Human blood is a prime necessity in modern surgery and treatment for certain diseases. Blood transfusion is needed in many cases and there is no substitute for human blood. To meet the increasing demand for this life-saving blood, it is necessary for us all to know that donating blood does no harm to the donor. A person has approximately 200 to 250 ounces of blood in the body. To give 10 to 20 ounces will do no harm and this deficiency can be made up in 2 or 3 weeks time.