In the past, a blood transfusion used whole blood. Today whole blood is often separated into is component parts, and only those needed are used.

A patient with anemia or an acute blood loss might receive concentrated (packed) red blood cells; a person with too few platelets might be given platelets; a person with cancer whose treatment has depleted the white blood cell count might be given a white blood cell preparation. Similarly, blood plasma might be used to replace lost blood volume, or to provide clotting factors.

Before donor blood is used to obtain blood components, it is tested for particular viruses, including those that cause hepatitis B and AIDS . Then the blood components must be stored properly. For example, packed red blood cells can be stored for several years if they are frozen, but can be used for only about a month if they are not frozen. After thawing, however, such cells must be used within about a day. Platelet preparations must be used within five days, and white blood cell concentrates must be used immediately.

Synthetic blood--all still experimental--try to duplicate blood's oxygen-carrying ability. They are variations on chemicals that bind oxygen, mimicking the hemoglobin carried in red blood cells.

The first such synthetic blood was silicone oil. A mouse immersed into a beaker of silicone oil, for example, can "breathe" the oxygen in the oil. Researchers have tried related chemicals containing fluorine and carbon, which bind oxygen similarly to silicone oil. In 1990, the first red blood cell substitute was approved for use to maintain localized blood flow during certain surgical procedures. The product consists of two fluorine compounds, a mild detergent, and lipid form egg yolk.

In another approach, hemoglobin molecules are linked together and administered alone, rather than in cells as occurs in the circulatory system. Yet another type of artificial blood consists of hemoglobin packaged into synthetic microcapsules, which are tiny fat bubbles currently used in carbonless carbon paper and paper that releases an odor when scratched. However, these "neohemocytes" so far provoke an immune attack from the animals in which they have been tested. Artificial bloods based on hemoglobin do not offer the freedom from contamination possible with a completely synthetic product.

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