Obtaining The Blood
There are two main methods of obtaining blood from a donor. The most frequent is to simply take the blood from a vein as whole blood. This blood is typically separated into parts, usually red blood cells and plasma, since most recipients need only a specific component for transfusions. A typical donation is 450 millilitres (or approximately one US pint) of whole blood, though 500 millilitre donations are also common. Historically, blood donors in India would donate only 250 or 350 millilitre and donors in the People's Republic of China would donate only 200 millilitres, though larger 300 and 400 millilitre donations have become more common.
The other method is to draw blood from the donor, separate it using a centrifuge or a filter, store the desired part, and return the rest to the donor. This process is called apheresis, and it is often done with a machine specifically designed for this purpose. This process is especially common for plasma and platelets.
For direct transfusions a vein can be used but the blood may be taken from an artery instead. In this case, the blood is not stored, but is pumped directly from the donor into the recipient. This was an early method for blood transfusion and is rarely used in modern practice. It was phased out during World War II because of problems with logistics, and doctors returning from treating wounded soldiers set up banks for stored blood when they returned to civilian life.
Read more about this topic: Blood Donation
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