The number of milliliters of blood in a person’s body is closely related to the individual’s body weight. The weight of a person’s own blood is directly proportional to their own body weight. Normally, the weight of one’s own blood is about 7-8% of one’s own body weight. So a person who weighs about 70kg, the total amount of own blood is about 5.0kg, which means that the volume of own blood is about 5000ml. But if a child or teenager weighs about 20kg, the weight of blood in the body is about 1.5kg, and the number of milliliters of blood, about 1500ml. Therefore, the volume of blood in a person’s body is closely related to his or her own weight, and because of this, there are clear requirements on the weight of the donor when donating blood in a clinical setting, and only adult males weighing no less than 50kg and adult females weighing no less than 45kg can donate blood.