When brain cells are dehydrated by increased extracellular fluid osmolarity, it can cause a series of symptoms of central nervous system dysfunction, including drowsiness, muscle twitching, coma, and even death. It is hypertonic dehydration. When the brain volume is significantly reduced due to dehydration, the vascular tension between the skull and cerebral cortex increases, thus leading to local intracerebral hemorrhage and subarachnoid hemorrhage due to venous rupture. So how do you clinically check for increased extracellular fluid osmolality? The magnitude of plasma osmolarity is mainly related to the content of inorganic salts and proteins. Among the various inorganic salt ions that make up the extracellular fluid, the ones with a clear advantage in content are Na+ and Cl-, and 90% of the osmolarity of the extracellular fluid comes from Na+ and Cl-. At 37°C, the osmolarity of human plasma is about 770 KPa, which is equivalent to the osmolarity of the intracellular fluid . The osmotic pressure of plasma is approximately equal to that of 0.9% NaCl solution or 5% glucose solution, so 0.9% NaCl solution or 5% glucose solution is the isotonic solution of plasma, and the former is also called saline. Any higher than 0.9% NaCl solution is called a hypertonic solution, below which it becomes a hypotonic solution. Plasma osmolality has two components: one is the plasma crystal osmolality formed by small molecules of crystalline substances such as inorganic salts, glucose and urea in plasma; the other is the plasma colloid osmolality formed by large molecules such as plasma proteins. Since there are very many small and medium molecule crystalline substances in plasma, plasma osmolality is mainly crystal osmolality.