The loss of body fluids resulting in a decrease in extracellular fluid is called dehydration. When a person works under high temperature conditions, exercises vigorously or has certain diseases (such as violent vomiting or severe diarrhea), a large amount of water and inorganic salts (mainly sodium salts) will be lost, which will lead to a decrease in the body’s extracellular fluid osmolarity and symptoms such as decreased blood pressure, increased heart rate, four chills, and even coma in severe cases. Water balance disorders can be manifested as too little or too much overall water or little change in overall water, but there are significant differences in water distribution, i.e., an increase in intracellular water and a decrease in extracellular water, or a decrease in intracellular and an increase in extracellular water. Disturbances in water balance are often accompanied by changes in electrolytes in body fluids and changes in osmolality. The loss of body fluids resulting in a decrease in extracellular fluid is called dehydration. Dehydration can be divided into hypertonic, isotonic and hypotonic dehydration due to changes in plasma sodium concentration or not. 1, hypertonic dehydration dehydration is dominated by water loss, and less electrolyte loss compared with water, i.e., for every 1L of body fluid lost, about 300mOsm or less of electrolytes are lost, resulting in an increase in extracellular osmotic pressure, which is mostly seen in insufficient drinking water, such as high-temperature work with a lot of sweating, or the patient’s non-manifest water loss is still ongoing, resulting in an increase in water excretion. High osmotic dehydration is characterized by: ① increased electrolyte concentration in body fluids, with plasma Na+ concentration greater than 150 mOsm/L or the sum of CL- and HCO3- concentrations greater than 140 mmol/L; ② decreased amount of extracellular fluid; ③ transfer of intracellular fluid water to extracellular fluid, resulting in a significant decrease in intracellular fluid. The clinical symptoms are thirst, increase in body temperature and various neurological symptoms, as well as a decrease in urine volume and consequently a significant weight loss. 2. Isotonic dehydration mainly results in the loss of extracellular fluid. Since the loss of water and electrolytes is basically balanced, i.e., about 300mOsm/L electrolytes are lost for every 1L of body fluid lost, and thus the osmolarity of extracellular fluid remains normal, it is called isotonic dehydration. It is commonly seen in the loss of digestive fluid such as vomiting and diarrhea, when the electrolyte concentration of body fluid does not change. Normally, the plasma Na+ concentration is 130-150 mmol/L or the sum of Cl- and HCO3- concentration is 120-140 mmol/L; however, the amount of extracellular fluid is reduced and the amount of intracellular fluid is normal. Isotonic dehydration damages the body by reducing the amount of extracellular fluid and leading to insufficient blood volume, decreased blood pressure and impaired peripheral blood circulation. Hypotonic dehydration is mainly due to loss of electrolytes. Compared with water, the loss of electrolytes is more, i.e. for every 1L of body fluid lost, more than 300mOsm of electrolytes are lost at the same time, thus the osmolarity of extracellular fluid is lower than normal, so it is called hypotonic dehydration. The cause of hypotonic dehydration is caused by the loss of body fluids and the replacement of water without replenishing electrolytes, such as the loss of gastrointestinal digestive juices (diarrhea, vomiting, etc.) and the loss of electrolytes from digestive juices and sweat without replenishing water, resulting in hypotonic dehydration. In this case, the plasma Na+ concentration is less than 130 mmol/L or the sum of Cl- and HCO3- concentration is less than 120 mmol/L. The amount of extracellular fluid decreases, the amount of intracellular fluid increases, and there is a slight weight loss.