Treating blood loss
Blood loss may result from injury, major surgery, or diseases that destroy red blood cells or platelets, two important blood components. If too much blood is lost (low blood volume), your body cannot maintain a proper blood pressure, which results in shock. Blood loss can also reduce the number of oxygen-carrying red blood cells in the blood, which may prevent enough oxygen from reaching the rest of the body.
Whole blood is rarely given to treat blood loss. Instead, you are given the blood component you most need. If you have lost too many red blood cells or are not making enough of them, you are given packed red blood cells. If you have low blood volume, you are given plasma and/or other fluids to maintain blood pressure. If you have lost a great deal of blood, or if your clotting factors or platelets are low or abnormal, you may also need a transfusion of either of these to help control bleeding. Sometimes you may need replacements of some blood substances if your body does not make enough of them. For example, you may be given substances to help your blood clot (clotting factors) if you do not have enough of them naturally.
Blood lost during surgery sometimes can be recovered, cleaned, and returned to you as a transfusion. This greatly reduces the amount of blood you might otherwise need to receive. Receiving your own blood back is safer, because there is no chance of a reaction.
Replacing or supplementing blood components
One blood component that affects the blood's ability to clot is platelets. A reduced number of platelets (thrombocytopenia) or the failure of platelets to function properly increases the time it takes for bleeding to stop (increased bleeding time). Transfusion with platelets improves the clotting time, which reduces the risk of uncontrolled bleeding. This treatment does not cure the cause of platelet loss.
Anemia is a decrease in the number of oxygen-carrying red blood cells or a decrease in the amount of hemoglobin, the oxygen-carrying substance in the red blood cells. There are several types of anemia, each with a different cause, and each is treated differently. Severe anemia may be treated with a transfusion of packed red blood cells. This temporarily increases the number of oxygen-carrying red blood cells in circulation and may improve symptoms, but it does not treat the cause of the anemia.