The amount and speed of blood loss are different, and the influence of blood loss on human body is also different. If the blood loss is less than 10% of all blood, the human body only needs to mobilize the stored blood to participate in circulation, and there will be no obvious discomfort. If the blood loss reaches 20% of the total blood volume, the body is still insufficient to maintain the normal blood volume and blood pressure level after compensation, which will affect people's life activities. If the blood loss exceeds 30% in a short time, it will be life-threatening if the blood transfusion is not timely.

Under normal circumstances, an adult loses 500 ml of blood, and it is impossible to have obvious symptoms. When the blood loss exceeds 800 ml, there will be symptoms such as pale face, pale lips, cold sweat on the skin, cold hands and feet, shortness of breath, rapid and weak pulse. When the blood loss reaches more than 1500 ml, it can cause insufficient blood supply to the brain, and the injured person will have symptoms such as blurred vision, thirst, dizziness, unconsciousness or anxiety, and even coma.