Storing blood is much more difficult than simply freezing it. Blood for transfusion must be kept alive. Too cold and the cells freeze and die. Too hot and they die. 4 °C is just right. Like the three bears' porridge. Not too hot, not too cold. Blood bank refrigerators use the same recirculating air system as vaccine and pharmacy refrigerators. The evaporator is behind the back wall and a fan circulates the air over the evaporator and through the storage chamber where the blood bags are stored. Defrosting is automatic and since there is a wall between the evaporator being defrosted and the storage compartment, there is minimal disruption of the storage compartment temperature.
