How much blood is in a normal person’s body is closely related to his or her own body weight, because the weight of a person’s own blood is in direct proportion to his or her own body weight. The weight of blood in a normal person’s body accounts for about 7-8% of his or her own body weight; the larger the weight of a person, the more blood he or she has in his or her body, and the lighter the weight, the less blood he or she has in his or her body accordingly. An adult weighing about 70kg has a total body blood volume of about 5-5.5kg, which translates to a volume of about 5,000-5,500mL, on the contrary, a child weighing about 20kg has a body blood weight of about 1.5kg, which translates to a volume of about 1.5L. Therefore, the amount of blood in a person’s body is closely related to his or her body weight, and because of this, when donating blood in the clinic, people who are underweight are usually not asked to donate blood.