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":http://en.wikipedia.org/wiki/Human_blood_group_systems, depending upon inherited antigens. There is also the + and - status to indicate if the "Rh factor":http://en.wikipedia.org/wiki/Rh_blood_group_system (or Rhesus factor) is present.

Blood bags are often seen hanging from [[iv stand|intravenous stands]] in hospitals near the patient's bed.
blood_pack 血液バッグ 血液袋 혈액백