One unit of blood is about 120ml, because one unit of blood is the red blood cells separated from 200ml of whole blood. Generally speaking, red blood cells account for 40%-50% of all blood, so that the solvent of one unit of aqueous blood is between 80-100ml, while adding other anticoagulants, calcium ions, etc., the volume of one unit of blood is about 120ml. When transfusing blood in the clinic, it is calculated on a unit basis. For example, if a few units of red blood cells are transfused to an anemic patient, the patient’s hemoglobin can rise to a level of about 5-6g when one unit of red blood cells is transfused. Therefore, according to the degree of anemia of the patient and the number of units of blood transfused, it is possible to roughly calculate the number of grams of hemoglobin elevation of the patient after transfusion, which is also a common method to judge the effect of blood transfusion of the patient.