what is the central idea of this paragraph ?
Human blood is given to patients who need it via blood transfusions. A blood transfusion is a medical procedure that transfers a unit of blood from one person (a donor) to another (a recipient). Blood transfusions are usually well tolerated and safe. They’ve been used to save many lives and to improve the quality of many, many more. As a replacement for human blood, donated blood has been an unmatched success. Yet even blood is limited as a substitute for blood. Any two people cannot necessarily share blood. Special care must be taken to match the blood types of a donor and a recipient. Donated blood must also be screened to guarantee it’s free of transmittable diseases [...]. Packets of human blood have a limited shelf life. Even with refrigeration, blood lasts only about a month. Donors are continually needed to replenish stores of blood. Sometimes there are too few donors. This is particularly true in remote areas, during natural disasters, and on battlefields. In such situations, the demand for donated blood can exceed the supply.