Blood transfusions are performed by inserting a needle into the patient's blood vessel and connecting it to an intravenous line, or IV, the National Heart, Lung, and Blood Institute states. Blood passes from the IV line to the vessel and circulates into the patient's bloodstream, and the process lasts roughly one to four hours.
A blood transfusion replaces lost blood to help the body maintain a healthy supply, WebMD notes. Blood is most commonly sourced from donor contributions to blood banks, but in some cases, patients can have their own blood drawn and preserved in preparation for surgical procedures. Prior to donating, volunteers are screened for their blood type and possible health risks. After collecting donations, blood bank technicians conduct further testing to confirm each sample's type classification and search for signs of disease. Blood that doesn't meet safety requirements is discarded to prevent the spread of infectious contaminants.
Blood transfusions are most often performed when a patient experiences excessive blood loss during an injury or surgery, according to Mayo Clinic. The procedure is also used to counteract diseases that can interfere with blood production, such as cancer. In most cases, the components of blood a patient needs most, such as plasma or white bloods cells, are isolated for the transfusion. Matching patients and donors with the same blood type reduces the risk of an allergic reaction, but recipients can still suffer complications, including hives and fever.