The formula for calculating plasma osmolality is effective plasma osmolality = sodium + potassium mmol/L x 2 + values for glucose as well as urea nitrogen. The osmolality of the blood is mainly calculated based on the concentration of potassium, the concentration of sodium, as well as the concentration of glucose and urea nitrogen, the value is equivalent to about twice the value of the concentration of potassium ions, plus twice the value of the concentration of sodium ions, plus the concentration of urea nitrogen, as well as plus the concentration of glucose, about the normal osmolality of the blood is maintained in the range of 290 to 320 mmol / liter. Less than 280 mmol/liter is considered hypotonic, and greater than 310 mmol/liter is considered hypertonic. Plasma osmolality is composed of plasma colloid osmolality and plasma crystal osmolality. The magnitude of plasma osmolality is related to the concentration of crystalline and colloid substances within the plasma; therefore, by measuring the concentration of the above substances, the osmolality of plasma can be determined. Blood osmolality should be maintained at 290 to 320 mmol/l. If it is not in the normal range it is recommended that the patient go to the hospital promptly.