Hypotonic dehydration is also known as chronic dehydration or secondary dehydration. Water and sodium are lost simultaneously, but water deficiency is less than sodium loss, so serum sodium is below the normal range and the extracellular fluid is hypotonic. The body reduces the secretion of antidiuretic hormone, so that the reabsorption of water in the renal tubules is reduced and the urinary output is increased to raise the osmolarity of the extracellular fluid. However, the amount of extracellular fluid in turn decreases even more, and intertissue fluid enters the circulation, which partially compensates for the blood volume, but makes the decrease in intertissue fluid more than the decrease in plasma. Faced with a significant decrease in circulating blood volume, the body will no longer take into account the osmotic pressure and try to maintain blood volume. Excitation of the renin-aldosterone system leads to a decrease in renal sodium excretion and an increase in CI- and water reabsorption. Therefore, the sodium chloride content in the urine is significantly reduced. The decrease in blood volume in turn stimulates the posterior pituitary gland, resulting in increased secretion of antidiuretic hormone and increased water reabsorption, leading to oliguria. If the blood volume continues to decrease, shock will occur when the above compensatory functions are no longer able to maintain blood volume. This kind of shock caused by massive sodium loss is also called hyponatremic shock. The main causes of water and salt metabolism disorders are: continuous loss of gastrointestinal digestive juices, such as repeated vomiting, long-term gastrointestinal tract suction or chronic intestinal obstruction, resulting in the loss of sodium with a large amount of digestive juices; chronic oozing of large trauma; excessive renal excretion of water and sodium, such as the application of sodium-removing diuretics (clothianidin, diuretic acid, etc.), without attention to the amount of sodium supplementation, so that the body is relatively more sodium than water deficiency.