Hypotonic dehydration refers to the loss of both water and sodium, but the lack of water is less than the lack of sodium. The serum sodium is lower than the normal range, and the extracellular fluid is in a hypotonic state. The body reduces the secretion of antidiuretic hormone, which reduces the reabsorption of water in the renal tubules and increases urine excretion to increase the osmotic pressure of the extracellular fluid. However, the amount of extracellular fluid decreases further. Although the interstitial fluid enters the blood circulation and can partially compensate for the blood volume, the reduction in interstitial fluid exceeds the reduction in plasma. Faced with a significant reduction in circulating blood volume, the body will no longer take into account the osmotic pressure and try to maintain blood volume.