Clinically, blood transfusion is mainly used for patients with the following three major types of diseases: 1) acute internal bleeding due to chronic blood loss or trauma; 2) hematologic diseases, such as impaired coagulation factor synthesis or chronic anemia; 3) severe infections. The indications for clinical transfusion include medical treatment and surgical transfusion. The indications for medical transfusion mainly refer to blood transfusion when hemoglobin is less than 60g/L and the red blood cell pressure product is less than 20%; or if there is coagulation factor deficiency, transfusion of coagulation factor can be considered. For surgical transfusion, it mainly refers to transfusion of red blood cells when hemoglobin is less than 70g/L or erythrocyte pressure product is less than 30%, or when the patient has active bleeding after surgery or intraoperatively, and blood pressure continues to drop.