of body weight is severe dehydration.