Low blood volume refers to the whole body blood volume is lower than normal, mainly due to the loss of body fluids caused by thirst, urine, easy to fatigue, postural dizziness and other symptoms, serious can appear drowsiness, indifference, little urine or even no urine and other symptoms. Normal human blood volume is about 4000~5000mL, accounting for 7% of body weight. Low blood volume can be caused by physiological and pathological reasons. 1. If a normal person does not drink water for a long time, it can cause physiological low blood volume, and the blood volume can be restored to normal by replenishing water in time. 2. Certain diseases lead to the loss of body fluids, which can cause hypovolemia, and the main ways include the loss of body fluids through the gastrointestinal tract, skin, kidneys and other organs. (1) Loss through gastrointestinal tract, such as gastroenteritis, due to vomiting, diarrhea, a large number of body fluids can be lost through the gastrointestinal tract. Loss through the kidneys, such as the use of diuretics, large amounts of diuresis, loss through the skin, including sweating, etc., can cause blood volume reduction. (2) Trauma causing massive bleeding can also cause hypovolemia and even shock. (3) Fluid retention in the third interstitial space (e.g., thoracic cavity, abdominal cavity, etc.) and reduction of intravascular fluid can also cause hypovolemia. Low blood volume can also be seen in other diseases, after excluding physiological causes, should promptly go to the hospital to clarify the cause, under the guidance of the doctor to actively cooperate with the treatment.