In a blood transfusion, donated blood is added to your own blood. A blood transfusion may also be done to supplement various components of your blood with donated blood products. In rare cases, a blood transfusion is done with blood that you've donated ahead of time before you undergo surgery.
During a typical blood transfusion, certain parts of blood are delivered through an intravenous (IV) line that's placed in one of the veins in your arm. A blood transfusion usually takes one to two hours, though in an emergency it can be done much faster.
A blood transfusion boosts blood levels that are low, either because your body isn't making enough or because blood has been lost during surgery, injury or disease.