A small bag full of blood, which has been collected from the patient or from a blood donor in order to replace that patient's blood should there be a lot of blood loss during surgery, or from exsanguination due to a critical injury.
Different patients/donors' blood is collected, analyzed and grouped according to blood type, of which there are four: A, B, AB, and O, depending upon inherited antigens. There is also the + and - status to indicate if the Rh factor (or Rhesus factor) is present.
Blood bags are often seen hanging from intravenous stands in hospitals near the patient's bed.
